An Introduction to Geology

An Introduction to Geology

Free Textbook for College-Level Introductory Geology Courses

Chris Johnson; Matthew D. Affolter; Paul Inkenbrandt; and Cam Mosher

An Introduction to Geology by Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, Cam Mosher is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Contents

Introduction

Main Body

1 Understanding Science

2 Plate Tectonics

3 Minerals

4 Igneous Processes and Volcanoes

5 Weathering, Erosion, and Sedimentary Rocks

6 Metamorphic Rocks

7 Geologic Time

8 Earth History

9 Crustal Deformation and Earthquakes

10 Mass Wasting

11 Water

12 Coastlines

13 Deserts

14 Glaciers

15 Global Climate Change

16 Energy and Mineral Resources

1

Introduction

An Introduction to Geology

Written by

Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, Cam Mosher

Salt Lake Community College – 2017

Contact the authors at edits (at_) opengeology.org with edits, suggestions, or if adopting the book.

Faculty who adopt this text for their course should contact the authors at edits (at_) opengeology.org so that the authors can keep faculty users up to date of critical changes.

——————————————————————

Edited and Reviewed by

Johnathan Barnes – Salt Lake Community College
Jack Bloom – Rio Tinto Kennecott Copper (retired)
Deanna Brandau – Utah Museum of Natural History
Gereld Bryant – Dixie State University
Gregg Beukelman – Utah Geological Survey
Peter Davis – Pacific Lutheran University
Renee Faatz – Snow College
Gabriel Filippelli – Indiana University-Purdue University Indianapolis
Michelle Cooper Fleck – Utah State University
Colby Ford – Dixie State University
Michael Hylland – Utah Geological Survey
J. Lucy Jordan – Utah Geological Survey
Mike Kass – Salt Lake Community College
Ben Laabs – North Dakota State University
Tom Lachmar – Utah State University
Johnny MacLean – Southern Utah University
Erich Peterson – University of Utah
Tiffany Rivera – Westminster College
Leif Tapanila – Idaho State University

Special Thanks to

Jason Pickavance – Salt Lake Community College, Director of Educational Initiatives
R. Adam Dastrup – Salt Lake Community College, Geoscience Dept. Chair, Director of Opengeography.org

Open Educational Resources

This text is provided to you as an Open Educational Resource which you access online. It is designed to give you a comprehensive introduction to Geology at no or very nominal cost. It contains both written and graphic text material, intra-text links to other internal material which may aid in understanding topics and concepts, intra-text links to the appendices and glossary for tables and definitions of words, and extra-text links to videos and web material that clarifies and augments topics and concepts. Like any new or scientific subject, Geology has its own vocabulary for geological concepts. For you to converse effectively with this text and colleagues in this earth science course, you will use the language of geology, so comprehending these terms is important. Use the intra-text links to the Glossary and other related material freely to gain familiarity with this language.

Tips for study

Each chapter begins with a list of KEY CONCEPTS you should be able to grasp through effective study of the chapter material. These are stated as Student Learning Objectives (SLOs) in behavioral terms. In other words, when you have completed study of that chapter, you should be able to do stated things with your understanding. Within chapters, each section concludes with a set of questions, called “Did I Get it?” questions. After completing the section, you should get these key points and answer questions related to the student learning objectives. At the end of each chapter are summaries of the sections and chapter review questions so you can review each section in the context of the chapter.

1 Understanding Science

1 Understanding Science

STUDENT LEARNING OUTCOMES

At the end of this chapter, students should be able to:

Contrast objective versus subjective observations, and quantitative versus qualitative observations

Identify a pseudoscience based on its lack of falsifiability

Contrast the methods used by Aristotle and Galileo to describe the natural environment

Explain the scientific method and apply it to a problem or question

Describe the foundations of modern geology, such as the principle of uniformitarianism

Contrast uniformitarianism with catastrophism

Explain why studying geology is important

Identify how Earth materials are transformed by rock cycle processes

Describe the steps involved in a reputable scientific study

Explain rhetorical arguments used by science deniers

1.1 What is Science?

Scientists seek to understand the fundamental principles that explain natural patterns and processes. Science is more than just a body of knowledge, science provides a means to evaluate and create new knowledge without bias. Scientists use objective evidence over subjective evidence, to reach sound and logical conclusions.

An objective observation is without personal bias and the same by all individuals. Humans are biased by nature, so they cannot be completely objective; the goal is to be as unbiased as possible. A subjective observation is based on a person’s feelings and beliefs and is unique to that individual.

Another way scientists avoid bias is by using quantitative over qualitative measurements whenever possible. A quantitative measurement is expressed with a specific numerical value. Qualitative observations are general or relative descriptions. For example, describing a rock as red or heavy is a qualitative observation. Determining a rock’s color by measuring wavelengths of reflected light or its density by measuring the proportions of minerals it contains is quantitative. Numerical values are more precise than general descriptions, and they can be analyzed using statistical calculations. This is why quantitative measurements are much more useful to scientists than qualitative observations.

Establishing truth in science is difficult because all scientific claims are falsifiable, which means any initial hypothesis may be tested and proven false. Only after exhaustively eliminating false results, competing ideas, and possible variations does a hypothesis become regarded as a reliable scientific theory. This meticulous scrutiny reveals weaknesses or flaws in a hypothesis and is the strength that supports all scientific ideas and procedures. In fact, proving current ideas are wrong has been the driving force behind many scientific careers.

Falsifiability separates science from pseudoscience. Scientists are wary of explanations of natural phenomena that discourage or avoid falsifiability. An explanation that cannot be tested or does not meet scientific standards is not considered science, but pseudoscience. Pseudoscience is a collection of ideas that may appear scientific but does not use the scientific method. Astrology is an example of pseudoscience. It is a belief system that attributes the movement of celestial bodies to influencing human behavior. Astrologers rely on celestial observations, but their conclusions are not based on experimental evidence and their statements are not falsifiable. This is not to be confused with astronomy which is the scientific study of celestial bodies and the cosmos.

Science is also a social process. Scientists share their ideas with peers at conferences, seeking guidance and feedback. Research papers and data submitted for publication are rigorously reviewed by qualified peers, scientists who are experts in the same field. The scientific review process aims to weed out misinformation, invalid research results, and wild speculation. Thus, it is slow, cautious, and conservative. Scientists tend to wait until a hypothesis is supported by overwhelming amount of evidence from many independent researchers before accepting it as scientific theory.

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1.2 The Scientific Method

Modern science is based on the scientific method, a procedure that follows these steps:

Formulate a question or observe a problem

Apply objective experimentation and observation

Analyze collected data and Interpret results

Devise an evidence-based theory

Submit findings to peer review and/or publication

This has a long history in human thought but was first fully formed by Ibn al-Haytham over 1,000 years ago. At the forefront of the scientific method are conclusions based on objective evidence, not opinion or hearsay.

Step One: Observation, Problem, or Research Question

The procedure begins with identifying a problem or research question, such as a geological phenomenon that is not well explained in the scientific community’s collective knowledge. This step usually involves reviewing the scientific literature to understand previous studies that may be related to the question.

Step Two: Hypothesis

Once the problem or question is well defined, the scientist proposes a possible answer, a hypothesis, before conducting an experiment or field work. This hypothesis must be specific, falsifiable, and should be based on other scientific work. Geologists often develop multiple working hypotheses because they usually cannot impose strict experimental controls or have limited opportunities to visit a field location.

Step Three: Experiment and Hypothesis Revision

The next step is developing an experiment that either supports or refutes the hypothesis. Many people mistakenly think experiments are only done in a lab; however, an experiment can consist of observing natural processes in the field. Regardless of what form an experiment takes, it always includes the systematic gathering of objective data. This data is interpreted to determine whether it contradicts or supports the hypothesis, which may be revised and tested again. When a hypothesis holds up under experimentation, it is ready to be shared with other experts in the field.

Step Four: Peer Review, Publication, and Replication

Scientists share the results of their research by publishing articles in scientific journals, such as Science and Nature. Reputable journals and publishing houses will not publish an experimental study until they have determined its methods are scientifically rigorous and the conclusions are supported by evidence. Before an article is published, it undergoes a rigorous peer review by scientific experts who scrutinize the methods, results, and discussion. Once an article is published, other scientists may attempt to replicate the results. This replication is necessary to confirm the reliability of the study’s reported results. A hypothesis that seemed compelling in one study might be proven false in studies conducted by other scientists. New technology can be applied to published studies, which can aid in confirming or rejecting once-accepted ideas and/or hypotheses.

Step Five: Theory Development

In casual conversation, the word theory implies guesswork or speculation. In the language of science, an explanation or conclusion made in a theory carries much more weight because it is supported by experimental verification and widely accepted by the scientific community. After a hypothesis has been repeatedly tested for falsifiability through documented and independent studies, it eventually becomes accepted as a scientific theory.

While a hypothesis provides a tentative explanation before an experiment, a theory is the best explanation after being confirmed by multiple independent experiments. Confirmation of a theory may take years, or even longer. For example, the continental drift hypothesis first proposed by Alfred Wegener in 1912 was initially dismissed. After decades of additional evidence collection by other scientists using more advanced technology, Wegener’s hypothesis was accepted and revised as the theory of plate tectonics.

The theory of evolution by natural selection is another example. Originating from the work of Charles Darwin in the mid-19th century, the theory of evolution has withstood generations of scientific testing for falsifiability. While it has been updated and revised to accommodate knowledge gained by using modern technologies, the theory of evolution continues to be supported by the latest evidence.

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1.3 Early Scientific Thought

Western scientific thought began in the ancient city of Athens, Greece. Athens was governed as a democracy, which encouraged individuals to think independently, at a time when most civilizations were ruled by monarchies or military conquerors. Foremost among the early philosopher/scientists to use empirical thinking was Aristotle, born in 384 BCE. Empiricism emphasizes the value of evidence gained from experimentation and observation. Aristotle studied under Plato and tutored Alexander the Great. Alexander would later conquer the Persian Empire, and in the process spread Greek culture as far east as India.

Aristotle applied an empirical method of analysis called deductive reasoning, which applies known principles of thought to establish new ideas or predict new outcomes. Deductive reasoning starts with generalized principles and logically extends them to new ideas or specific conclusions. If the initial principle is valid, then it is highly likely the conclusion is also valid. An example of deductive reasoning is if A=B, and B=C, then A=C. Another example is if all birds have feathers, and a sparrow is a bird, then a sparrow must also have feathers. The problem with deductive reasoning is if the initial principle is flawed, the conclusion will inherit that flaw. Here is an example of a flawed initial principle leading to the wrong conclusion; if all animals that fly are birds, and bats also fly, then bats must also be birds.

This type of empirical thinking contrasts with inductive reasoning, which begins from new observations and attempts to discern underlying generalized principles. A conclusion made through inductive reasoning comes from analyzing measurable evidence, rather making a logical connection. For example, to determine whether bats are birds a scientist might list various characteristics observed in birds–the presence of feathers, a toothless beak, hollow bones, lack of forelegs, and externally laid eggs. Next, the scientist would check whether bats share the same characteristics, and if they do not, draw the conclusion that bats are not birds.

Both types of reasoning are important in science because they emphasize the two most important aspects of science: observation and inference. Scientists test existing principles to see if they accurately infer or predict their observations. They also analyze new observations to determine if the inferred underlying principles still support them.

Greek culture was spread by Alexander and then absorbed by the Romans, who help further extend Greek knowledge into Europe through their vast infrastructure of roads, bridges, and aqueducts. After the fall of the Roman Empire in 476 CE, scientific progress in Europe stalled. Scientific thinkers of medieval time had such high regard for Aristotle’s wisdom and knowledge they faithfully followed his logical approach to understanding nature for centuries. By contrast, science in the Middle East flourished and grew between 800 and 1450 CE, along with culture and the arts.

Near the end of the medieval period, empirical experimentation became more common in Europe. During the Renaissance, which lasted from the 14th through 17th centuries, artistic and scientific thought experienced a great awakening. European scholars began to criticize the traditional Aristotelian approach and by the end of the Renaissance period, empiricism was poised to become a key component of the scientific revolution that would arise in the 17th century.

An early example of how Renaissance scientists began to apply a modern empirical approach is their study of the solar system. In the second century, the Greek astronomer Claudius Ptolemy observed the Sun, Moon, and stars moving across the sky. Applying Aristotelian logic to his astronomical calculations, he deductively reasoned all celestial bodies orbited around the Earth, which was located at the center of the universe. Ptolemy was a highly regarded mathematician, and his mathematical calculations were widely accepted by the scientific community. The view of the cosmos with Earth at its center is called the geocentric model. This geocentric model persisted until the Renaissance period, when some revolutionary thinkers challenged the centuries-old hypothesis.

By contrast, early Renaissance scholars such as astronomer Nicolaus Copernicus (1473-1543) proposed an alternative explanation for the perceived movement of the Sun, Moon, and stars. Sometime between 1507 and 1515, he provided credible mathematical proof for a radically new model of the cosmos, one in which the Earth and other planets orbited around a centrally located Sun. After the invention of the telescope in 1608, scientists used their enhanced astronomical observations to support this heliocentric, Sun-centered, model.

Two scientists, Johannes Kepler and Galileo Galilei, are credited with jump-starting the scientific revolution. They accomplished this by building on Copernicus work and challenging long-established ideas about nature and science.

Johannes Kepler (1571-1630) was a German mathematician and astronomer who expanded on the heliocentric model—improving Copernicus’ original calculations and describing planetary motion as elliptical paths. Galileo Galilei (1564 – 1642) was an Italian astronomer who used the newly developed telescope to observe the four largest moons of Jupiter. This was the first piece of direct evidence to contradict the geocentric model, since moons orbiting Jupiter could not also be orbiting Earth.

Galileo strongly supported the heliocentric model and attacked the geocentric model, arguing for a more scientific approach to determine the credibility of an idea. Because of this he found himself at odds with prevailing scientific views and the Catholic Church. In 1633 he was found guilty of heresy and placed under house arrest, where he would remain until his death in 1642.

Galileo is regarded as the first modern scientist because he conducted experiments that would prove or disprove falsifiable ideas and based his conclusions on mathematical analysis of quantifiable evidence—a radical departure from the deductive thinking of Greek philosophers such as Aristotle . His methods marked the beginning of a major shift in how scientists studied the natural world, with an increasing number of them relying on evidence and experimentation to form their hypotheses. It was during this revolutionary time that geologists such as James Hutton and Nicolas Steno also made great advances in their scientific fields of study.

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1.4 Foundations of Modern Geology

As part of the scientific revolution in Europe, modern geologic principles developed in the 17th and 18th centuries. One major contributor was Nicolaus Steno (1638-1686), a Danish priest who studied anatomy and geology. Steno was the first to propose the Earth’s surface could change over time. He suggested sedimentary rocks, such as sandstone and shale, originally formed in horizontal layers with the oldest on the bottom and progressively younger layers on top.

In the 18th century, Scottish naturalist James Hutton (1726–1797) studied rivers and coastlines and compared the sediments they left behind to exposed sedimentary rock strata. He hypothesized the ancient rocks must have been formed by processes like those producing the features in the oceans and streams. Hutton also proposed the Earth was much older than previously thought. Modern geologic processes operate slowly. Hutton realized if these processes formed rocks, then the Earth must be very old, possibly hundreds of millions of years old.

Hutton’s idea is called the principle of uniformitarianism and states that natural processes operate the same now as in the past, i.e. the laws of nature are uniform across space and time. Geologist often state “the present is the key to the past,” meaning they can understand ancient rocks by studying modern geologic processes.

Prior to the acceptance of uniformitarianism, scientists such as German geologist Abraham Gottlob Werner (1750-1817) and French anatomist Georges Cuvier (1769-1832) thought rocks and landforms were formed by great catastrophic events. Cuvier championed this view, known as catastrophism, and stated, “The thread of operation is broken; nature has changed course, and none of the agents she employs today would have been sufficient to produce her former works.” He meant processes that operate today did not operate in the past. Known as the father of vertebrate paleontology, Cuvier made significant contributions to the study of ancient life and taught at Paris’s Museum of Natural History. Based on his study of large vertebrate fossils, he was the first to suggest species could go extinct. However, he thought new species were introduced by special creation after catastrophic floods.

Hutton’s ideas about uniformitarianism and Earth’s age were not well received by the scientific community of his time. His ideas were falling into obscurity when Charles Lyell, a British lawyer and geologist (1797-1875), wrote the Principles of Geology in the early 1830s and later, Elements of Geology. Lyell’s books promoted Hutton’s principle of uniformitarianism, his studies of rocks and the processes that formed them, and the idea that Earth was possibly over 300 million years old. Lyell and his three-volume Principles of Geology had a lasting influence on the geologic community and public at large, who eventually accepted uniformitarianism and millionfold age for the Earth. The principle of uniformitarianism became so widely accepted, that geologists regarded catastrophic change as heresy. This made it harder for ideas like the sudden demise of the dinosaurs by asteroid impact to gain traction.

A contemporary of Lyell, Charles Darwin (1809-1882) took Principles of Geology on his five-year trip on the HMS Beagle. Darwin used uniformitarianism and deep geologic time to develop his initial ideas about evolution. Lyell was one of the first to publish a reference to Darwin’s idea of evolution.

The next big advancement, and perhaps the largest in the history of geology, is the theory of plate tectonics and continental drift. Dogmatic acceptance of uniformitarianism inhibited the progress of this idea, mainly because of the permanency placed on the continents and their positions. Ironically, slow and steady movement of plates would fit well into a uniformitarianism model. However, much time passed and a great deal of scientific resistance had to be overcome before the idea took hold. This happened for several reasons. Firstly, the movement was so slow it was overlooked. Secondly, the best evidence was hidden under the ocean. Finally, the accepted theories were anchored by a large amount of inertia. Instead of being bias free, scientists resisted and ridiculed the emerging idea of plate tectonics. This example of dogmatic thinking is still to this day a tarnish on the geoscience community.

Plate tectonics is most commonly attributed to Alfred Wegener, the first scientist to compile a large data set supporting the idea of continents shifting places over time. He was mostly ignored and ridiculed for his ideas, but later workers like Marie Tharp, Bruce Heezen, Harry Hess, Laurence Morley, Frederick Vine, Drummond Matthews, Kiyoo Wadati, Hugo Benioff, Robert Coats, and J. Tuzo Wilson benefited from advances in sub-sea technologies. They discovered, described, and analyzed new features like the mid-ocean ridge, alignment of earthquakes, and magnetic striping. Gradually these scientists introduced a paradigm shift that revolutionized geology into the science we know today.

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1.5 The Study of Geology

Geologists apply the scientific method to learn about Earth’s materials and processes. Geology plays an important role in society; its principles are essential to locating, extracting, and managing natural resources; evaluating environmental impacts of using or extracting these resources; as well as understanding and mitigating the effects of natural hazards.

Geology often applies information from physics and chemistry to the natural world, like understanding the physical forces in a landslide or the chemical interaction between water and rocks. The term comes from the Greek word geo, meaning Earth, and logos, meaning to think or reckon with.

1.5.1 Why Study Geology?

Geology plays a key role in how we use natural resources—any naturally occurring material that can be extracted from the Earth for economic gain. Our developed modern society, like all societies before it, is dependent on geologic resources. Geologists are involved in extracting fossil fuels, such as coal and petroleum; metals such as copper, aluminum, and iron; and water resources in streams and underground reservoirs inside soil and rocks. They can help conserve our planet’s finite supply of nonrenewable resources, like petroleum, which are fixed in quantity and depleted by consumption. Geologists can also help manage renewable resources that can be replaced or regenerated, such as solar or wind energy, and timber.

Resource extraction and usage impacts our environment, which can negatively affect human health. For example, burning fossil fuels releases chemicals into the air that are unhealthy for humans, especially children. Mining activities can release toxic heavy metals, such as lead and mercury, into the soil and waterways. Our choices will have an effect on Earth’s environment for the foreseeable future. Understanding the remaining quantity, extractability, and renewability of geologic resources will help us better sustainably manage those resources.

Geologists also study natural hazards created by geologic processes. Natural hazards are phenomena that are potentially dangerous to human life or property. No place on Earth is completely free of natural hazards, so one of the best ways people can protect themselves is by understanding geology. Geology can teach people about the natural hazards in an area and how to prepare for them. Geologic hazards include landslides, earthquakes, tsunamis, floods, volcanic eruptions, and sea-level rise.

Finally, geology is where other scientific disciplines intersect in the concept known as Earth System Science. In science, a system is a group of interactive objects and processes. Earth System Science views the entire planet as a combination of systems that interact with each other via complex relationships. This geology textbook provides an introduction to science in general and will often reference other scientific disciplines.

Earth System Science includes five basic systems (or spheres), the Geosphere (the solid body of the Earth), the Atmosphere (the gas envelope surrounding the Earth), the Hydrosphere (water in all its forms at and near the surface of the Earth), the Cryosphere (frozen water part of Earth), and the Biosphere (life on Earth in all its forms and interactions, including humankind).

Rather than viewing geology as an isolated system, earth system scientists study how geologic processes shape not only the world, but all the spheres it contains. They study how these multidisciplinary spheres relate, interact, and change in response to natural cycles and human-driven forces. They use elements from physics, chemistry, biology, meteorology, environmental science, zoology, hydrology, and many other sciences.

1.5.2 Rock Cycle

The most fundamental view of Earth materials is the rock cycle, which describes the major materials that comprise the Earth, the processes that form them, and how they relate to each other. It usually begins with hot molten liquid rock called magma or lava. Magma forms under the Earth’s surface in the crust or mantle. Lava is molten rock that erupts onto the Earth’s surface. When magma or lava cools, it solidifies by a process called crystallization in which minerals grow within the magma or lava. The rocks resulting rocks are igneous rocks. Ignis is Latin for fire.

Igneous rocks, as well as other types of rocks, on Earth’s surface are exposed to weathering and erosion, which produces sediments. Weathering is the physical and chemical breakdown of rocks into smaller fragments. Erosion is the removal of those fragments from their original location. The broken-down and transported fragments or grains are considered sediments, such as gravel, sand, silt, and clay. These sediments may be transported by streams and rivers, ocean currents, glaciers, and wind.

Sediments come to rest in a process known as deposition. As the deposited sediments accumulate—often under water, such as in a shallow marine environment—the older sediments get buried by the new deposits. The deposits are compacted by the weight of the overlying sediments and individual grains are cemented together by minerals in groundwater. These processes of compaction and cementation are called lithification. Lithified sediments are considered a sedimentary rock, such as sandstone and shale. Other sedimentary rocks are made by direct chemical precipitation of minerals rather than eroded sediments, and are known as chemical sedimentary rocks.

Pre-existing rocks may be transformed into a metamorphic rock; meta- means change and -morphos means form or shape. When rocks are subjected to extreme increases in temperature or pressure, the mineral crystals are enlarged or altered into entirely new minerals with similar chemical make up. High temperatures and pressures occur in rocks buried deep within the Earth’s crust or that come into contact with hot magma or lava. If the temperature and pressure conditions melt the rocks to create magma and lava, the rock cycle begins anew with the creation of new rocks.

1.5.3 Plate Tectonics and Layers of Earth

The theory of plate tectonics is the fundamental unifying principle of geology and the rock cycle. Plate tectonics describes how Earth’s layers move relative to each other, focusing on the tectonic or lithospheric plates of the outer layer. Tectonic plates float, collide, slide past each other, and split apart on an underlying mobile layer called the asthenosphere. Major landforms are created at the plate boundaries, and rocks within the tectonic plates move through the rock cycle. Plate tectonics is discussed in more detail in Chapter 2.

Earth’s three main geological layers can be categorized by chemical composition or the chemical makeup: crust, mantle, and core. The crust is the outermost layer and composed of mostly silicon, oxygen, aluminum, iron, and magnesium. There are two types, continental crust and oceanic crust. Continental crust is about 50 km (30 mi) thick, composed of low-density igneous and sedimentary rocks, Oceanic crust is approximately 10 km (6 mi) thick and made of high-density igneous basalt-type rocks. Oceanic crust makes up most of the ocean floor, covering about 70% of the planet. Tectonic plates are made of crust and a portion the upper mantle, forming a rigid physical layer called the lithosphere.

The mantle, the largest chemical layer by volume, lies below the crust and extends down to about 2,900 km (1,800 mi) below the Earth’s surface. The mostly solid mantle is made of peridotite, a high-density composed of silica, iron, and magnesium. The upper part of mantel is very hot and flexible, which allows the overlying tectonic plates to float and move about on it. Under the mantle is the Earth’s core, which is 3,500 km (2,200 mi) thick and made of iron and nickel. The core consists of two parts, a liquid outer core and solid inner core. Rotations within the solid and liquid metallic core generate Earth’s magnetic field (see figure).

1.5.4 Geologic Time and Deep Time

“The result, therefore, of our present enquiry is, that we find no vestige of a beginning; no prospect of an end.” (James Hutton, 1788)

One of the early pioneers of geology, James Hutton, wrote this about the age of the Earth after many years of geological study. Although he wasn’t exactly correct—there is a beginning and will be an end to planet Earth—Hutton was expressing the difficulty humans have in perceiving the vastness of geological time. Hutton did not assign an age to the Earth, although he was the first to suggest the planet was very old.
Today we know Earth is approximately 4.54 ± 0.05 billion years old. This age was first calculated by Caltech professor Clair Patterson in 1956, who measured the half-lives of lead isotopes to radiometrically date a meteorite recovered in Arizona. Studying geologic time, also known as deep time, can help us overcome a perspective of Earth that is limited to our short lifetimes. Compared to the geologic scale, the human lifespan is very short, and we struggle to comprehend the depth of geologic time and slowness of geologic processes. For example, the study of earthquakes only goes back about 100 years; however, there is geologic evidence of large earthquakes occurring thousands of years ago. And scientific evidence indicates earthquakes will continue for many centuries into the future.

Eons are the largest divisions of time, and from oldest to youngest are named Hadean, Archean, Proterozoic, and Phanerozoic. The three oldest eons are sometimes collectively referred to as Precambrian time.

Life first appeared more than 3,800 million of years ago (Ma). From 3,500 Ma to 542 Ma, or 88% of geologic time, the predominant life forms were single-celled organisms such as bacteria. More complex organisms appeared only more recently, during the current Phanerozoic Eon, which includes the last 542 million years or 12% of geologic time.

The name Phanerozoic comes from phaneros, which means visible, and zoic, meaning life. This eon marks the proliferation of multicellular animals with hard body parts, such as shells, which are preserved in the geological record as fossils. Land-dwelling animals have existed for 360 million years, or 8% of geologic time. The demise of the dinosaurs and subsequent rise of mammals occurred around 65 Ma, or 1.5% of geologic time. Our human ancestors belonging to the genus Homo have existed since approximately 2.2 Ma—0.05% of geological time or just 1/2,000th the total age of Earth.

The Phanerozoic Eon is divided into three eras: Paleozoic, Mesozoic, and Cenozoic. Paleozoic means ancient life, and organisms of this era included invertebrate animals, fish, amphibians, and reptiles. The Mesozoic (middle life) is popularly known as the Age of Reptiles and is characterized by the abundance of dinosaurs, many of which evolved into birds. The mass extinction of the dinosaurs and other apex predator reptiles marked the end of the Mesozoic and beginning of the Cenozoic. Cenozoic means new life and is also called the Age of Mammals, during which mammals evolved to become the predominant land-dwelling animals. Fossils of early humans, or hominids, appear in the rock record only during the last few million years of the Cenozoic. The geologic time scale, geologic time, and geologic history are discussed in more detail in chapters 7 and 8.

1.5.5 The Geologist’s Tools

In its simplest form, a geologist’s tool may be a rock hammer used for sampling a fresh surface of a rock. A basic tool set for fieldwork might also include:

Magnifying lens for looking at mineralogical details

Compass for measuring the orientation of geologic features

Map for documenting the local distribution of rocks and minerals

Magnet for identifying magnetic minerals like magnetite

Dilute solution of hydrochloric acid to identify carbonate-containing minerals like calcite or limestone.

In the laboratory, geologists use optical microscopes to closely examine rocks and soil for mineral composition and grain size. Laser and mass spectrometers precisely measure the chemical composition and geological age of minerals. Seismographs record and locate earthquake activity, or when used in conjunction with ground penetrating radar, locate objects buried beneath the surface of the earth. Scientists apply computer simulations to turn their collected data into testable, theoretical models. Hydrogeologists drill wells to sample and analyze underground water quality and availability. Geochemists use scanning electron microscopes to analyze minerals at the atomic level, via x-rays. Other geologists use gas chromatography to analyze liquids and gases trapped in glacial ice or rocks.

Technology provides new tools for scientific observation, which leads to new evidence that helps scientists revise and even refute old ideas. Because the ultimate technology will never be discovered, the ultimate observation will never be made. And this is the beauty of science—it is ever-advancing and always discovering something new.

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1.6 Science Denial and Evaluating Sources

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Introductory science courses usually deal with accepted scientific theory and do not include opposing ideas, even though these alternate ideas may be credible. This makes it easier for students to understand the complex material. Advanced students will encounter more controversies as they continue to study their discipline.

Some groups of people argue that some established scientific theories are wrong, not based on their scientific merit but rather on the ideology of the group. This section focuses on how to identify evidence based information and differentiate it from pseudoscience.

1.6.1 Science Denial

Science denial happens when people argue that established scientific theories are wrong, not based on scientific merit but rather on subjective ideology—such as for social, political, or economic reasons. Organizations and people use science denial as a rhetorical argument against issues or ideas they oppose. Three examples of science denial versus science are: 1) teaching evolution in public schools, 2) linking tobacco smoke to cancer, and 3) linking human activity to climate change. Among these, denial of climate change is strongly connected with geology. A climate denier specifically denies or doubts the objective conclusions of geologists and climate scientists.

Science denial generally uses three false arguments. The first argument tries to undermine the credibility of the scientific conclusion by claiming the research methods are flawed or the theory is not universally accepted—the science is unsettled. The notion that scientific ideas are not absolute creates doubt for non-scientists; however, a lack of universal truths should not be confused with scientific uncertainty. Because science is based on falsfiabiity, scientists avoid claiming universal truths and use language that conveys uncertainty. This allows scientific ideas to change and evolve as more evidence is uncovered.

The second argument claims the researchers are not objective and motivated by an ideology or economic agenda. This is an ad hominem argument in which a person’s character is attacked instead of the merit of their argument. They claim results have been manipulated so researchers can justify asking for more funding. They claim that because the researchers are funded by a federal grant, they are using their results to lobby for expanded government regulation.

The third argument is to demand a balanced view, equal time in media coverage and educational curricula, to engender the false illusion of two equally valid arguments. Science deniers frequently demand equal coverage of their proposals, even when there is little scientific evidence supporting their ideology. For example, science deniers might demand religious explanations be taught as an alternative to the well-established theory of evolution [zotpressInText item=”{X9U8B54N},{W934C3CR}” format=”%num%” brackets=”yes”]. Or that all possible causes of climate change be discussed as equally probable, regardless of the body of evidence. Conclusions derived using the scientific method should not be confused with those based on ideologies.

Furthermore, conclusions about nature derived from ideologies have no place in science research and education. For example, it would be inappropriate to teach the flat earth model in a modern geology course because this idea has been disproved by the scientific method. Unfortunately, widespread scientific illiteracy allows these arguments to be used to suppress scientific knowledge and spread misinformation.

The formation of new conclusions based on the scientific method is the only way to change scientific conclusions. We wouldn’t teach Flat Earth geology along with plate tectonics because Flat Earthers don’t follow the scientific method. The fact that scientists avoid universal truths and change their ideas as more evidence is uncovered shouldn’t be seen as meaning that the science is unsettled. Because of widespread scientific illiteracy, these arguments are used by those who wish to suppress science and misinform the general public.

In a classic case of science denial, beginning in the 1960s and for the next three decades, the tobacco industry and their scientists used rhetorical arguments to deny a connection between tobacco usage and cancer. Once it became clear scientific studies overwhelmingly found that using tobacco dramatically increased a person’s likelihood of getting cancer, their next strategy was to create a sense of doubt about on the science. The tobacco industry suggested the results were not yet fully understood and more study was needed. They used this doubt to lobby for delaying legislative action that would warn consumers of the potential health hazards [zotpressInText item=”{X9U8B54N},{CBD5438R}” format=”%num%” brackets=”yes”]. This same tactic is currently being employed by those who deny the significance of human involvement in climate change.

1.6.2 Evaluating Sources of Information

In the age of the internet, information is plentiful. Geologists, scientists, or anyone exploring scientific inquiry must discern valid sources of information from pseudoscience and misinformation. This evaluation is especially important in scientific research because scientific knowledge is respected for its reliability. Textbooks such as this one can aid this complex and crucial task. At its roots, quality information comes from the scientific method, beginning with the empirical thinking of Aristotle. The application of the scientific method helps produce unbiased results. A valid inference or interpretation is based on objective evidence or data. Credible data and inferences are clearly labeled, separated, and differentiated. Anyone looking over the data can understand how the author’s conclusion was derived or come to an alternative conclusion. Scientific procedures are clearly defined so the investigation can be replicated to confirm the original results or expanded further to produce new results. These measures make a scientific inquiry valid and its use as a source reputable. Of course, substandard work occasionally slips through and retractions are published from time to time. An infamous article linking the MMR vaccine to autism appeared in the highly reputable journal Lancet in 1998. Journalists discovered the author had multiple conflicts of interest and fabricated data, and the article was retracted in 2010.

In addition to methodology, data, and results, the authors of a study should be investigated. When looking into any research, the author(s) should be investigated. An author’s credibility is based on multiple factors, such as having a degree in a relevant topic or being funded from an unbiased source.

The same rigor should be applied to evaluating the publisher, ensuring the results reported come from an unbiased process. The publisher should be easy to discover. Good publishers will show the latest papers in the journal and make their contact information and identification clear. Reputable journals show their peer review style. Some journal are predatory, where they use unexplained and unnecessary fees to submit and access journals. Reputable journals have recognizable editorial boards. Often, a reliable journal will associate with a trade, association, or recognized open source initiative.

One of the hallmarks of scientific research is peer review. Research should be transparent to peer review. This allows the scientific community to reproduce experimental results, correct and retract errors, and validate theories. This allows reproduction of experimental results, corrections of errors, and proper justification of the research to experts.

Citation is not only imperative to avoid plagiarism, but also allows readers to investigate an author’s line of thought and conclusions. When reading scientific works, it is important to confirm the citations are from reputable scientific research. Most often, scientific citations are used to reference paraphrasing rather than quotes. The number of times a work is cited is said to measure of the influence an investigation has within the scientific community, although this technique is inherently biased.

Take this quiz to check your comprehension of this section.

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Summary

Science is a process, with no beginning and no end. Science is never finished because a full truth can never be known. However, science and the scientific method are the best way to understand the universe we live in. Scientists draw conclusions based on objective evidence; they consolidate these conclusions into unifying models. Geologists likewise understand studying the Earth is an ongoing process, beginning with James Hutton who declared the Earth has “…no vestige of a beginning, no prospect of an end.” Geologists explore the 4.5 billion-year history of Earth, its resources, and its many hazards. From a larger viewpoint, geology can teach people how to develop credible conclusions, as well as identify and stop misinformation.

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2 Plate Tectonics

2 Plate Tectonics

KEY CONCEPTS

At the end of this chapter, students should be able to:

Describe how the ideas behind plate tectonics started with Alfred Wegener’s hypothesis of continental drift

Describe the physical and chemical layers of the Earth and how they affect plate movement

Explain how movement at the three types of plate boundaries causes earthquakes, volcanoes, and mountain building

Identify convergent boundaries, including subduction and collisions, as places where plates come together

Identify divergent boundaries, including rifts and mid-ocean ridges, as places where plates separate

Explain transform boundaries as places where adjacent plates shear past each other

Describe the Wilson Cycle, beginning with continental rifting, ocean basin creation, plate subduction, and ending with ocean basin closure

Explain how the tracks of hotspots, places that have continually rising magma, is used to calculate plate motion

Revolution is a word usually reserved for significant political or social changes. Several of these idea revolutions forced scientists to re-examine their entire field, triggering a paradigm shift that shook up their conventionally held knowledge. Charles Darwin’s book on evolution, On the Origin of Species, published in 1859; Gregor Mendel’s discovery of the genetic principles of inheritance in 1866; and James Watson, Francis Crick, and Rosalind Franklin’s model for the structure of DNA in 1953 did that for biology. Albert Einstein’s relativity and quantum mechanics concepts in the early twentieth century did the same for Newtonian physics.

The concept of plate tectonics was just as revolutionary for geology. The theory of plate tectonics attributes the movement of massive sections of the Earth’s outer layers with creating earthquakes, mountains, and volcanoes. Many earth processes make more sense when viewed through the lens of plate tectonics. Because it is so important in understanding how the world works, plate tectonics is the first topic of discussion in this textbook.

2.1 Alfred Wegener’s Continental Drift Hypothesis

Alfred Wegener (1880-1930) was a German scientist who specialized in meteorology and climatology. His knack for questioning accepted ideas started in 1910 when he disagreed with the explanation that the Bering Land Bridge was formed by isostasy, and that similar land bridges once connected the continents. After reviewing the scientific literature, he published a hypothesis stating the continents were originally connected, and then drifted apart. While he did not have the precise mechanism worked out, his hypothesis was backed up by a long list of evidence.

2.1.1 Early Evidence for Continental Drift Hypothesis

Wegener’s first piece of evidence was that the coastlines of some continents fit together like pieces of a jigsaw puzzle. People noticed the similarities in the coastlines of South America and Africa on the first world maps, and some suggested the continents had been ripped apart. Antonio Snider-Pellegrini did preliminary work on continental separation and matching fossils in 1858.

What Wegener did differently was synthesize a large amount of data in one place. He used true edges of the continents, based on the shapes of the continental shelves. This resulted in a better fit than previous efforts that traced the existing coastlines.

Wegener also compiled evidence by comparing similar rocks, mountains, fossils, and glacial formations across oceans. For example, the fossils of the primitive aquatic reptile Mesosaurus were found on the separate coastlines of Africa and South America. Fossils of another reptile, Lystrosaurus, were found on Africa, India, and Antarctica. He pointed out these were land-dwelling creatures could not have swum across an entire ocean.

Opponents of continental drift insisted trans-oceanic land bridges allowed animals and plants to move between continents. The land bridges eventually eroded away, leaving the continents permanently separated. The problem with this hypothesis is the improbability of a land bridge being tall and long enough to stretch across a broad, deep ocean.

More support for continental drift came from the puzzling evidence that glaciers once existed in normally very warm areas in southern Africa, India, Australia, and Arabia. These climate anomalies could not be explained by land bridges. Wegener found similar evidence when he discovered tropical plant fossils in the frozen region of the Arctic Circle. As Wegener collected more data, he realized the explanation that best fit all the climate, rock, and fossil observations involved moving continents.

2.1.2 Proposed Mechanism for Continental Drift

Wegener’s work was considered a fringe science theory for his entire life. One of the biggest flaws in his hypothesis was an inability to provide a mechanism for how the continents moved. Obviously, the continents did not appear to move, and changing the conservative minds of the scientific community would require exceptional evidence that supported a credible mechanism. Other pro-continental drift followers used expansion, contraction, or even the moon’s origin to explain how the continents moved. Wegener used centrifugal forces and precession, but this model was proven wrong. He also speculated about seafloor spreading, with hints of convection, but could not substantiate these proposals. As it turns out, current scientific knowledge reveals convection is one the major forces in driving plate movements, along with gravity and density.

2.1.3 Development of Plate Tectonic Theory

Wegener died in 1930 on an expedition in Greenland. Poorly respected in his lifetime, Wegener and his ideas about moving continents seemed destined to be lost in history as fringe science. However, in the 1950s, evidence started to trickle in that made continental drift a more viable idea. By the 1960s, scientists had amassed enough evidence to support the missing mechanism—namely, seafloor spreading—for Wegener’s hypothesis of continental drift to be accepted as the theory of plate tectonics. Ongoing GPS and earthquake data analyses continue to support this theory. The next section provides the pieces of evidence that helped transform one man’s wild notion into a scientific theory.

Mapping of the Ocean Floors

In 1947 researchers started using an adaptation of SONAR to map a region in the middle of the Atlantic Ocean with poorly-understood topographic and thermal properties. Using this information, Bruce Heezen and Marie Tharp created the first detailed map of the ocean floor to reveal the Mid-Atlantic Ridge, a basaltic mountain range that spanned the length of the Atlantic Ocean, with rock chemistry and dimensions unlike the mountains found on the continents. Initially scientists thought the ridge was part of a mechanism that explained the expanding Earth or ocean-basin growth hypotheses. In 1959, Harry Hess proposed the hypothesis of seafloor spreading – that the mid-ocean ridges represented tectonic plate factories, where new oceanic plate was issuing from these long volcanic ridges. Scientists later included transform faults perpendicular to the ridges to better account for varying rates of movement between the newly formed plates. When earthquake epicenters were discovered along the ridges, the idea that earthquakes were linked to plate movement took hold.

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Seafloor sediment, measured by dredging and drilling, provided another clue. Scientists once believed sediment accumulated on the ocean floors over a very long time in a static environment. When some studies showed less sediment than expected, these results were initially used to argue against continental movement. With more time, researchers discovered these thinner sediment layers were located close to mid-ocean ridges, indicating the ridges were younger than the surrounding ocean floor. This finding supported the idea that the sea floor was not fixed in one place.

Paleomagnetism

The seafloor was also mapped magnetically. Scientists had long known of strange magnetic anomalies that formed a striped pattern of symmetrical rows on both sides of mid-oceanic ridges. What made these features unusual was the north and south magnetic poles within each stripe was reversed in alternating rows. By 1963, Harry Hess and other scientists used these magnetic reversal patterns to support their model for seafloor spreading (see also Lawrence W. Morley).

Paleomagnetism is the study of magnetic fields frozen within rocks, basically a fossilized compass. In fact, the first hard evidence to support plate motion came from paleomagnetism.

Igneous rocks containing magnetic minerals like magnetite typically provide the most useful data. In their liquid state as magma or lava, the magnetic poles of the minerals align themselves with the Earth’s magnetic field. When the rock cools and solidifies, this alignment is frozen into place, creating a permanent paleomagnetic record that includes magnetic inclination related to global latitude, and declination related to magnetic north.

Scientists had noticed for some time the alignment of magnetic north in many rocks was nowhere close to the earth’s current magnetic north. Some explained this away are part of the normal movement of earth’s magnetic north pole. Eventually, scientists realized adding the idea of continental movement explained the data better than pole movement alone.

Wadati-Benioff Zones

Around the same time mid-ocean ridges were being investigated, other scientists linked the creation of ocean trenches and island arcs to seismic activity and tectonic plate movement. Several independent research groups recognized earthquake epicenters traced the shapes of oceanic plates sinking into the mantle. These deep earthquake zones congregated in planes that started near the surface around ocean trenches and angled beneath the continents and island arcs. Today these earthquake zones called Wadati-Benioff zones.

Based on the mounting evidence, the theory plate tectonics continued to take shape. J. Tuzo Wilson was the first scientist to put the entire picture together by proposing that the opening and closing of the ocean basins. Before long, scientists proposed other models showing plates moving with respect to each other, with clear boundaries between them. Others started piecing together complicated histories of tectonic plate movement. The plate tectonic revolution had taken hold.

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Take this quiz to check your comprehension of this section.

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2.2 Layers of the Earth

In order to understand the details of plate tectonics, it is essential to first understand the layers of the earth. Firsthand information about what is below the surface is very limited; most of what we know is pieced together from hypothetical models, and analyzing seismic wave data and meteorite materials. In general, the Earth can be divided into layers based on chemical composition and physical characteristics.

2.2.1 Chemical Layers

Certainly the earth is composed of a countless combination of elements. Regardless of what elements are involved two major factors—temperature and pressure—are responsible for creating three distinct chemical layers.

Crust

The outermost chemical layer and the one we currently reside on, is the crust. There are two types of crust. Continental crust has a relatively low density and composition similar to granite. Oceanic crust has a relatively high density, especially when cold and old, and composition similar to basalt. The surface levels of crust are relatively brittle. The deeper parts of the crust are subjected to higher temperatures and pressure, which makes them more ductile. Ductile materials are like soft plastics or putty, they move under force. Brittle materials are like solid glass or pottery, they break under force, especially when it is applied quickly. Earthquakes, generally occur in the upper crust and are caused by the rapid movement of relatively brittle materials.

The base of the crust is characterized by a large increase in seismic velocity, which measures how fast earthquake waves travel through solid matter. Called the Mohorovičić Discontinuity, or Moho for short, this zone was discovered by Andrija Mohorovičić (pronounced mo-ho-ro-vee-cheech; audio pronunciation) in 1909 after studying earthquake wave paths in his native Croatia. The change in wave direction and speed is caused by dramatic chemical differences of the crust and mantle. Underneath the oceans, the Moho is found roughly 5 km below the ocean floor. Under the continents, it is located about 30-40 km below the surface. Near certain large mountain-building events known as orogenies, the continental Moho depth is doubled.

Mantle

The mantle sits below the crust and above the core. It is the largest chemical layer by volume, extending from the base of the crust to a depth of about 2900 km. Most of what we know about the mantle comes from seismic wave analysis, though information is gathered by studying ophiolites and xenoliths. Ophiolites are pieces of mantle that have risen through the crust until they are exposed as part of the ocean floor. Xenoliths are carried within magma and brought to the Earth’s surface by volcanic eruptions. Most xenoliths are made of peridotite, an ultramafic class of igneous rock (see chapter 4.2 for explanation). Because of this, scientists hypothesize most of the mantle is made of peridotite.

Core

The core of the Earth, which has both liquid and solid layers, and consists mostly of iron, nickel, and possibly some oxygen. Scientists looking at seismic data first discovered this innermost chemical layer in 1906. Through a union of hypothetical modeling, astronomical insight, and hard seismic data, they concluded the core is mostly metallic iron. Scientists studying meteorites, which typically contain more iron than surface rocks, have proposed the earth was formed from meteoric material. They believe the liquid component of the core was created as the iron and nickel sank into the center of the planet, where it was liquefied by intense pressure.

2.2.2 Physical Layers

The Earth can also be broken down into five distinct physical layers based on how each layer responds to stress. While there is some overlap in the chemical and physical designations of layers, specifically the core–mantle boundary, there are significant differences between the two systems.

Lithosphere

Lithos is Greek for stone, and the lithosphere is the outermost physical layer of the Earth. It is grouped into two types: oceanic and continental. Oceanic lithosphere is thin and relatively rigid. It ranges in thickness from nearly zero in new plates found around mid-ocean ridges, to an average of 140 km in most other locations. Continental lithosphere is generally thicker and considerably more plastic, especially at the deeper levels. Its thickness ranges from 40 to 280 km. The lithosphere is not continuous. It is broken into segments called plates. A plate boundary is where two plates meet and move relative to each other. Plate boundaries are where we see plate tectonics in action—mountain building, triggering earthquakes, and generating volcanic activity.

Asthenosphere

The asthenosphere is the layer below the lithosphere. Astheno- means lacking strength, and the most distinctive property of the asthenosphere is movement. Because it is mechanically weak, this layer moves and flows due to convection currents created by heat coming from the earth’s core cause. Unlike the lithosphere that consists of multiple plates, the asthenosphere is relatively unbroken. Scientists have determined this by analyzing seismic waves that pass through the layer. The depth of at which the asthenosphere is found is temperature-dependent. It tends to lie closer to the earth’s surface around mid-ocean ridges and much deeper underneath mountains and the centers of lithospheric plates.

Mesosphere

General perovskite structure. Perovskite silicates (i.e.

Bridgmenite,

(Mg,Fe)SiO3) are thought to be the main component of the lower mantle, making it the most common mineral in or on Earth.

The mesosphere, sometimes known as the lower mantle, is more rigid and immobile than the asthenosphere. Located at a depth of approximately 410 and 660 km below the earth’s surface, the mesosphere is subjected to very high pressures and temperatures. These extreme conditions create a transition zone in the upper mesosphere where minerals continuously change into various forms, or pseudomorphs. Scientists identify this zone by changes in seismic velocity and sometimes physical barriers to movement. Below this transitional zone, the mesosphere is relatively uniform until it reaches the core.

Inner and Outer Core

The outer core is the only entirely liquid layer within the Earth. It starts at a depth of 2,890 km and extends to 5,150 km, making it about 2,300 km thick. In 1936, the Danish geophysicist Inge Lehmann analyzed seismic data and was the first to prove a solid inner core existed within a liquid outer core . The solid inner core is about 1,220 km thick, and the outer core is about 2,300 km thick.

It seems like a contradiction that the hottest part of the Earth is solid, as the minerals making up the core should be liquified or vaporized at this temperature. Immense pressure keeps the minerals of the inner core in a solid phase. The inner core grows slowly from the lower outer core solidifying as heat escapes the interior of the Earth and is dispersed to the outer layers.

The earth’s liquid outer core is critically important in maintaining a breathable atmosphere and other environmental conditions favorable for life. Scientists believe the earth’s magnetic field is generated by the circulation of molten iron and nickel within the outer core. If the outer core were to stop circulating or become solid, the loss of the magnetic field would result in Earth getting stripped of life-supporting gases and water. This is what happened, and continues to happen, on Mars.

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2.2.3 Plate Tectonic Boundaries

At passive margins the plates don’t move—the continental lithosphere transitions into oceanic lithosphere and forms plates made of both types. A tectonic plate may be made of both oceanic and continental lithosphere connected by a passive margin. North and South America’s eastern coastlines are examples of passive margins. Active margins are places where the oceanic and continental lithospheric tectonic plates meet and move relative to each other, such as the western coasts of North and South America. This movement is caused by frictional drag created between the plates and differences in plate densities. The majority of mountain-building events, earthquake activity and active volcanism on the Earth’s surface can be attributed to tectonic plate movement at active margins.

In a simplified model, there are three categories of tectonic plate boundaries. Convergent boundaries are places where plates move toward each other. At divergent boundaries, the plates move apart. At transform boundaries, the plates slide past each other.

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2.3 Convergent Boundaries

Convergent boundaries, also called destructive boundaries, are places where two or more plates move toward each other. . Convergent boundary movement is divided into two types, subduction and collision, depending on the density of the involved plates. Continental lithosphere is of lower density and thus more buoyant than the underlying asthenosphere. Oceanic lithosphere is more dense than continental lithosphere, and, when old and cold, may even be more dense than asthenosphere.

When plates of different densities converge, the higher density plate is pushed beneath the more buoyant plate in a process called subduction. When continental plates converge without subduction occurring, this process is called collision.

2.3.1. Subduction

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Video showing continental–oceanic subduction, causing volcanism. By Tanya Atwater and John Iwerks.

Subduction occurs when a dense oceanic plate meets a more buoyant plate, like a continental plate or warmer/younger oceanic plate, and descends into the mantle. The worldwide average rate of oceanic plate subduction is 25 miles per million years, about a half-inch per year. As an oceanic plate descends, it pulls the ocean floor down into a trench. These trenches can be more than twice as deep as the average depth of the adjacent ocean basin, which is usually three to four km. The Mariana Trench, for example, approaches a staggering 11 km.

Within the trench, ocean floor sediments are scraped together and compressed between the subducting and overriding plates. This feature is called the accretionary wedge, mélange, or accretionary prism. Fragments of continental material, including microcontinents, riding atop the subducting plate may become sutured to the accretionary wedge and accumulate into a large area of land called a terrane. Vast portions of California are comprised of accreted terranes.

When the subducting oceanic plate, or slab, sinks into the mantle, the immense heat and pressure pushes volatile materials like water and carbon dioxide into an area below the continental plate and above the descending plate called the mantle wedge. The volatiles are released mostly by hydrated minerals that revert to non-hydrated minerals in these higher temperature and pressure conditions. When mixed with asthenospheric material above the plate, the volatile lower the melting point of the mantle wedge, and through a process called flux melting it becomes liquid magma. The molten magma is more buoyant than the lithospheric plate above it and migrates to the Earth’s surface where it emerges as volcanism. The resulting volcanoes frequently appear as curved mountain chains, volcanic arcs, due to the curvature of the earth. Both oceanic and continental plates can contain volcanic arcs.

How subduction is initiated is still a matter of scientific debate. It is generally accepted that subduction zones start as passive margins, where oceanic and continental plates come together, and then gravity initiates subduction and converts the passive margin into an active one. One hypothesis is gravity pulls the denser oceanic plate down or the plate can start to flow ductility at a low angle. Scientists seeking to answer this question have collected evidence that suggests a new subduction zone is forming off the coast of Portugal. Some scientists have proposed large earthquakes like the 1755 Lisbon earthquake may even have something to do with this process of creating a subduction zone, although the evidence is not definitive. Another hypothesis proposes subduction happens at transform boundaries involving plates of different densities.

Some plate boundaries look like they should be active, but show no evidence of subduction. The oceanic lithospheric plates on either side of the Atlantic Ocean for example, are denser than the underlying asthenosphere and are not subducting beneath the continental plates. One hypothesis is the bond holding the oceanic and continental plates together is stronger than the downwards force created by the difference in plate densities.

Subduction zones are known for having the largest earthquakes and tsunamis; they are the only places with fault surfaces large enough to create magnitude-9 earthquakes. These subduction-zone earthquakes not only are very large, but also are very deep. When a subducting slab becomes stuck and cannot descend, a massive amount of energy builds up between the stuck plates. If this energy is not gradually dispersed, it may force the plates to suddenly release along several hundred kilometers of the subduction zone. Because subduction-zone faults are located on the ocean floor, this massive amount of movement can generate giant tsunamis such as those that followed the 2004 Indian Ocean Earthquake and 2011 Tōhoku Earthquake in Japan.

All subduction zones have a forearc basin, a feature of the overriding plate found between the volcanic arc and oceanic trench. The forearc basin experiences a lot of faulting and deformation activity, particularly within the accretionary wedge.

In some subduction zones, tensional forces working on the continental plate create a backarc basin on the interior side of the volcanic arc. Some scientists have proposed a subduction mechanism called oceanic slab rollback creates extension faults in the overriding plates. In this model, the descending oceanic slab does not slide directly under the overriding plate but instead rolls back, pulling the overlying plate seaward. The continental plate behind the volcanic arc gets stretched like pizza dough until the surface cracks and collapses to form a backarc basin. If the extension activity is extensive and deep enough, a backarc basin can develop into a continental rifting zone. These continental divergent boundaries may be less symmetrical than their mid-ocean ridge counterparts.

In places where numerous young buoyant oceanic plates are converging and subducting at a relatively high velocity, they may force the overlying continental plate to buckle and crack. This is called back-arc faulting. Extensional back-arc faults pull rocks and chunks of plates apart. Compressional back-arc faults, also known as thrust faults, push them together.

The dual spines of the Andes Mountain range include a example of compressional thrust faulting. The western spine is part of a volcanic arc. Thrust faults have deformed the non-volcanic eastern spine, pushing rocks and pieces of continental plate on top of each other.

There are two styles of thrust fault deformation: thin-skinned faults that occur in superficial rocks lying on top of the continental plate and thick-skinned faults that reach deeper into the crust. The Sevier Orogeny in the western U.S. is a notable thin-skinned type of deformation created during the Cretaceous Period. The Laramide Orogeny, a thick-skinned type of deformation, occurred near the end of and slightly after the Sevier Orogeny in the same region.

Flat-slab, or shallow, subduction caused the Laramide Orogeny. When the descending slab subducts at a low angle, there is more contact between the slab and the overlying continental plate than in a typical subduction zone. The shallowly-subducting slab pushes against the overriding plate and creates an area of deformation on the overriding plate many kilometers away from the subduction zone.

Oceanic-Continental subduction

Oceanic-continental subduction occurs when an oceanic plate dives below a continental plate. This convergent boundary has a trench and mantle wedge and frequently, a volcanic arc. Well-known examples of continental volcanic arcs are the Cascade Mountains in the Pacific Northwest and western Andes Mountains in South America.

Oceanic-Oceanic Subduction

The boundaries of oceanic-oceanic subduction zones show very different activity from those involving oceanic–continental plates. Since both plates are made of oceanic lithosphere, it is usually the older plate that subducts because it is colder and denser. The volcanism on the overlying oceanic plate may remain hidden underwater.. If the volcanoes rise high enough the reach the ocean surface, the chain of volcanism forms an island arc. Examples of these island arcs include the Aleutian Islands in the northern Pacific Ocean, Lesser Antilles in the Caribbean Sea, and numerous island chains scattered throughout the western Pacific Ocean.

2.3.2. Collisions

When continental plates converge, during the closing of an ocean basin for example, subduction is not possible between the equally buoyant plates. Instead of one plate descending beneath another, the two masses of continental lithosphere slam together in a process known as collision. Without subduction, there is no magma formation and no volcanism. Collision zones are characterized by tall, non-volcanic mountains; a broad zone of frequent, large earthquakes; and very little volcanism.

When oceanic crust connected by a passive margin to continental crust completely subducts beneath a continent, an ocean basin closes, and continental collision begins. Eventually, as ocean basins close, continents join together to form a massive accumulation of continents called a supercontinent, a process that has taken place in ~500 million year old cycles over earth’s history.

The process of collision created Pangea, the supercontinent envisioned by Wegener as the key component of his continental drift hypothesis. Geologists now have evidence that continental plates have been continuously converging into supercontinents and splitting into smaller basin-separated continents throughout Earth’s existence, calling this process the supercontinent cycle, a process that takes place in approximately 500 million years. For example, they estimate Pangea began separating 200 million years ago. Pangea was preceded by an earlier supercontinents, one of which being Rodinia, which existed 1.1 billion years ago and started breaking apart 800 million to 600 million years ago.

A foreland basin is a feature that develops near mountain belts, as the combined mass of the mountains forms a depression in the lithospheric plate. While foreland basins may occur at subduction zones, they are most commonly found at collision boundaries. The Persian Gulf is possibly the best modern example, created entirely by the weight of the nearby Zagros Mountains.

If continental and oceanic lithosphere are fused on the same plate, it can partially subduct but its buoyancy prevents it from fully descending. In very rare cases, part of a continental plate may become trapped beneath a descending oceanic plate in a process called obduction. When a portion of the continental crust is driven down into the subduction zone, due to its buoyancy it returns to the surface relatively quickly.

As pieces of the continental lithosphere break loose and migrate upward through the obduction zone, they bring along bits of the mantle and ocean floor and amend them on top of the continental plate. Rocks composed of this mantle and ocean-floor material are called ophiolites and they provide valuable information about the composition of the mantle.

The area of collision-zone deformation and seismic activity usually covers a broader area because continental lithosphere is plastic and malleable. Unlike subduction-zone earthquakes, which tend to be located along a narrow swath near the convergent boundary, collision-zone earthquakes may occur hundreds of kilometers from the boundary between the plates.

The Eurasian continent has many examples of collision-zone deformations covering vast areas. The Pyrenees mountains begin in the Iberian Peninsula and cross into France. Also, there are the Alps stretching from Italy to central Europe; the Zagros mountains from Arabia to Iran; and Himalaya mountains from the Indian subcontinent to central Asia.

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Animation of India crashing into Asia, by Tanya Atwater.

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2.4 Divergent Boundaries

At divergent boundaries, sometimes called constructive boundaries, lithospheric plates move away from each other. There are two types of divergent boundaries, categorized by where they occur: continental rift zones and mid-ocean ridges. Continental rift zones occur in weak spots in the continental lithospheric plate. A mid-ocean ridge usually originates in a continental plate as a rift zone that expands to the point of splitting the plate apart, with seawater filling in the gap. The separate pieces continue to drift apart and become individual continents. This process is known as rift-to-drift.

2.4.1. Continental Rifting

In places where the continental plates are very thick, they reflect so much heat back into the mantle it develops strong convection currents that push super-heated mantle material up against the overlying plate, softening it. Tensional forces created by this convective upwelling begin to pull the weakened plate apart. As it stretches, it becomes thinner and develops deep cracks called extension or normal faults. Eventually plate sections located between large faults drop into deep depressions known as rift valleys, which often contain keystone-shaped blocks of down-dropped crust known as grabens. The shoulders of these grabens are called horsts. If only one side of a section drops, it is called a half-graben. Depending on the conditions, rifts can grow into very large lakes and even oceans.

While seemingly occurring at random, rifting is dictated by two factors. Rifting does not occur in continents with older and more stable interiors, known as cratons. When continental rifting does occur, the break-up pattern resembles the seams of a soccer ball, also called a truncated icosahedron. This is the most common surface-fracture pattern to develop on an evenly expanding sphere because it uses the least amount of energy.

Using the soccer ball model, rifting tends to lengthen and expand along a particular seam while fizzling out in the other directions. These seams with little or no tectonic activity are called failed rift arms. A failed rift arm is still a weak spot in the continental plate; even without the presence of active extension faults, it may develop into a called an aulacogen. One example of a failed rift arm is the Mississippi Valley Embayment, a depression through which the upper end of the Mississippi River flows. Occasionally connected rift arms do develop concurrently, creating multiple boundaries of active rifting. In places where the rift arms do not fail, for example the Afar Triangle, three divergent boundaries can develop near each other forming a triple junction.

Rifts come in two types: narrow and broad. Narrow rifts are characterized by a high density of highly active divergent boundaries. The East African Rift Zone, where the horn of Africa is pulling away from the mainland, is an excellent example of an active narrow rift. Lake Baikal in Russia is another. Broad rifts also have numerous fault zones, but they are distributed over wide areas of deformation. The Basin and Range region located in the western United States is a type of broad rift. The Wasatch Fault, which also created the Wasatch Mountain Range in the state of Utah, forms the eastern divergent boundary of this broad rift (Animation 1 and Animation 2).

Rifts have earthquakes, although not of the magnitude and frequency of other boundaries. They may also exhibit volcanism. Unlike the flux-melted magma found in subduction zones, rift-zone magma is created by decompression melting. As the continental plates are pulled apart, they create a region of low pressure that melts the lithosphere and draws it upwards. When this molten magma reaches the weakened and fault-riddled rift zone, it migrates to surface by breaking through the plate or escaping via an open fault. Examples of young rift volcanoes dot the Basin and Range region in the United States. Rift-zone activity is responsible for generating some unique volcanism, such as the Ol Doinyo Lengai in Tanzania. This volcano erupts lava consisting largely of carbonatite, a relatively cold, liquid carbonate mineral.

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South America and Africa rift, forming the Atlantic. Video by Tanya Atwater.

2.4.2. Mid-ocean ridges

As rifting and volcanic activity progress, the continental lithosphere becomes more mafic (see Chapter 4) and thinner, with the eventual result transforming the plate under the rifting area into oceanic lithosphere. This is the process that gives birth to a new ocean, much like the narrow Red Sea emerged with the movement of Arabia away from Africa. As the oceanic lithosphere continues to diverge, a mid-ocean ridge is formed.

Mid-ocean ridges, also known as spreading centers, have several distinctive features. They are the only places on earth that create new oceaniclithosphere. Decompression melting in the rift zone changes asthenosphere material into new lithosphere, which oozes up through cracks in oceanic plate. The amount of new lithosphere being created at mid-ocean ridges is highly significant. These undersea rift volcanoes produce more lava than all other types of volcanism combined. Despite this, most mid-oceanic ridge volcanism remains unmapped because the volcanoes are located deep on the ocean floor.

In rare cases, such as a few locations in Iceland, rift zones display the type of volcanism, spreading, and ridge formation found on the ocean floor.

The ridge feature is created by the accumulation of hot lithosphere material, which is lighter than the dense underlying asthenosphere. This chunk of isostatically buoyant lithosphere sits partially submerged and partially exposed on the asthenosphere, like an ice cube floating in a glass of water.

As the ridge continues to spread, the lithosphere material is pulled away from the area of volcanism and becomes colder and denser. As it continues to spread and cool, the lithosphere settles into wide swathes of relatively featureless topography called abyssal plains with lower topography.

This model of ridge formation suggests the sections of lithosphere furthest away from the mid-ocean ridges will be the oldest. Scientists have tested this idea by comparing the age of rocks located in various locations on the ocean floor. Rocks found near ridges are younger than those found far away from any ridges. Sediment accumulation patterns also confirm the idea of sea-floor spreading. Sediment layers tend to be thinner near mid-ocean ridges, indicating it has had less time to build up.

As mentioned in the section on paleomagnetism and the development of plate tectonic theory, scientists noticed mid-ocean ridges contained unique magnetic anomalies that show up as symmetrical striping on both sides of the ridge. The Vine-Matthews-Morley hypothesis proposes these alternating reversals are created by the earth’s magnetic field being imprinted into magma

after it emerges from the ridge. Very hot magma has no magnetic field. As the oceanic plates get pulled apart, the magma cools below the Curie point, the temperature below which a magnetic field gets locked into magnetic minerals. The alternating magnetic reversals in the rocks reflects the periodic swapping of earth’s magnetic north and south poles. This paleomagnetic pattern provides a great historical record of ocean-floor movement, and is used to reconstruct past tectonic activity and determine rates of ridge spreading.

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Video of the breakup of Pangea and formation of the northern Atlantic Ocean. By Tanya Atwater.

Thanks to their distinctive geology, mid-ocean ridges are home to some of the most unique ecosystems ever discovered. The ridges are often studded with hydrothermal vents, deep fissures that allow seawater to circulate through the upper portions of the oceanic plate and interact with hot rock. The super-heated seawater rises back up to the surface of the plate, carrying dissolved gasses and minerals, and small particulates. The resulting emitted hydrothermal water looks like black underwater smoke.

Scientists had known about these geothermal areas on the ocean floor for some time. However, it was not until 1977, when scientists piloting a deep submergence vehicle, the Alvin, discovered a thriving community of organisms clustered around these hydrothermal vents. These unique organisms, which include 10-foot-long tube worms taller than people, live in the complete darkness of the ocean floor deprived of oxygen and sunlight. They use geothermal energy provided by the vents and a process called bacterial chemosynthesis to feed on sulfur compounds. Before this discovery, scientists believed life on earth could not exist without photosynthesis, a process that requires sunlight. Some scientists suggest this type of environment could have been the origin of life on Earth, and perhaps even extraterrestrial life elsewhere in the galaxy, such as on Jupiter’s moon Europa.

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2.5 Transform Boundaries

A transform boundary, sometimes called a strike-slip or conservative boundary, is where the lithospheric plates slide past each other in the horizontal plane. This movement is described based on the perspective of an observer standing on one of the plates, looking across the boundary at the opposing plate. Dextral, also known as right-lateral, movement describes the opposing plate moving to the right. Sinistral, also known as left lateral, movement describe the opposing plate moving to the left.

Most transform boundaries are found on the ocean floor, around mid-ocean ridges. These boundaries form aseismic fracture zones, filled with earthquake-free transform faults, to accommodate different rates of spreading occurring at the ridge.

Some transform boundaries produce significant seismic activity, primarily as earthquakes, with very little mountain-building or volcanism. This type of transform boundary may contain a single fault or series of faults, which develop in places where plate tectonic stresses are transferred to the surface. As with other types of active boundaries, if the plates are unable to shear past each other the tectonic forces will continue to build up. If the built up energy between the plates is suddenly released, the result is an earthquake.

In the eyes of humanity, the most significant transform faults occur within continental plates, and have a shearing motion that frequently produces moderate-to-large magnitude earthquakes. Notable examples include the San Andreas Fault in California, Northern and Eastern Anatolian Faults in Turkey, Altyn Tagh Fault in central Asia, and Alpine Fault in New Zealand.

2.5.1. Transpression and Transtension

Bends along transform faults may create compressional or extensional forces that cause secondary faulting zones. Transpression occurs where there is a component of compression in addition to the shearing motion. These forces build up around the area of the bend, where the opposing plates are restricted from sliding past each other. As the forces continue to build up, they create mountains in the restraining bend around the fault. The Big Bend area, located in the southern part of the San Andreas Fault includes a large area of transpression where many mountains have been built, moved, and even rotated.

Transtension zones require a fault that includes a releasing bend, where the plates are being pulled apart by extensional forces. Depressions and sometimes volcanism develop in the releasing bend, along the fault. The Dead Sea found between Israel and Jordan, and the Salton Sea of California are examples of basins formed by transtensional forces.

2.5.2. Piercing Points

When a geological feature is cut by a fault, it is called a piercing point. Piercing points are very useful for recreating past fault movement, especially along transform boundaries. Transform faults are unique because their horizontal motion keeps a geological feature relatively intact, preserving the record of what happened. Other types of faults—normal and reverse —tend to be more destructive, obscuring or destroying these features. The best type of piercing point includes unique patterns that are used to match the parts of a geological feature separated by fault movement. Detailed studies of piercing points show the San Andreas Fault has experienced over 225 km of movement in the last 20 million years, and this movement occurred at three different fault traces.

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Video of the origin of the San Andreas fault. As the mid-ocean ridge subducts, the relative motion between the remaining plates become transform, forming the fault system. Note that because the motion of the plates is not exactly parallel to the fault, it causes divergent motion in the interior of North America. By Tanya Atwater.

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2.6 The Wilson Cycle

The Wilson Cycle is named for J. Tuzo Wilson who first described it in 1966, and it outlines the ongoing origin and breakup of supercontinents, such as Pangea and Rodinia. Scientists have determined this cycle has been operating for at least three billion years and possibly earlier.

There are a number of hypotheses about how the Wilson Cycle works. One mechanism proposes that rifting happens because continental plates reflect the heat much better than oceanic plates. When continents congregate together, they reflect more of the Earth’s heat back into the mantle, generating more vigorous convection currents that then start the continental rifting process. Some geologists believe mantle plumes are remnants of these periods of increased mantle temperature and convection upwelling, and study them for clues about the origin of continental rifting.

The mechanism behind how supercontinents are created is still largely a mystery. There are three schools of thought about what continues to drive the continents further apart and eventually bring them together. The ridge-push hypothesis suggests after the initial rifting event, plates continue to be pushed apart by mid-ocean spreading centers and their underlying convection currents. Slab-pull proposes the plates are pulled apart by descending slabs in the subduction zones of the oceanic–continental margins. A third idea, gravitational sliding, attributes the movement to gravitational forces pulling the lithospheric plates down from the elevated mid-ocean ridges and across the underlying asthenosphere. Current evidence seems to support slab pull more than ridge push or gravitational sliding.

2.7 Hotspots

The Wilson Cycle provides a broad overview of tectonic plate movement. To analyze plate movement more precisely, scientists study hotspots. First postulated by J. Tuzo Wilson in 1963, a hotspot is an area in the lithospheric plate where molten magma breaks through and creates a volcanic center, islands in the ocean and mountains on land. As the plate moves across the hotspot, the volcano center becomes extinct because it is no longer over an active magma source. Instead, the magma emerges through another area in the plate to create a new active volcano. Over time, the combination of moving plate and stationary hotspot creates a chain of islands or mountains. The classic definition of hotspots states they do not move, although recent evidence suggests that there may be exceptions.

Hotspots are the only types of volcanism not associated with subduction or rifting zones at plate boundaries; they seem totally disconnected from any plate tectonics processes, such as earthquakes. However, there are relationships between hotspots and plate tectonics. There are several hotspots, current and former, that are believed to have begun at the time of rifting. Also, scientists use the age of volcanic eruptions and shape of the chain to quantify the rate and direction of plate movement relative to the hotspot.

Scientists are divided over how magma is generated in hotspots. Some suggest that hotspots originate from super-heated material from as deep as the core that reaches the Earth’s crust as a mantle plume. Others argue the molten material that feeds hotspots is sourced from the mantle. Of course, it is difficult to collect data from these deep-Earth features due to the extremely high pressure and temperature.

How hotspots are initiated is another highly debated subject. The prevailing mechanism has hotspots starting in divergent boundaries during supercontinent rifting. Scientists have identified a number of current and past hotspots believed to have begun this way. Subducting slabs have also been named as causing mantle plumes and hot-spot volcanism. Some geologists have suggested another geological process not involving plate tectonics may be involved, such as a large space objects crashing into Earth. Regardless of how they are formed, dozens are on the Earth. Some well-known examples include the Tahiti Islands, Afar Triangle, Easter Island, Iceland, Galapagos Islands, and Samoan Islands. The United States is home to two of the largest and best-studied hotspots: Hawaii and Yellowstone.

2.7.1 Hawaiian hotspot

The active volcanoes in Hawaii represent one of the most active hotspot sites on earth. Scientific evidence indicates the Hawaiian hotspot is at least 80 million years old. Geologists believe it is actually much older; however any rocks with proof of this have been subducted under the ocean floor. The big island of Hawaii sits atop a large mantle plume that marks the active hotspot. The Kilauea volcano is the main vent for this hotspot and has been actively erupting since 1983.

This enormous volcanic island chain, much of which is underwater, stretches across the Pacific for almost 6,000 km. The seamount chain’s most striking feature is a sharp 60-degree bend located at the midpoint, which marks a significant change in plate movement direction that occurred 50 million years ago. The change in direction has been more often linked to a plate reconfiguration, but also to other things like plume migration.

In an attempt to map the Hawaiian mantle plume as far down as the lower mantle, scientists have used tomography, a type of three-dimensional seismic imaging. This information—along with other evidence gathered from rock ages, vegetation types, and island size—indicate the oldest islands in the chain are located the furthest away from the active hotspot.

2.7.2 Yellowstone hotspot

Like the Hawaiian version, the Yellowstone hotspot is formed by magma rising through the lithosphere. What makes it different is this hotspot is located under a thick, continental plate. Hawaii sits on a thin oceanic plate, which is easily breached by magma coming to the surface. At Yellowstone, the thick continental plate presents a much more difficult barrier for magma to penetrate. When it does emerge, the eruptions are generally much more violent. Thankfully they are also less frequent.

Over 15 million years of eruptions by this hotspot have carved a curved path across the western United States. It has been suggested the Yellowstone hotspot is connected to the much older Columbia River flood basalts and even to 70 million-year-old volcanism found in the Yukon region of Canada.

The most recent major eruption of this hotspot created the Yellowstone Caldera and Lava Creek tuff formation approximately 631,000 years ago. The eruption threw 1,000 cubic kilometers of ash and magma into the atmosphere, some of which was found as far away as Mississippi. Should the hotspot erupt again, scientists predict it will be another massive event. This would be a calamity reaching far beyond the western United States. These super volcanic eruptions fill the earth’s atmosphere with so much gas and ash, they block sunlight from reaching the earth. Not only would this drastically alter climates and environments around the globe, it could affect worldwide food production.

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Summary

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Plate tectonics is a unifying theory; it explains nearly all of the major geologic processes on Earth. Since its early inception in the 1950s and 1960s, geologists have been guided by this revolutionary perception of the world. The theory of plate tectonics states the surface layer of the Earth is broken into a network of solid, relatively brittle plates. Underneath the plates is a much hotter and more ductile layer that contains zones of convective upwelling generated by the interior heat of Earth. These convection currents move the surface plates around—bringing them together, pulling them apart, and shearing them side-by-side. Earthquakes and volcanoes form at the boundaries where the plates interact, with the exception of volcanic hotspots, which are not caused by plate movement.

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3 Minerals

3 Minerals

KEYCONCEPTS

At the end of this chapter, students should be able to:

Define mineral.

Describe the basic structure of the atom.

Derive basic atomic information from the Periodic Table of Elements.

Describe chemical bonding related to minerals.

Describe the main ways minerals form.

Describe the silicon-oxygen tetrahedron and how it forms common silicate minerals.

List common non-silicate minerals in oxide, sulfide, sulfate, and carbonate groups.

Identify minerals using physical properties and identification tables.

The term “minerals” as used in nutrition labels and pharmaceutical products is not the same as a mineral in a geological sense. In geology, the classic definition of a mineral is: 1) naturally occurring, 2) inorganic, 3) solid at room temperature, 4) regular crystal structure, and 5) defined chemical composition. Some natural substances technically should not be considered minerals, but are included by exception. For example, water and mercury are liquid at room temperature. Both are considered minerals because they were classified before the room-temperature rule was accepted as part of the definition. Calcite is quite often formed by organic processes, but is considered a mineral because it is widely found and geologically important. Because of these discrepancies, the International Mineralogical Association in 1985 amended the definition to: “A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes.” This means that the calcite in the shell of a clam is not considered a mineral. But once that clam shell undergoes burial, diagenesis, or other geological processes, then the calcite is considered a mineral. Typically, substances like coal, pearl, opal, or obsidian that do not fit the definition of mineral are called mineraloids.

A rock is a substance that contains one or more minerals or mineraloids. As is discussed in later chapters, there are three types of rocks composed of minerals: igneous (rocks crystallizing from molten material), sedimentary (rocks composed of products of mechanical weathering (sand, gravel, etc.) and chemical weathering (things precipitated from solution), and metamorphic (rocks produced by alteration of other rocks by heat and pressure.

3.1 Chemistry of Minerals

Rocks are composed of minerals that have a specific chemical composition. To understand mineral chemistry, it is essential to examine the fundamental unit of all matter, the atom.

3.1.1 The Atom

Matter is made of atoms. Atoms consists of subatomic particles—protons, neutrons, and electrons. A simple model of the atom has a central nucleus composed of protons, which have positive charges, and neutrons which have no charge. A cloud of negatively charged electrons surrounds the nucleus, the number of electrons equaling the number of protons thus balancing the positive charge of the protons for a neutral atom. Protons and neutrons each have a mass number of 1. The mass of an electron is less than 1/1000th that of a proton or neutron, meaning most of the atom’s mass is in the nucleus.

3.1.2 Periodic Table of the Elements

Matter is composed of elements which are atoms that have a specific number of protons in the nucleus. This number of protons is called the Atomic Number for the element. For example, an oxygen atom has 8 protons and an iron atom has 26 protons. An element cannot be broken down chemically into a simpler form and retains unique chemical and physical properties. Each element behaves in a unique manner in nature. This uniqueness led scientists to develop a periodic table of the elements, a tabular arrangement of all known elements listed in order of their atomic number.

The first arrangement of elements into a periodic table was done by Dmitri Mendeleev in 1869 using the elements known at the time. In the periodic table, each element has a chemical symbol, name, atomic number, and atomic mass. The chemical symbol is an abbreviation for the element, often derived from a Latin or Greek name for the substance. The atomic number is the number of protons in the nucleus. The atomic mass is the number of protons and neutrons in the nucleus, each with a mass number of one. Since the mass of electrons is so much less than the protons and neutrons, the atomic mass is effectively the number of protons plus neutrons.

The atomic mass of natural elements represents an average mass of the atoms comprising that substance in nature and is usually not a whole number as seen on the periodic table, meaning that an element exists in nature with atoms having different numbers of neutrons. The differing number of neutrons affects the mass of an element in nature and the atomic mass number represents this average. This gives rise to the concept of isotope. Isotopesare forms of an element with the same number of protons but different numbers of neutrons. There are usually several isotopes for a particular element. For example, 98.9% of carbon atoms have 6 protons and 6 neutrons. This isotope of carbon is called carbon-12 (12C). A few carbon atoms, carbon-13 (13C), have 6 protons and 7 neutrons. A trace amount of carbon atoms, carbon-14 (14C), has 6 protons and 8 neutrons.

Among the 118 known elements, the heaviest are fleeting human creations known only in high energy particle accelerators, and they decay rapidly. The heaviest naturally occurring element is uranium, atomic number 92. The eight most abundant elements in Earth’s continental crust are shown in Table 1. These elements are found in the most common rock forming minerals.

Element	Symbol	Abundance %
Oxygen	O	47%
Silicon	Si	28%
Aluminum	Al	8%
Iron	Fe	5%
Calcium	Ca	4%
Sodium	Na	3%
Potassium	K	3%
Magnesium	Mg	2%

Table 1. Eight Most Abundant Elements in the Earth’s Continental Crust % by weight (source: USGS). All other elements are less than 1%.

3.1.3 Chemical Bonding

Most substances on Earth are compounds containing multiple elements. Chemical bonding describes how these atoms attach with each other to form compounds, such as sodium and chlorine combining to form NaCl, common table salt. Compounds that are held together by chemical bonds are called molecules. Water is a compound of hydrogen and oxygen in which two hydrogen atoms are covalently bonded with one oxygen making the water molecule. The oxygen we breathe is formed when one oxygen atom covalently bonds with another oxygen atom to make the molecule O2. The subscript 2 in the chemical formula indicates the molecule contains two atoms of oxygen.

Most minerals are also compounds of more than one element. The common mineral calcite has the chemical formula CaCO3 indicating the molecule consists of one calcium, one carbon, and three oxygen atoms. In calcite, one carbon and three oxygen atoms are held together by covalent bonds to form a molecular ion, called carbonate, which has a negative charge. Calcium as an ion has a positive charge of plus two. The two oppositely charged ions attract each other and combine to form the mineral calcite, CaCO3. The name of the chemical compound is calcium carbonate, where calcium is Ca and carbonate refers to the molecular ion CO3-2.

The mineral olivine has the chemical formula (Mg,Fe)2SiO4, in which one silicon and four oxygen atoms are bonded with two atoms of either magnesium or iron. The comma between iron (Fe) and magnesium (Mg) indicates the two elements can occupy the same location in the crystal structure and substitute for one another.

3.1.3.1 Valence and Charge

The electrons around the atom’s nucleus are located in shells representing different energy levels. The outermost shell is called the valence shell. Electrons in the valence shell are involved in chemical bonding. In 1913, Niels Bohr proposed a simple model of the atom that states atoms are more stable when their outermost shell is full. Atoms of most elements thus tend to gain or lose electrons so the outermost or valence shell is full. In Bohr’s model, the innermost shell can have a maximum of two electrons and the second and third shells can have a maximum of eight electrons. When the innermost shell is the valence shell, as in the case of hydrogen and helium, it obeys the octet rule when it is full with two electrons. For elements in higher rows, the octet rule of eight electrons in the valence shell applies.

The rows in the periodic table present the elements in order of atomic number and the columns organize elements with similar characteristics, such as the same number of electrons in their valence shells. Columns are often labeled from left to right with Roman numerals I to VIII, and Arabic numerals 1 through 18. The elements in columns I and II have 1 and 2 electrons in their respective valence shells and the elements in columns VI and VII have 6 and 7 electrons in their respective valence shells.

In row 3 and column I, sodium (Na) has 11 protons in the nucleus and 11 electrons in three shells—2 electrons in the inner shell, 8 electrons in the second shell, and 1 electron in the valence shell. To maintain a full outer shell of 8 electrons per the octet rule, sodium readily gives up that 1 electron so there are 10 total electrons. With 11 positively charged protons in the nucleus and 10 negatively charged electrons in two shells, sodium when forming chemical bonds is an ion with an overall net charge of +1.

All elements in column I have a single electron in their valence shell and a valence of 1. These other column I elements also readily give up this single valence electron and thus become ions with a +1 charge. Elements in column II readily give up 2 electrons and end up as ions with a charge of +2. Note that elements in columns I and II which readily give up their valence electrons, often form bonds with elements in columns VI and VII which readily take up these electrons. Elements in columns 3 through 15 are usually involved in covalent bonding. The last column 18 (VIII) contains the noble gases. These elements are chemically inert because the valence shell is already full with 8 electrons, so they do not gain or lose electrons. An example is the noble gas helium which has 2 valence electrons in the first shell. Its valence shell is therefore full. All elements in column VIII possess full valence shells and do not form bonds with other elements.

As seen above, an atom with a net positive or negative charge as a result of gaining or losing electrons is called an ion. In general the elements on the left side of the table lose electrons and become positive ions, called cations because they are attracted to the cathode in an electrical device. The elements on the right side tend to gain electrons. These are called anions because they are attracted to the anode in an electrical device. The elements in the center of the periodic table, columns 3 through 15, do not consistently follow the octet rule. These are called transition elements. A common example is iron, which has a +2 or +3 charge depending on the oxidation state of the element. Oxidized Fe+3 carries a +3 charge and reduced Fe+2 is +2. These two different oxidation states of iron often impart dramatic colors to rocks containing their minerals—the oxidized form producing red colors and the reduced form producing green.

3.1.3.2 Ionic Bonding

Ionic bonds, also called electron-transfer bonds, are formed by the electrostatic attraction between atoms having opposite charges. Atoms of two opposite charges attract each other electrostatically and form an ionic bond in which the positive ion transfers its electron (or electrons) to the negative ion which takes them up. Through this transfer both atoms thus achieve a full valence shell. For example one atom of sodium (Na+1) and one atom of chlorine (Cl-1) form an ionic bond to make the compound sodium chloride (NaCl). This is also known as the mineral halite or common table salt. Another example is calcium (Ca+2) and chlorine (Cl-1) combining to make the compound calcium chloride (CaCl2). The subscript 2 indicates two atoms of chlorine are ionically bonded to one atom of calcium.

3.1.3.3 Covalent Bonding

Ionic bonds are usually formed between a metal and a nonmetal. Another type, called a covalent or electron-sharing bond, commonly occurs between nonmetals. Covalent bonds share electrons between ions to complete their valence shells. For example, oxygen (atomic number 8) has 8 electrons—2 in the inner shell and 6 in the valence shell. Gases like oxygen often form diatomic molecules by sharing valence electrons. In the case of oxygen, two atoms attach to each other and share 2 electrons to fill their valence shells to become the common oxygen molecule we breathe (O2). Methane (CH4) is another covalently bonded gas. The carbon atom needs 4 electrons and each hydrogen needs 1. Each hydrogen shares its electron with the carbon to form a molecule as shown in the figure.

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3.2 Formation of Minerals

Minerals form when atoms bond together in a crystalline arrangement. Three main ways this occurs in nature are: 1) precipitation directly from an aqueous (water) solution with a temperature change, 2) crystallization from a magma with a temperature change, and 3) biological precipitation by the action of organisms.

3.2.1 Precipitation from aqueous solution

Solutions consist of ions or molecules, known as solutes, dissolved in a medium or solvent. In nature this solvent is usually water. Many minerals can be dissolved in water, such as halite or table salt, which has the composition sodium chloride, NaCl. The Na+1 and Cl-1 ions separate and disperse into the solution.

Precipitation is the reverse process, in which ions in solution come together to form solid minerals. Precipitation is dependent on the concentration of ions in solution and other factors such as temperature and pressure. The point at which a solvent cannot hold any more solute is called saturation. Precipitation can occur when the temperature of the solution falls, when the solute evaporates, or with changing chemical conditions in the solution. An example of precipitation in our homes is when water evaporates and leaves behind a rind of minerals on faucets, shower heads, and drinking glasses.

In nature, changes in environmental conditions may cause the minerals dissolved in water to form bonds and grow into crystals or cement grains of sediment together. In Utah, deposits of tufa formed from mineral-rich springs that emerged into the ice age Lake Bonneville. Now exposed in dry valleys, this porous tufa was a natural insulation used by pioneers to build their homes with a natural protection against summer heat and winter cold. The travertine terraces at Mammoth Hot Springs in Yellowstone Park are another example formed by calcite precipitation at the edges of the shallow spring-fed ponds.

Another example of precipitation occurs in the Great Salt Lake, Utah, where the concentration of sodium chloride and other salts is nearly eight times greater than in the world’s oceans [zotpressInText item=”{DU5CMSHJ}” format=”%num%” brackets=”yes”]. Streams carry salt ions into the lake from the surrounding mountains. With no other outlet, the water in the lake evaporates and the concentration of salt increases until saturation is reached and the minerals precipitate out as sediments. Similar salt deposits include halite and other precipitates, and occur in other lakes like Mono Lake in California and the Dead Sea.

3.2.2 Crystallization from Magma

Heat is energy that causes atoms in substances to vibrate. Temperature is a measure of the intensity of the vibration. If the vibrations are violent enough, chemical bonds are broken and the crystals melt releasing the ions into the melt. Magma is molten rock with freely moving ions. When magma is emplaced at depth or extruded onto the surface (then called lava), it starts to cool and mineral crystals can form.

3.2.3 Precipitation by Organisms

Many organisms build bones, shells, and body coverings by extracting ions from water and precipitating minerals biologically. The most common mineral precipitated by organisms is calcite, or calcium carbonate (CaCO3). Calcite is often precipitated by organisms as a polymorph called aragonite. Polymorphsare crystals with the same chemical formula but different crystal structures. Marine invertebrates such as corals and clams precipitate aragonite or calcite for their shells and structures. Upon death, their hard parts accumulate on the ocean floor as sediments, and eventually may become the sedimentary rock limestone. Though limestone can form inorganically, the vast majority is formed by this biological process. Another example is marine organisms called radiolaria, which are zooplankton that precipitate silica for their microscopic external shells. When the organisms die, the shells accumulate on the ocean floor and can form the sedimentary rock chert. An example of biologic precipitation from the vertebrate world is bone, which is composed mostly of a type of apatite, a mineral in the phosphate group. The apatite found in bones contains calcium and water in its structure and is called hydroxycarbonate apatite, Ca5(PO4)3(OH). As mentioned above, such substances are not technically minerals until the organism dies and these hard parts become fossils.

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3.3 Silicate Minerals

Minerals are categorized based on their composition and structure. Silicate minerals are built around a molecular ion called the silicon-oxygen tetrahedron. A tetrahedron has a pyramid-like shape with four sides and four corners. Silicate minerals form the largest group of minerals on Earth, comprising the vast majority of the Earth’s mantle and crust. Of the nearly four thousand known minerals on Earth, most are rare. There are only a few that make up most of the rocks likely to be encountered by surface dwelling creatures like us. These are generally called the rock-forming minerals.

The silicon-oxygen tetrahedron (SiO4) consists of a single silicon atom at the center and four oxygen atoms located at the four corners of the tetrahedron. Each oxygen ion has a -2 charge and the silicon ion has a +4 charge. The silicon ion shares one of its four valence electrons with each of the four oxygen ions in a covalent bond to create a symmetrical geometric four-sided pyramid figure. Only half of the oxygen’s valence electrons are shared, giving the silicon-oxygen tetrahedron an ionic charge of -4. This silicon-oxygen tetrahedron forms bonds with many other combinations of ions to form the large group of silicate minerals.

The silicon ion is much smaller than the oxygen ions (see the figures) and fits into a small space in the center of the four large oxygen ions, seen if the top ball is removed (as shown in the figure to the right). Because only one of the valence electrons of the corner oxygens is shared, the silicon-oxygen tetrahedron has chemically active corners available to form bonds with other silica tetrahedra or other positively charged ions such as Al+3, Fe+2,+3, Mg+2, K+1, Na+1, and Ca+2. Depending on many factors, such as the original magma chemistry, silica-oxygen tetrahedra can combine with other tetrahedra in several different configurations. For example, tetrahedra can be isolated, attached in chains, sheets, or three dimensional structures. These combinations and others create the chemical structure in which positively charged ions can be inserted for unique chemical compositions forming silicate mineral groups.

3.3.1 The dark ferromagnesian silicates

The Olivine Family

Olivine is the primary mineral component in mantle rock such as peridotite and basalt. It is characteristically green when not weathered. The chemical formula is (Fe,Mg)2SiO4. As previously described, the comma between iron (Fe) and magnesium (Mg) indicates these two elements occur in a solid solution. Not to be confused with a liquid solution, a solid solution occurs when two or more elements have similar properties and can freely substitute for each other in the same location in the crystal structure.

Olivine is referred to as a mineral family because of the ability of iron and magnesium to substitute for each other. Iron and magnesium in the olivine family indicates a solid solution forming a compositional series within the mineral group which can form crystals of all iron as one end member and all mixtures of iron and magnesium in between to all magnesium at the other end member. Different mineral names are applied to compositions between these end members. In the olivine series of minerals, the iron and magnesium ions in the solid solution are about the same size and charge, so either atom can fit into the same location in the growing crystals. Within the cooling magma, the mineral crystals continue to grow until they solidify into igneous rock. The relative amounts of iron and magnesium in the parent magma determine which minerals in the series form. Other rarer elements with similar properties to iron or magnesium, like manganese (Mn), can substitute into the olivine crystalline structure in small amounts. Such ionic substitutions in mineral crystals give rise to the great variety of minerals and are often responsible for differences in color and other properties within a group or family of minerals. Olivine has a pure iron end-member (called fayalite) and a pure magnesium end-member (called forsterite). Chemically, olivine is mostly silica, iron, and magnesium and therefore is grouped among the dark-colored ferromagnesian (iron=ferro, magnesium=magnesian) or mafic minerals, a contraction of their chemical symbols Ma and Fe. Mafic minerals are also referred to as dark-colored ferromagnesian minerals. Ferro means iron and magnesian refers to magnesium. Ferromagnesian silicates tend to be more dense than non-ferromagnesian silicates. This difference in density ends up being important in controlling the behavior of the igneous rocks that are built from these minerals: whether a tectonic plate subducts or not is largely governed by the density of its rocks, which are in turn controlled by the density of the minerals that comprise them.

The crystal structure of olivine is built from independent silica tetrahedra. Minerals with independent tetrahedral structures are called neosilicates (or orthosilicates). In addition to olivine, other common neosilicate minerals include garnet, topaz, kyanite, and zircon.

Two other similar arrangements of tetrahedra are close in structure to the neosilicates and grade toward the next group of minerals, the pyroxenes. In a variation on independent tetrahedra called sorosilicates, there are minerals that share one oxygen between two tetrahedra, and include minerals like pistachio-green epidote, a gemstone. Another variation are the cyclosilicates, which as the name suggests, consist of tetrahedral rings, and include gemstones such as beryl, emerald, aquamarine, and tourmaline

3.3.2 Pyroxene Family

Pyroxene is another family of dark ferromagnesian minerals, typically black or dark green in color. Members of the pyroxene family have a complex chemical composition that includes iron, magnesium, aluminum, and other elements bonded to polymerized silica tetrahedra. Polymers are chains, sheets, or three-dimensional structures, and are formed by multiple tetrahedra covalently bonded via their corner oxygen atoms. Pyroxenes are commonly found in mafic igneous rocks such as peridotite, basalt, and gabbro, as well as metamorphic rocks like eclogite and blue schist.

Pyroxenes are built from long, single chains of polymerized silica tetrahedra in which tetrahedra share two corner oxygens. The silica chains are bonded together into the crystal structures by metal cations. A common member of the pyroxene family is augite, itself containing several solid solution series with a complex chemical formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6 that gives rise to a number of individual mineral names.

This single-chain crystalline structure bonds with many elements, which can also freely substitute for each other. The generalized chemical composition for pyroxene is XZ(Al,Si)2O6. X represents the ions Na, Ca, Mg, or Fe, and Z represents Mg, Fe, or Al. These ions have similar ionic sizes, which allows many possible substitutions among them. Although the cations may freely substitute for each other in the crystal, they carry different ionic charges that must be balanced out in the final crystalline structure. For example Na has a charge of +1, but Ca has charge of +2. If a Na+ ion substitutes for a Ca+2 ion, it creates an unequal charge that must be balanced by other ionic substitutions elsewhere in the crystal. Note that ionic size is more important than ionic charge for substitutions to occur in solid solution series in crystals.

3.3.3 Amphibole Family

Amphiboleminerals are built from polymerized double silica chains and they are also referred to as inosilicates. Imagine two pyroxene chains that connect together by sharing a third oxygen on each tetrahedra. Amphiboles are usually found in igneous and metamorphic rocks and typically have a long-bladed crystal habit. The most common amphibole, hornblende, is usually black; however, they come in a variety of colors depending on their chemical composition. The metamorphic rock, amphibolite, is primarily composed of amphibole minerals.

Amphiboles are composed of iron, magnesium, aluminum, and other cations bonded with silica tetrahedra. These dark ferromagnesian minerals are commonly found in gabbro, baslt, diorite, and often form the black specks in granite. Their chemical formula is very complex and generally written as (RSi4O11)2, where R represents many different cations. For example, it can also be written more exactly as AX2Z5((Si,Al,Ti)8O22)(OH,F,Cl,O)2. In this formula A may be Ca, Na, K, Pb, or blank; X equals Li, Na, Mg, Fe, Mn, or Ca; and Z is Li, Na, Mg, Fe, Mn, Zn, Co, Ni, Al, Cr, Mn, V, Ti, or Zr. The substitutions create a wide variety of colors such as green, black, colorless, white, yellow, blue, or brown. Amphibole crystals can also include hydroxide ions (OH–), which occurs from an interaction between the growing minerals and water dissolved in magma.

3.3.4 Sheet Silicates

Sheet silicates are built from tetrahedra which share all three of their bottom corner oxygens thus forming sheets of tetrahedra with their top corners available for bonding with other atoms. Micas and clays are common types of sheet silicates, also known as phyllosilicates. Mica minerals are usually found in igneous and metamorphic rocks, while clay minerals are more often found in sedimentary rocks. Two frequently found micas are dark-colored biotite, frequently found in granite, and light-colored muscovite, found in the metamorphic rock called schist.

Chemically, sheet silicates usually contain silicon and oxygen in a 2:5 ratio (Si4O10). Micas contain mostly silica, aluminum, and potassium. Biotite mica has more iron and magnesium and is considered a ferromagnesian silicate mineral. Muscovite micas belong to the felsic silicate minerals. Felsic is a contraction formed from feldspar, the dominant mineral in felsic rocks.

The illustration of the crystalline structure of mica shows the corner O atoms bonded with K, Al, Mg, Fe, and Si atoms, forming polymerized sheets of linked tetrahedra, with an octahedral layer of Fe, Mg, or Al, between them. The yellow potassium ions form Van der Waals bonds (attraction and repulsion between atoms, molecules, and surfaces) and hold the sheets together. Van der Waals bonds differ from covalent and ionic bonds, and exist here between the sandwiches, holding them together into a stack of sandwiches. The Van der Waals bonds are weak compared to the bonds within the sheets, allowing the sandwiches to be separated along the potassium layers. This gives mica its characteristic property of easily cleaving into sheets.

Clays minerals occur in sediments formed by the weathering of rocks and are another family of silicate minerals with a tetrahedral sheet structure. Clay minerals form a complex family, and are an important component of many sedimentary rocks. Other sheet silicates include serpentine and chlorite, found in metamorphic rocks.

Clay minerals are composed of hydrous aluminum silicates. One type of clay, kaolinite, has a structure like an open-faced sandwich, with the bread being a single layer of silicon-oxygen tetrahedra and a layer of aluminum as the spread in an octahedral configuration with the top oxygens of the sheets.

3.3.5 Framework Silicates

Quartz and feldspar are the two most abundant minerals in the continental crust. In fact, feldspar itself is the single most abundant mineral in the Earth’s crust. There are two types of feldspar, one containing potassium and abundant in felsic rocks of the continental crust, and the other with sodium and calcium abundant in the mafic rocks of oceanic crust. Together with quartz, these minerals are classified as framework silicates. They are built with a three-dimensional framework of silica tetrahedra in which all four corner oxygens are shared with adjacent tetrahedra. Within these frameworks in feldspar are holes and spaces into which other ions like aluminum, potassium, sodium, and calcium can fit giving rise to a variety of mineral compositions and mineral names.

Feldspars are usually found in igneous rocks, such as granite, rhyolite, and basalt as well as metamorphic rocks and detrital sedimentary rocks. Detrital sedimentary rocks are composed of mechanically weathered rock particles, like sand and gravel. Quartz is especially abundant in detrital sedimentary rocks because it is very resistant to disintegration by weathering. While quartz is the most abundant mineral on the Earth’s surface, due to its durability, the feldspar minerals are the most abundant minerals in the Earth’s crust, comprising roughly 50% of the total minerals that make up the crust.

Quartz is composed of pure silica, SiO2, with the tetrahedra arranged in a three dimensional framework. Impurities consisting of atoms within this framework give rise to many varieties of quartz among which are gemstones like amethyst, rose quartz, and citrine. Feldspars are mostly silica with aluminum, potassium, sodium, and calcium. Orthoclase feldspar (KAlSi3O8), also called potassium feldspar or K-spar, is made of silica, aluminum, and potassium. Quartz and orthoclase feldspar are felsic minerals. Felsic is the compositional term applied to continental igneous minerals and rocks that contain an abundance of silica. Another feldspar is plagioclase with the formula (Ca,Na)AlSi3O8, the solid solution (Ca,Na) indicating a series of minerals, one end of the series with calcium CaAl2Si2O8, called anorthite, and the other end with sodium NaAlSi3O8, called albite. Note how the mineral accommodates the substitution of Ca++ and Na+. Minerals in this solid solution series have different mineral names.

Note that aluminum, which has a similar ionic size to silicon, can substitute for silicon inside the tetrahedra (see figure). Because potassium ions are so much larger than sodium and calcium ions, which are very similar in size, the inability of the crystal lattice to accommodate both potassium and sodium/calcium gives rise to the two families of feldspar, orthoclase and plagioclase respectively. Framework silicates are called tectosilicates and include the alkali metal-rich feldspathoids and zeolites.

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3.4 Non-Silicate Minerals

The crystal structure of non-silicate minerals (see table) does not contain silica-oxygen tetrahedra. Many non-silicate minerals are economically important and provide metallic resources such as copper, lead, and iron. They also include valuable non-metallic products such as salt, construction materials, and fertilizer.

Mineral Group	Examples	Formula	Uses
Nativeelements	gold, silver, copper	Au, Ag, Cu	Jewelry, coins, industry
Carbonates	calcite, dolomite	CaCO3, CaMg(CO3)2	Lime, Portland cement
Oxides	hematite, magnetite, bauxite	Fe2O3, Fe3O4, a mixture of aluminum oxides	Ores of iron & aluminum, pigments
Halides	halite, sylvite	NaCl, KCl	Table salt, fertilizer
Sulfides	galena, chalcopyrite, cinnabar	PbS, CuFeS2, HgS	Ores of lead, copper, mercury
Sulphates	gypsum, epsom salts	CaSo4·2H2O, MgSO4·7H2O	Sheetrock, therapeutic soak
Phosphates	apatite	Ca5(PO4)3(F,Cl,OH)	Fertilizer, teeth, bones

Common non-silicate mineral groups.

3.4.1 Carbonates

Calcite (CaCO3) and dolomite (CaMg(CO3)2) are the two most frequently occurring carbonateminerals, and usually occur in sedimentary rocks, such as limestone and dolostone rocks, respectively. Some carbonate rocks, such calcite and dolomite, are formed via evaporation and precipitation. However, most carbonate-rich rocks, such as limestone, are created by the lithification of fossilized marine organisms. These organisms, including those we can see and many microscopic organisms, have shells or exoskeletons consisting of calcium carbonate (CaCO3). When these organisms die, their remains accumulate on the floor of the water body in which they live and the soft body parts decompose and dissolve away. The calcium carbonate hard parts become included in the sediments, eventually becoming the sedimentary rock called limestone. While limestone may contain large, easy to see fossils, most limestones contain the remains of microscopic creatures and thus originate from biological processes.

Calcite crystals show an interesting property called birefringence, meaning they polarize light into two wave components vibrating at right angles to each other. As the two light waves pass through the crystal, they travel at different velocities and are separated by refraction into two different travel paths. In other words, the crystal produces a double image of objects viewed through it. Because they polarize light, calcite crystals are used in special petrographic microscopes for studying minerals and rocks.

Many non-silicate minerals are referred to as salts. The term salts used here refers to compounds made by replacing the hydrogen in natural acids. The most abundant natural acid is carbonic acid that forms by the solution of carbon dioxide in water. Carbonate minerals are salts built around the carbonate ion (CO3-2) where calcium and/or magnesium replace the hydrogen in carbonic acid (H2CO3). Calcite and a closely related polymorph aragonite are secreted by organisms to form shells and physical structures like corals. Many such creatures draw both calcium and carbonate from dissolved bicarbonate ions (HCO3–) in ocean water. As seen in the mineral identification section below, calcite is easily dissolved in acid and thus effervesces in dilute hydrochloric acid (HCl). Small dropper bottles of dilute hydrochloric acid are often carried by geologists in the field as well as used in mineral identification labs.

Other salts include halite (NaCl) in which sodium replaces the hydrogen in hydrochloric acid and gypsum (Ca[SO4] • 2 H2O) in which calcium replaces the hydrogen in sulfuric acid. Note that some water molecules are also included in the gypsum crystal. Salts are often formed by evaporation and are called evaporite minerals.

The figure shows the crystal structure of calcite (CaCO3). Like silicon, carbon has four valence electrons. The carbonate unit consists of carbon atoms (tiny white dots) covalently bonded to three oxygen atoms (red), one oxygen sharing two valence electrons with the carbon and the other two sharing one valence electron each with the carbon, thus creating triangular units with a charge of -2. The negatively charged carbonate unit forms an ionic bond with the Ca ion (blue), which as a charge of +2.

3.4.2 Oxides, Halides, and Sulfides

After carbonates, the next most common non-silicate minerals are the oxides, halides, and sulfides.

Oxides consist of metal ions covalently bonded with oxygen. The most familiar oxide is rust, which is a combination of iron oxides (Fe2O3) and hydrated oxides. Hydrated oxides form when iron is exposed to oxygen and water. Iron oxides are important for producing metallic iron. When iron oxide or ore is smelted, it produces carbon dioxide (CO2) and metallic iron.

The red color in rocks is usually due to the presence of iron oxides. For example, the red sandstone cliffs in Zion National Park and throughout Southern Utah consist of white or colorless grains of quartz coated with iron oxide which serve as cementing agents holding the grains together.

Other iron oxides include limonite, magnetite, and hematite. Hematite occurs in many different crystal forms. The massive form shows no external structure. Botryoidal hematite shows large concentric blobs. Specular hematite looks like a mass of shiny metallic crystals. Oolitic hematite looks like a mass of dull red fish eggs. These different forms of hematite are polymorphs and all have the same formula, Fe2O3.

Other common oxide minerals include:

ice (H2O), an oxide of hydrogen

bauxite (Al2H2O4), hydrated oxides of aluminum, an ore for producing metallic aluminum

corundum (Al2O3), which includes ruby and sapphire gemstones.

The halides consist of halogens in column VII, usually fluorine or chlorine, ionically bonded with sodium or other cations. These include halite or sodium chloride (NaCl), common table salt; sylvite or potassium chloride (KCl); and fluorite or calcium fluoride (CaF2).

Halideminerals usually form from the evaporation of sea water or other isolated bodies of water. A well-known example of halide mineral deposits created by evaporation is the Bonneville Salt Flats, located west of the Great Salt Lake in Utah (see figure).

Many important metal ores are sulfides, in which metals are bonded to sulfur. Significant examples include: galena (lead sulfide), sphalerite (zinc sulfide), pyrite

(iron sulfide, sometimes called “fool’s gold”), and chalcopyrite (iron-copper sulfide). Sulfides are well known for being important ore minerals. For example, galena is the main source of lead, sphalerite is the main source of zinc, and chalcopyrite is the main copper ore mineral mined in porphyry deposits like the Bingham mine (see chapter 16). The largest sources of nickel, antimony, molybdenum, arsenic, and mercury are also sulfides.

3.4.3 Sulfates

Sulfateminerals contain a metal ion, such as calcium, bonded to a sulfate ion. The sulfate ion is a combination of sulfur and oxygen (SO4–2). The sulfate mineral gypsum (CaSO4ᐧ2H2O) is used in construction materials such as plaster and drywall. Gypsum is often formed from evaporating water and usually contains water molecules in its crystalline structure. The ᐧ2H2O in the formula indicates the water molecules are whole H2O. This is different from minerals like amphibole, which contain a hydroxide ion (OH–) that is derived from water, but is missing a hydrogen ion (H+). The calcium sulfate without water is a different mineral than gypsum called anhydrite (CaSO4).

3.4.4 Phosphates

Phosphateminerals have a tetrahedral phosphate unit (PO4-3) combined with various anions and cations. In some cases arsenic or vanadium can substitute for phosphorus. Phosphates are an important ingredient of fertilizers as well as detergents, paint, and other products. The best known phosphate mineral is apatite, Ca5(PO4)3(F,Cl,OH), variations of which are found in teeth and bones. The gemstone turquoise [CuAl6(PO4)4(OH)8·4H2O ] is a copper-rich phosphate mineral that, like gypsum, contains water molecules.

3.4.5 Native Element Minerals

Native element minerals, usually metals, occur in nature in a pure or nearly pure state. Gold is an example of a nativeelementmineral; it is not very reactive and rarely bonds with other elements so it is usually found in an isolated or pure state. The non-metallic and poorly-reactive mineral carbon is often found as a native element, such as graphite and diamonds. Mildly reactive metals like silver, copper, platinum, mercury, and sulfur sometimes occur as native element minerals. Reactive metals such as iron, lead, and aluminum almost always bond to other elements and are rarely found in a native state.

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3.5 Identifying Minerals

Geologists identify minerals by their physical properties. In the field, where geologists may have limited access to advanced technology and powerful machines, they can still identify minerals by testing several physical properties: luster and color, streak, hardness, crystal habit, cleavage and fracture, and some special properties. Only a few common minerals make up the majority of Earth’s rocks and are usually seen as small grains in rocks. Of the several properties used for identifying minerals, it is good to consider which will be most useful for identifying them in small grains surrounded by other minerals.

3.5.1 Luster and Color

The first thing to notice about a mineral is its surface appearance, specifically luster and color. Luster describes how the mineral looks. Metallic luster looks like a shiny metal such as chrome, steel, silver, or gold. Submetallic luster has a duller appearance. Pewter, for example, shows submetallic luster.

Nonmetallicluster doesn’t look like a metal and may be described as vitreous (glassy), earthy, silky, pearly, and other surface qualities. Nonmetallic minerals may be shiny, although their vitreous shine is different from metallic luster. See the table for descriptions and examples of nonmetallic luster.

Luster	Image	Description
Vitreous/glassy	Quartz crystals	Surface is shiny like glass
Earthy/dull	Kaolin specimen showing dull or earthy luster	Dull, like dried mud or clay
Silky	Specimen showing silky luster	Soft shine like silk fabric
Pearly	Specimen showing pearly luster	Like the inside of a clam shell or mother-of-pearl
Submetallic	Submetallic luster on sphalerite	Has the appearance of dull metal, like pewter. These minerals would usually still be considered metallic . Submetallic appearance can occur in metallic minerals because of weathering .

Surface color may be helpful in identifying minerals, although it can be quite variable within the same mineral family. Mineral colors are affected by the main elements as well as impurities in the crystals. These impurities may be rare elements—like manganese, titanium, chromium, or lithium—even other molecules that are not normally part of the mineral formula. For example, the incorporation of water molecules gives quartz, which is normally clear, a milky color.

Some minerals predominantly show a single color. Malachite and azurite are green and blue, respectively, because of their copper content. Other minerals have a predictable range of colors due to elemental substitutions, usually via a solid solution. Feldspars, the most abundant minerals in the earth’s crust, are complex, have solid solution series, and present several colors including pink, white, green, gray and others. Other minerals also come in several colors, influenced by trace amounts of several elements. The same element may show up as different colors, in different minerals. With notable exceptions, color is usually not a definitive property of minerals. For identifying many minerals. a more reliable indicator is streak, which is the color of the powdered mineral.

3.5.2 Streak

Streak examines the color of a powdered mineral, and can be seen when a mineral sample is scratched or scraped on an unglazed porcelain streak plate. A paper page in a field notebook may also be used for the streak of some minerals. Minerals that are harder than the streak plate will not show streak, but will scratch the porcelain. For these minerals, a streak test can be obtained by powdering the mineral with a hammer and smearing the powder across a streak plate or notebook paper.

While mineral surface colors and appearances may vary, their streak colors can be diagnostically useful. An example of this property is seen in the iron-oxide mineral hematite. Hematite occurs in a variety of forms, colors and lusters, from shiny metallic silver to earthy red-brown, and different physical appearances. A hematite streak is consistently reddish brown, no matter what the original specimen looks like. Iron sulfide or pyrite, is a brassy metallic yellow. Commonly named fool’s gold, pyrite has a characteristic black to greenish-black streak.

3.5.3 Hardness

Hardness measures the ability of a mineral to scratch other substances. The Mohs Hardness Scale gives a number showing the relative scratch-resistance of minerals when compared to a standardized set of minerals of increasing hardness. The Mohs scale was developed by German geologist Fredrick Mohs in the early 20th century, although the idea of identifying minerals by hardness goes back thousands of years. Mohs hardness values are determined by the strength of a mineral’s atomic bonds.

The figure shows the minerals associated with specific hardness values, together with some common items readily available for use in field testing and mineral identification. The hardness values run from 1 to 10, with 10 being the hardest; however, the scale is not linear. Diamond defines a hardness of 10 and is actually about four times harder than corundum, which is 9. A steel pocketknife blade, which has a hardness value of 5.5, separates between hard and soft minerals on many mineral identification keys.

3.5.4 Crystal Habit

Minerals can be identified by crystal habit, how their crystals grow and appear in rocks. Crystal shapes are determined by the arrangement of the atoms within the crystal structure. For example, a cubic arrangement of atoms gives rise to a cubic-shaped mineral crystal. Crystal habit refers to typically observed shapes and characteristics; however, they can be affected by other minerals crystallizing in the same rock. When minerals are constrained so they do not develop their typical crystal habit, they are called anhedral. Subhedral crystals are partially formed shapes. For some minerals characteristic crystal habit is to grow crystal faces even when surrounded by other crystals in rock. An example is garnet. Minerals grown freely where the crystals are unconstrained and can take characteristic shapes often form crystal faces. A euhedral crystal has a perfectly formed, unconstrained shape. Some minerals crystallize in such tiny crystals, they do not show a specific crystal habit to the naked eye. Other minerals, like pyrite, can have an array of different crystal habits, including cubic, dodecahedral, octahedral, and massive. The table lists typical crystal habits of various minerals.

Habit	Image	Examples
Bladed long and flat crystals	Bladed kyanite	kyanite, amphibole , gypsum
Botryoidal/mammillary blobby, circular crystals	Malachite from the Congo	hematite, malachite, smithsonite
Coating/ laminae /druse crystals that are small and coat surfaces	Quartz (var. amethyst) geode	quartz , calcite , malachite, azurite
Cubic cube-shaped crystals	Cubic crystals of galena	pyrite, galena, halite
Dodecahedral 12-sided polygon shapes	Pyrite crystals with dodecahedral habit	garnet, pyrite
Dendritic branching crystals	Manganese dendrites, scale in mm.	Mn- oxides , copper, gold
Equant crystals that do not have a long direction	Large olivine crystal	olivine , garnet, pyroxene
Fibrous thin, very long crystals	Tremolite, a type of amphibole	serpentine, amphibole , zeolite
Layered, sheets stacked, very thin, flat crystals	Sheet crystals of muscovite	mica ( biotite , muscovite , etc.)
Lenticular/platy crystals that are plate-like	Orange wulfenite on calcite	selenite roses, wulfenite, calcite
Hexagonal crystals with six sides	Hexagonal hanksite	quartz , hanksite, corundum
Massive /granular Crystals with no obvious shape, microscopic crystals	Limonite, a hydrated oxide of iron	limonite, pyrite, azurite, bornite
Octahedral 4-sided double pyramid crystals	Octagonal cleavage in fluorite	diamond, fluorite, magnetite, pyrite
Prismatic/columnar very long, cylindrical crystals	Columnar tourmaline	tourmaline, beryl, barite
Radiating crystals that grow from a point and fan out	Pyrophyllite	pyrite “suns”, pyrophyllite
Rhombohedral crystals shaped like slanted cubes	Calcite crystal in shape of rhomb.	calcite , dolomite
Tabular/blocky/stubby sharp-sided crystals with no long direction	Crystals of diopside, a member of the pyroxene family	feldspar , pyroxene , calcite
Tetrahedral three-sided, pyramid-shaped crystals	Tetrahedrite	magnetite, spinel, tetrahedrite

Another crystal habit that may be used to identify minerals is striations, which are dark and light parallel lines on a crystal face. Twinning is another, which occurs when the crystal structure replicates in mirror images along certain directions in the crystal.

Striations and twinning are related properties in some minerals including plagioclase feldspar. Striations are optical lines on a cleavage surface. Because of twinning in the crystal, striations show up on one of the two cleavage faces of the plagioclase crystal.

3.5.5 Cleavage and Fracture

Minerals often show characteristic patterns of breaking along specific cleavage planes or show characteristic fracture patterns. Cleavage planes are smooth, flat, parallel planes within the crystal. The cleavage planes may show as reflective surfaces on the crystal, as parallel cracks that penetrate into the crystal, or show on the edge or side of the crystal as a series of steps like rice terraces. Cleavage arises in crystals where the atomic bonds between atomic layers are weaker along some directions than others, meaning they will break preferentially along these planes. Because they develop on atomic surfaces in the crystal, cleavage planes are optically smooth and reflect light, although the actual break on the crystal may appear jagged or uneven. In such cleavages, the cleavage surface may appear like rice terraces on a mountainside that all reflect sunlight from a particular sun angle. Some minerals have a strong cleavage, some minerals only have weak cleavage or do not typically demonstrate cleavage.

For example, quartz and olivine rarely show cleavage and typically break into conchoidal fracture patterns.

Graphite has its carbon atoms arranged into layers with relatively strong bonds within the layer and very weak bonds between the layers. Thus graphite cleaves readily between the layers and the layers slide easily over one another giving graphite its lubricating quality.

Mineralfracture surfaces may be rough and uneven or they may be show conchoidal fracture. Uneven fracture patterns are described as irregular, splintery, fibrous. A conchoidal fracture has a smooth, curved surface like a shallow bowl or conch shell, often with curved ridges. Natural volcanic glass, called obsidian, breaks with this characteristic conchoidal pattern

To work with cleavage, it is important to remember that cleavage is a result of bonds separating along planes of atoms in the crystal structure. On some minerals, cleavage planes may be confused with crystal faces. This will usually not be an issue for crystals of minerals that grew together within rocks. The act of breaking the rock to expose a fresh face will most likely break the crystals along cleavage planes. Some cleavage planes are parallel with crystal faces but many are not. Cleavage planes are smooth, flat, parallel planes within the crystal. The cleavage planes may show as parallel cracks that penetrate into the crystal (see amphibole below), or show on the edge or side of the crystal as a series of steps like rice terraces. For some minerals characteristic crystal habit is to grow crystal faces even when surrounded by other crystals in rock. An example is garnet. Minerals grown freely where the crystals are unconstrained and can take characteristic shapes often form crystal faces (see quartz below).

In some minerals, distinguishing cleavage planes from crystal faces may be challenging for the student. Understanding the nature of cleavage and referring to the number of cleavage planes and cleavage angles on identification keys should provide the student with enough information to distinguish cleavages from crystal faces. Cleavage planes may show as multiple parallel cracks or flat surfaces on the crystal. Cleavage planes may be expressed as a series of steps like terraced rice paddies. See the cleavage surfaces on galena above or plagioclase below. Cleavage planes arise from the tendency of mineral crystals to break along specific planes of weakness within the crystal favored by atomic arrangements. The number of cleavage planes, the quality of the cleavage surfaces, and the angles between them are diagnostic for many minerals and cleavage is one of the most useful properties for identifying minerals. Learning to recognize cleavage is an especially important and useful skill in studying minerals.

As an identification property of minerals, cleavage is usually given in terms of the quality of the cleavage (perfect, imperfect, or none), the number of cleavage surfaces, and the angles between the surfaces. The most common number of cleavage plane directions in the common rock-forming minerals are: one perfect cleavage (as in mica), two cleavage planes (as in feldspar, pyroxene, and amphibole), and three cleavage planes (as in halite, calcite, and galena). One perfect cleavage (as in mica) develops on the top and bottom of the mineral specimen with many parallel cracks showing on the sides but no angle of intersection. Two cleavage planes intersect at an angle. Common cleavage angles are 60°, 75°, 90°, and 120°. Amphibole has two cleavage planes at 60° and 120°. Galena and halite have three cleavage planes at 90° (cubic cleavage). Calcite cleaves readily in three directions producing a cleavage figure called a rhomb that looks like a cube squashed over toward one corner giving rise to the approximately 75° cleavage angles. Pyroxene has an imperfect cleavage with two planes at 90°.

Cleavages on common rock-forming minerals

Quartz—none (conchoidal fracture)

Olivine—none (conchoidal fracture)

Mica—1 perfect

Feldspar—2 perfect at 90°

Pyroxene—2 imperfect at 90°

Amphibole—2 perfect at 60°/120°

Calcite—3 perfect at approximately 75°

Halite, galena, pyrite—3 perfect at 90°

3.5.6 Special Properties

Special properties are unique and identifiable characteristics used to identify minerals or that allow some minerals to be used for special purposes. Ulexite has a fiber-optic property that can project images through the crystal like a high-definition television screen (see figure). A simple identifying special property is taste, such as the salty flavor of halite or common table salt (NaCl). Sylvite is potassium chloride (KCl) and has a more bitter taste.

Another property geologists may use to identify minerals is a property related to density called specific gravity. Specific gravity measures the weight of a mineral specimen relative to the weight of an equal volume of water. The value is expressed as a ratio between the mineral and water weights. To measure specific gravity, a mineral specimen is first weighed in grams then submerged in a graduated cylinder filled with pure water at room temperature. The rise in water level is noted using the cylinder’s graduated scale. Since the weight of water at room temperature is 1 gram per cubic centimeter, the ratio of the two weight numbers gives the specific gravity. Specific gravity is easy to measure in the laboratory but is less useful for mineral identification in the field than other more easily observed properties, except in a few rare cases such as the very dense galena or native gold. The high density of these minerals gives rise to a qualitative property called “heft.” Experienced geologists can roughly assess specific gravity by heft, a subjective quality of how heavy the specimen feels in one’s hand relative to its size.

A simple test for identifying calcite and dolomite is to drop a bit of dilute hydrochloric acid (10-15% HCl) on the specimen. If the acid drop effervesces or fizzes on the surface of the rock, the specimen is calcite. If it does not, the specimen is scratched to produce a small amount of powder and test with acid again. If the acid drop fizzes slowly on the powdered mineral, the specimen is dolomite. The difference between these two minerals can be seen in the video. Geologists who work with carbonate rocks carry a small dropper bottle of dilute HCl in their field kit. Vinegar, which contains acetic acid, can be used for this test and is used to distinguish non-calcite fossils from limestone. While acidic, vinegar produces less of a fizzing reaction because acetic acid is a weaker acid.

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Some iron-oxide minerals are magnetic and are attracted to magnets. A common name for a naturally magnetic iron oxide is lodestone. Others include magnetite (Fe3O4) and ilmenite (FeTiO3). Magnetite is strongly attracted to magnets and can be magnetized. Ilmenite and some types of hematite are weakly magnetic.

Some minerals and mineraloids scatter light via a phenomenon called iridescence. This property occurs in labradorite (a variety of plagioclase) and opal. It is also seen in biologically created substances like pearls and seashells. Cut diamonds show iridescence and the jeweler’s diamond cut is designed to maximize this property.

Striations on mineral cleavage faces are an optical property that can be used to separate plagioclase feldspar from potassium feldspar (K-spar). A process called twinning creates parallel zones in the crystal that are repeating mirror images. The actual cleavage angle in plagioclase is slightly different than 90o and the alternating mirror images in these twinned zones produce a series of parallel lines on one of plagioclase’s two cleavage faces. Light reflects off these twinned lines at slightly different angles which then appear as light and dark lines called striations on the cleavage surface. Potassium feldspar does not exhibit twinning or striations but may show linear features called exsolution lamellae, also known as perthitic lineation or simply perthite. Because sodium and potassium do not fit into the same feldspar crystal structure, the lines are created by small amounts of sodium feldspar (albite) separating from the dominant potassium feldspar (K-spar) within the crystal structure. The two different feldspars crystallize out into roughly parallel zones within the crystal, which are seen as these linear markings.

One of the most interesting special mineral properties is fluorescence. Certain minerals, or trace elements within them, give off visible light when exposed to ultraviolet radiation or black light. Many mineral exhibits have a fluorescence room equipped with black lights so this property can be observed. An even rarer optical property is phosphorescence. Phosphorescent minerals absorb light and then slowly release it, much like a glow-in-the-dark sticker.

Take this quiz to check your comprehension of this section.

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Summary

Minerals are the building blocks of rocks and essential to understanding geology. Mineral properties are determined by their atomic bonds. Most minerals begin in a fluid, and either crystallize out of cooling magma or precipitate as ions and molecules out of a saturated solution. The silicates are largest group of minerals on Earth, by number of varieties and relative quantity, making up a large portion of the crust and mantle. Based on the silicon-oxygen tetrahedra, the crystal structure of silicates reflects the fact that silicon and oxygen are the top two of Earth’s most abundant elements. Non-silicate minerals are also economically important, and providing many types of construction and manufacturing materials. Minerals are identified by their unique physical properties, including luster, color, streak, hardness, crystal habit, fracture, cleavage, and special properties.

Take this quiz to check your comprehension of this Chapter.

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References

Clarke, F.W.H.S.W., 1927, The Composition of the Earth’s Crust: Professional Paper, United States Geological Survey, Professional Paper.

Gordon, L.M., and Joester, D., 2011, Nanoscale chemical tomography of buried organic-inorganic interfaces in the chiton tooth: Nature, v. 469, no. 7329, p. 194–197.

Hans Wedepohl, K., 1995, The composition of the continental crust: Geochim. Cosmochim. Acta, v. 59, no. 7, p. 1217–1232.

Lambeck, K., 1986, Planetary evolution: banded iron formations: v. 320, no. 6063, p. 574–574.

metallicbond | chemistry.

Scerri, E.R., 2007, The Periodic Table: Its Story and Its Significance: Oxford University Press, USA.

Thomson, J.J., 1897, XL. Cathode Rays: Philosophical Magazine Series 5, v. 44, no. 269, p. 293–316.

Trenn, T.J., Geiger, H., Marsden, E., and Rutherford, E., 1974, The Geiger-Marsden Scattering Results and Rutherford’s Atom, July 1912 to July 1913: The Shifting Significance of Scientific Evidence: Isis, v. 65, no. 1, p. 74–82.

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4 Igneous Processes and Volcanoes

4 Igneous Processes and Volcanoes

KEY CONCEPTS

By the end of this chapter, students should be able to:

Explain the origin of magma it relates to plate tectonics

Describe how the Bowen’s Reaction Series relates mineral crystallization and melting temperatures

Explain how cooling of magma leads to rock compositions and textures, and how these are used to classify igneous rocks

Analyze the features of common igneous landforms and how they relate to their origin

Explain partial melting and fractionation, and how they change magma compositions

Describe how silica content affects magma viscosity and eruptive style of volcanoes

Describe volcano types, eruptive styles, composition, and their plate tectonic settings

Describe volcanic hazards

Igneous rock is formed when liquid rock freezes into a solid rock. This molten material is called magma when it is in the ground and lava when it is on the surface. Only the Earth’s outer core is liquid; the Earth’s mantle and crust is naturally solid. However, there are a few minor pockets of magma that form near the surface where geologic processes cause melting. It is this magma that becomes the source for volcanoes and igneous rocks. This chapter will describe the classification of igneous rocks, the unique processes that form magmas, types of volcanoes and volcanic processes, volcanic hazards, and igneous landforms.

Lava cools quickly on the surface of the earth and forms tiny microscopic crystals. These are known as fine-grained extrusive, or volcanic, igneous rocks. Extrusive rocks are often vesicular, filled with holes from escaping gas bubbles. Volcanism is the process in which lava is erupted. Depending on the properties of the lava that is erupted, the volcanism can be drastically different, from smooth and gentle to dangerous and explosive. This leads to different types of volcanoes and different volcanic hazards.

In contrast, magma that cools slowly below the earth’s surface forms larger crystals which can be seen with the naked eye. These are known as coarse-grained intrusive, or plutonic, igneous rocks. This relationship between cooling rates and grain sizes of the solidified minerals in igneous rocks is important for interpreting the rock’s geologic history.

4.1 Classification of Igneous Rocks

Igneous rocks are classified based on texture and composition. Texture describes the physical characteristics of the minerals, such as grain size. This relates to the cooling history of the molten magma from which it came. Composition refers to the rock’s specific mineralogy and chemical composition. Cooling history is also related to changes that can occur to the composition of igneous rocks.

4.1.1 Texture

If magma cools slowly, deep within the crust, the resulting rock is called intrusive or plutonic. The slow cooling process allows crystals to grow large, giving intrusive igneous rock a coarse-grained or phaneritic texture. The individual crystals in phaneritic texture are readily visible to the unaided eye.

When lava is extruded onto the surface, or intruded into shallow fissures near the surface and cools, the resulting igneous rock is called extrusive or volcanic. Extrusive igneous rocks have a fine-grained or aphanitic texture, in which the grains are too small to see with the unaided eye. The fine-grained texture indicates the quickly cooling lava did not have time to grow large crystals. These tiny crystals can be viewed under a petrographic microscope. In some cases, extrusive lava cools so rapidly it does not develop crystals at all. This non-crystalline material is not classified as minerals, but as volcanic glass. This is a common component of volcanic ash and rocks like obsidian.

Some igneous rocks have a mix of coarse-grained minerals surrounded by a matrix of fine-grained material in a texture called porphyritic. The large crystals are called phenocrysts and the fine-grained matrix is called the groundmass or matrix. Porphyritic texture indicates the magma body underwent a multi-stage cooling history, cooling slowly while deep under the surface and later rising to a shallower depth or the surface where it cooled more quickly.

Residual molten material expelled from igneous intrusions may form veins or masses containing very large crystals of minerals like feldspar, quartz, beryl, tourmaline, and mica. This texture, which indicates a very slow crystallization, is called pegmatitic. A rock that chiefly consists of pegmatitic texture is known as a pegmatite. To give an example of how large these crystals can get, transparent cleavage sheets of pegmatitic muscovite mica were used as windows during the Middle Ages.

All magmas contain gases dissolved in solution called volatiles. As the magma rises to the surface, the drop in pressure causes the dissolved volatiles to come bubbling out of solution, like the fizz in an opened bottle of soda. The gas bubbles become trapped in the solidifying lava to create a vesicular texture, with the holes specifically called vesicles. The type of volcanic rock with common vesicles is called scoria.

An extreme version of scoria occurs when volatile-rich lava is very quickly quenched and becomes a meringue-like froth of glass called pumice. Some pumice is so full of vesicles that the density of the rock drops low enough that it will float.

Lava that cools extremely quickly may not form crystals at all, even microscopic ones. The resulting rock is called volcanic glass. Obsidian is a rock consisting of volcanic glass. Obsidian as a glassy rock shows an excellent example of conchoidal fracture similar to the mineral quartz (see Chapter 3).

When volcanoes erupt explosively, vast amounts of lava, rock, ash, and gases are thrown into the atmosphere. The solid parts, called tephra, settle back to earth and cool into rocks with pyroclastic textures. Pyro, meaning fire, refers to the igneous source of the tephra and clasticrefers to the rock fragments. Tephra fragments are named based on size—ash (<2 mm), lapilli (2-64 mm), and bombs or blocks (>64 mm). Pyroclastic texture is usually recognized by the chaotic mix of crystals, angular glass shards, and rock fragments. Rock formed from large deposits of tephra fragments is called tuff. If the fragments accumulate while still hot, the heat may deform the crystals and weld the mass together, forming a welded tuff.

4.1.2 Composition

Composition refers to a rock’s chemical and mineral make-up . For igneous rock, composition is divided into four groups: felsic, intermediate, mafic, and ultramafic. These groups refer to differing amounts of silica, iron, and magnesium found in the minerals that make up the rocks. It is important to realize these groups do not have sharp boundaries in nature, but rather lie on a continuous spectrum with many transitional compositions and names that refer to specific quantities of minerals. As an example, granite is a commonly-used term, but has a very specific definition which includes exact quantities of minerals like feldspar and quartz. Rocks labeled as ‘granite‘ in laymen applications can be several other rocks, including syenite, tonalite, and monzonite. To avoid these complications, the following figure presents a simplified version of igneous rock nomenclature focusing on the four main groups, which is adequate for an introductory student.

Felsic refers to a predominance of the light-colored (felsic) minerals feldspar and silica in the form of quartz. These light-colored minerals have more silica as a proportion of their overall chemical formula. Minor amounts of dark-colored (mafic) minerals like amphibole and biotite mica may be present as well. Felsic igneous rocks are rich in silica (in the 65-75% range, meaning the rock would be 65-75% weight percent SiO2) and poor in iron and magnesium.

Intermediate is a composition between felsic and mafic. It usually contains roughly-equal amounts of light and dark minerals, including light grains of plagioclase feldspar and dark minerals like amphibole. It is intermediate in silica in the 55-60% range.

Mafic refers to a abundance of ferromagnesian minerals (with magnesium and iron, chemical symbols Mg and Fe) plus plagioclase feldspar. It is mostly made of dark minerals like pyroxene and olivine, which are rich in iron and magnesium and relatively poor in silica. Mafic rocks are low in silica, in the 45-50% range.

Ultramafic refers to the extremely mafic rocks composed of mostly olivine and some pyroxene which have even more magnesium and iron and even less silica. These rocks are rare on the surface, but make up peridotite, the rock of the upper mantle. It is poor in silica, in the 40% or less range.

On the figure above, the top row has both plutonic and volcanic igneous rocks arranged in a continuous spectrum from felsic on the left to intermediate, mafic, and ultramafic toward the right. Rhyolite thus refers to the volcanic and felsic igneous rocks, and granitethus refer to intrusive and felsic igneous rocks. Andesiteand diorite likewise refer to extrusive and intrusive intermediate rocks (with dacite and granodiorite applying to those rocks with composition between felsic and intermediate). Basaltand gabbroare the extrusive and intrusive names for mafic igneous rocks, and peridotiteis ultramafic, with komatiite as the fine-grained extrusive equivalent. Komatiite is a rare rock because volcanic material that comes direct from the mantle is not common, although some examples can be found in ancient Archean rocks. Nature rarely has sharp boundaries and the classification and naming of rocks often imposes what appear to be sharp boundary names onto a continuous spectrum.

Aphanitic/Phaneritic Rock Types with images

FelsicComposition
Granite from Cape Cod, Massachusetts.	Rhyolite (source: Michael C. Rygel via Wikimedia Commons)
Granite is a course-crystalline felsic intrusive rock. The presence of quartz is a good indicator of granite . Granite commonly has large amounts of salmon pink potassium feldspar and white plagioclase crystals that have visible cleavage planes. Granite is a good approximation for the continental crust , both in density and composition .	Rhyolite is a fine-crystalline felsicextrusive rock. Rhyolite is commonly pink and will often have glassy quartz phenocrysts . Because felsic lavas are less mobile, it is less common than granite . Examples of rhyolite include several lava flows in Yellowstone National Park and the altered rhyolite that makes up the Grand Canyon of the Yellowstone.
IntermediateComposition
Diorite	Andesite
Diorite is a coarse-crystalline intermediate intrusive igneous rock . Diorite is identifiable by it’s Dalmatian-like appearance of black hornblende and biotite and white plagioclase feldspar . It is found in its namesake, the Andes Mountains as well as the Henry and Abajo mountains of Utah.	Andesite is a fine crystalline intermediate extrusive rock. It is commonly grey and porphyritic . It can be found in the Andes Mountains and in some island arcs (see Chapter 2 ). It is the fine grained compositional equivalent of diorite .
MaficComposition
Gabbro	Vesicular Basalt
Gabbro is a coarse-grained mafic igneous rock , made with mainly mafic minerals like pyroxene and only minor plagioclase . Because mafic lava is more mobile, it is less common than basalt . Gabbro is a major component of the lower oceanic crust .	Basalt is a fine-grained mafic igneous rock . It is commonly vesicular and aphanitic . When porphyritic , it often has either olivine or plagioclase phenocrysts . Basalt is the main rock which is formed at mid-ocean ridges , and is therefore the most common rock on the Earth’s surface, making up the entirety of the ocean floor (except where covered by sediment ).

4.1.3 Igneous Rock Bodies

Igneous rocks are common in the geologic record, but surprisingly, it is the intrusive rocks that are more common. Extrusive rocks, because of their small crystals and glass, are less durable. Plus, they are, by definition, exposed to the elements of erosion immediately. Intrusive rocks, forming underground with larger, stronger crystals, are more likely to last. Therefore, most landforms and rock groups that owe their origin to igneous rocks are intrusive bodies. A significant exception to this is active volcanoes, which are discussed in a later section on volcanism. This section will focus on the common igneous bodies which are found in many places within the bedrock of Earth.

When magma intrudes into a weakness like a crack or fissure and solidifies, the resulting cross-cutting feature is called a dike (sometimes spelled dyke). Because of this, dikes are often vertical or at an angle relative to the pre-existing rock layers that they intersect. Dikes are therefore discordant intrusions, not following any layering that was present. Dikes are important to geologists, not only for the study of igneous rocks themselves but also for dating rock sequences and interpreting the geologic history of an area. The dike is younger than the rocks it cuts across and, as discussed in the chapter on Geologic Time (Chapter 7), may be used to assign actual numeric ages to sedimentary sequences, which are notoriously difficult to age date.

Sills are another type of intrusive structure. A sill is a concordant intrusion that runs parallel to the sedimentary layers in the country rock. They are formed when magma exploits a weakness between these layers, shouldering them apart and squeezing between them. As with dikes, sills are younger than the surrounding layers and may be radioactively dated to study the age of sedimentary strata.

A magma chamber is a large underground reservoir of molten rock. The path of rising magma is called a diapir. The processes by which a diapir intrudes into the surrounding native or country rock are not well understood and are the subject of ongoing geological inquiry. For example, it is not known what happens to the pre-existing country rock as the diapir intrudes. One theory is the overriding rock gets shouldered aside, displaced by the increased volume of magma. Another is the native rock is melted and consumed into the rising magma or broken into pieces that settle into the magma, a process known as stoping. It has also been proposed that diapirs are not a real phenomenon, but just a series of dikes that blend into each other. The dikes may be intruding over millions of years, but since they may be made of similar material, they would be appearing to be formed at the same time. Regardless, when a diapir cools, it forms an mass of intrusive rock called a pluton. Plutons can have irregular shapes, but can often be somewhat round.

When many plutons merge together in an extensive single feature, it is called a batholith. Batholiths are found in the cores of many mountain ranges, including the granite formations of Yosemite National Park in the Sierra Nevada of California. They are typically more than 100 km2 in area, associated with subduction zones, and mostly felsic in composition. A stock is a type of pluton with less surface exposure than a batholith, and may represent a narrower neck of material emerging from the top of a batholith. Batholiths and stocks are discordant intrusions that cut across and through surrounding country rock.

Laccoliths are blister-like, concordant intrusions of magma that form between sedimentary layers. The Henry Mountains of Utah are a famous topographic landform formed by this process. Laccoliths bulge upwards; a similar downward-bulging intrusion is called a lopolith.

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Click on the plus signs the illustration for descriptions of several igneous features.

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4.2 Bowen’s Reaction Series

Bowen’s Reaction Series describes the temperature at which minerals crystallize when cooled, or melt when heated. The low end of the temperature scale where all minerals crystallize into solid rock, is approximately 700°C (1292°F). The upper end of the range where all minerals exist in a molten state, is approximately 1,250°C (2,282°F). These numbers reference minerals that crystallize at standard sea-level pressure, 1 bar. The values will be different for minerals located deep below the Earth’s surface due to the increased pressure, which affects crystallization and melting temperatures (see Chapter 4.4). However, the order and relationships are maintained.

In the figure, the righthand column lists the four groups of igneous rock from top to bottom: ultramafic, mafic, intermediate, and felsic. The down-pointing arrow on the far right shows increasing amounts of silica, sodium, aluminum, and potassium as the mineral composition goes from ultramafic to felsic. The up-pointing arrow shows increasing ferromagnesian components, specifically iron, magnesium, and calcium. To the far left of the diagram is a temperature scale. Minerals near the top of diagram, such as olivine and anorthite (a type of plagioclase), crystallize at higher temperatures. Minerals near the bottom, such as quartz and muscovite, crystalize at lower temperatures.

The most important aspect of Bowen's Reaction Series is to notice the relationships between minerals and temperature. Norman L. Bowen (1887-1956) was an early 20th Century geologist who studied igneous rocks. He noticed that in igneous rocks, certain minerals always occur together and these mineral assemblages exclude other minerals. Curious as to why, and with the hypothesis in mind that it had to do with the temperature at which the rocks cooled, he set about conducting experiments on igneous rocks in the early 1900s. He conducted experiments on igneous rock—grinding combinations of rocks into powder, sealing the powders into metal capsules, heating them to various temperatures, and then cooling them.

When he opened the quenched capsules, he found a glass surrounding mineral crystals that he could identify under his petrographic microscope. The results of many of these experiments, conducted at different temperatures over a period of several years, showed that the common igneous minerals crystallize from magma at different temperatures. He also saw that minerals occur together in rocks with others that crystallize within similar temperature ranges, and never crystallize with other minerals. This relationship can explain the main difference between mafic and felsic igneous rocks. Mafic igneous rocks contain more mafic minerals, and therefore, crystallize at higher temperatures than felsic igneous rocks. This is even seen in lava flows, with felsic lavas erupting hundreds of degrees cooler than their mafic counterparts. Bowen’s work laid the foundation for understanding igneous petrology (the study of rocks) and resulted in his book, The Evolution of the Igneous Rocks in 1928.

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4.3 Magma Generation

Magma and lava contain three components: melt, solids, and volatiles. The melt is made of ions from minerals that have liquefied. The solids are made of crystallized minerals floating in the liquid melt. These may be minerals that have already cooled Volatiles are gaseous components—such as water vapor, carbon dioxide, sulfur, and chlorine—dissolved in the magma. The presence and amount of these three components affect the physical behavior of the magma and will be discussed more below.

4.3.1 Geothermal Gradient

Below the surface, the temperature of the Earth rises. This heat is caused by residual heat left from the formation of Earth and ongoing radioactive decay. The rate at which temperature increases with depth is called the geothermal gradient. The average geothermal gradient in the upper 100 km (62 mi) of the crust is about 25°C per kilometer of depth. So for every kilometer of depth, the temperature increases by about 25°C.

The depth-temperature graph (see figure) illustrates the relationship between the geothermal gradient (geotherm, red line) and the start of rock melting (solidus, green line). The geothermal gradient changes with depth (which has a direct relationship to pressure) through the crust into upper mantle. The area to the left of the green line includes solid components; to the right is where liquid components start to form. The increasing temperature with depth makes the depth of about 125 kilometers (78 miles) where the natural geothermal gradient is closest to the solidus.

The temperature at 100 km (62 mi) deep is about 1,200°C (2,192°F). At bottom of the crust, 35 km (22 mi) deep, the pressure is about 10,000 bars. A bar is a measure of pressure, with 1 bar being normal atmospheric pressure at sea level. At these pressures and temperatures, the crust and mantle are solid. To a depth of 150 km (93 mi), the geothermal gradient line stays to the left of the solidus line. This relationship continues through the mantle to the core–mantle boundary, at 2,880 km (1,790 mi).

The solidus line slopes to the right because the melting temperature of any substance depends on pressure. The higher pressure created at greater depth increases the temperature needed to melt rock. In another example, at sea level with an atmospheric pressure close to 1 bar, water boils at 100°C. But if the pressure is lowered, as shown on the video below, water boils at a much lower temperature.

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There are three principal ways rock behavior crosses to the right of the green solidus line to create molten magma: 1) decompression melting caused by lowering the pressure, 2) flux melting caused by adding volatiles (see more below), and 3) heat-induced melting caused by increasing the temperature. The Bowen’s Reaction Series shows that minerals melt at different temperatures. Since magma is a mixture of different minerals, the solidus boundary is more of a fuzzy zone rather than a well-defined line; some minerals are melted and some remain solid. This type of rock behavior is called partial melting and represents real-world magmas, which typically contain solid, liquid, and volatile components.

The figure below uses P-T diagrams to show how melting can occur at three different plate tectonic settings. The green line is called the solidus, the melting point temperature of the rock at that pressure. Setting A is a situation (called “normal”) in the middle of a stable plate in which no magma is generated. In the other three situations, rock at a lettered location with a temperature at the geothermal gradient is moved to a new P-T situation on the diagram. This shift is indicated by the arrow and its temperature relative to the solidus is shown by the red line. Partial melting occurs where the red line temperature of the rock crosses the green solidus on the diagram. Setting B is at a mid-ocean ridge (decompression melting) where reduction of pressure carries the rock at its temperature across the solidus. Setting C is a hotspot where decompression melting plus addition of heat carries the rock across the solidus, and setting D is a subduction zone where a process called flux melting takes place where the solidus (melting point) is actually shifted to below the temperature of the rock.

Graphs A-D below, along with the side view of the Earth’s layers in various tectonic settings (see figure), show how melting occurs in different situations. Graph A illustrates a normal situation, located in the middle of a stable plate, where no melted rock can be found. The remaining three graphs illustrate rock behavior relative to shifts in the geothermal gradient or solidus lines. Partial melting occurs when the geothermal gradient line crosses the solidus line. Graph B illustrates behavior of rock located at a mid-ocean ridge, labeled X in the graph and side view. Reduced pressure shifts the geotherm to the right of the solidus, causing decompression melting. Graph C and label Y illustrate a hotspot situation. Decompression melting, plus an addition of heat, shifts the geotherm across the solidus. Graph D and label Z show a subduction zone, where an addition of volatiles lowers the melting point, shifting the solidus to the left of the geothermal gradient. B, C, and D all show different ways the Earth produces intersections of the geothermal gradient and the solidus, which results in melting each time.

4.3.2 Decompression Melting

Magma is created at mid-ocean ridges via decompression melting. Strong convection currents cause the solid asthenosphere to slowly flow beneath the lithosphere. The upper part of the lithosphere (crust) is a poor heat conductor, so the temperature remains about the same throughout the underlying mantle material. Where the convection currents cause mantle material to rise, the pressure decreases, which causes the melting point to drop. In this situation, the rock at the temperature of the geothermal gradient is rising toward the surface, thus hotter rock is now shallower, at a lower pressure, and the rock, still at the temperature of the geothermal gradient at its old location, shifts past the its melting point (shown as the red line crossing over the solidus or green line in example B in previous figure) and partial melting starts. As this magma continues to rise, it cools and crystallizes to form new lithospheric crust.

4.3.3 Flux Melting

Flux melting or fluid-induced melting occurs in island arcs and subduction zones when volatile gases are added to mantle material (see figure: graph D, label Z). Flux-melted magma produces many of the volcanoes in the circum-Pacific subduction zones, also known as the Ring of Fire. The subducting slab contains oceanic lithosphere and hydrated minerals. As covered in Chapter 2, these hydrated forms are created when water ions bond with the crystal structure of silicate minerals. As the slab descends into the hot mantle, the increased temperature causes the hydrated minerals to emit water vapor and other volatile gases, which are expelled from the slab like water being squeezed out of a sponge. The volatiles dissolve into the overlying asthenospheric mantle and decrease its melting point. In this situation the applied pressure and temperature have not changed, the mantle‘s melting point has been lowered by the addition of volatile substances. The previous figure (graph D) shows the green solidus line shifting to the left of and below the red geothermal gradient line, and melting begins. This is analogous to adding salt to an icy roadway. The salt lowers the freezing temperature of the solid ice so it turns into liquid water.

4.3.4 Heat-Induced Melting

Heat-induced melting, transforming solid mantle into liquid magma by simply applying heat, is the least common process for generating magma (see figure: graph C, label Y). Heat-induced melting occurs at a mantle plumes or hotspots. The rock surrounding the plume is exposed to higher temperatures, the geothermal gradient crosses to the right of the green solidus line, and the rock begins to melt. The mantle plume includes rising mantle material, meaning some decompression melting is occurring as well. A small amount of magma is also generated by intense regional metamorphism (see Chapter 6). This magma becomes a hybrid metamorphic–igneous rock called migmatite.

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4.4 Partial Melting and Crystallization

Even though all magmas originate from similar mantle rocks, and start out as similar magma, other things, like partial melting and crystallization processes like magmatic differentiation, can change the chemistry of the magma. This explains the wide variety of resulting igneous rocks that are found all over Earth.

4.4.1 Partial Melting

Because the mantle is composed of many different minerals, it does not melt uniformly. As minerals with lower melting points turn into liquid magma, those with higher melting points remain as solid crystals. This is known as partial melting. As magma slowly rises and cools into solid rock, it undergoes physical and chemical changes in a process called magmatic differentiation.

According to Bowen’s Reaction Series (Section 4.2), each mineral has a unique melting and crystallization temperature. Since most rocks are made of many different minerals, when they start to melt, some minerals begin melting sooner than others. This is known as partial melting, and creates magma with a different composition than the original mantle material.

The most important example occurs as magma is generated from mantle rocks (as discussed in Section 4.3). The chemistry of mantle rock (peridotite) is ultramafic, low in silicates and high in iron and magnesium. When peridotite begins to melt, the silica-rich portions melt first due to their lower melting point. If this continues, the magma becomes increasingly silica-rich, turning ultramafic mantle into mafic magma, and mafic mantle into intermediate magma. The magma rises to the surface because it is more buoyant than the mantle.

Partial melting also occurs as existing crustal rocks melt in the presence of heat from magmas. In this process, existing rocks melt, allowing the magma formed to be more felsic and less mafic than the pre-existing rock. Early in the Earth’s history when the continents were forming, silica-rich magmas formed and rose to the surface and solidified into granitic continents. In the figure, the old granitic cores of the continents, called shields, are shown in orange.

4.4.2 Crystallization and Magmatic Differentiation

Liquid magma is less dense than the surrounding solid rock, so it rises through the mantle and crust. As magma begins to cool and crystallize, a process known as magmatic differentiation changes the chemistry of the resultant rock towards a more felsic composition. This happens via two main methods: assimilation and fractionation.

During assimilation, pieces of country rock with a different, often more felsic, composition are added to the magma. These solid pieces may melt, which changes the composition of the original magma. At times, the solid fragments may remain intact within the cooling magma and only partially melt. The unmelted country rocks within an igneous rock mass are called xenoliths.

Xenoliths are also common in the processes of magma mixing and rejuvenation, two other processes that can contribute to magmatic differentiation. Magma mixing occurs when two different magmas come into contact and mix, though at times, the magmas can remain heterogeneous and create xenoliths, dikes, and other features. Magmatic rejuvenation happens when a cooled and crystallized body of rock is remelted and pieces of the original rock may remain as xenoliths.

Much of the continental lithosphere is felsic (i.e. granitic), and normally more buoyant than the underlying mafic/ultramafic mantle. When mafic magma rises through thick continental crust, it does so slowly, more slowly than when magma rises through oceanic plates. This gives the magma lots of time to react with the surrounding country rock. The mafic magma tends to assimilate felsic rock, becoming more silica-rich as it migrates through the lithosphere and changing into intermediate or felsic magma by the time it reaches the surface. This is why felsic magmas are much more common within continents.

Fractionation or fractional crystallization is another process that increase magma silica content, making it more felsic. As the temperature drops within a magma diapir rising through the crust, some minerals will crystallize and settle to the bottom of the magma chamber, leaving the remaining melt depleted of those ions. Olivine is a mafic mineral at the top of the Bowen’s Reaction series with a high melting point and a smaller percentage of silica verses other common igneous minerals. When ultramafic magma cools, the olivine crystallizes first and settles to the bottom of the magma chamber (see figure). This means the remaining melt becomes more silica-rich and felsic. As the mafic magma further cools, the next minerals on Bowen's Reaction Series (plagioclase and pyroxene) crystallize next, removing even more low-silica components from the magma, making it even more felsic. This crystal fractionation can occur in oceanic lithosphere, but the formation of more differentiated, highly evolved felsic magmas is largely confined to continental regions where the longer time to the surface allows more fractionation to occur.

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4.5 Volcanism

When magma emerges onto the Earth’s surface, the molten rock is called lava. A volcano is a type of land formation created when lava solidifies into rock. Volcanoes have been an important part of human society for centuries, though their understanding has greatly increased as our understanding of plate tectonics has made them less mysterious. This section describes volcano location, type, hazards, and monitoring.

4.5.1. Distribution and Tectonics

Most volcanoes are interplate volcanoes. Interplate volcanoes are located at active plate boundaries created by volcanism at mid-ocean ridges, subduction zones, and continental rifts. The prefix “inter-“ means between. Some volcanoes are intraplate volcanoes. The prefix “intra-“ means within, and intraplate volcanoes are located within tectonic plates, far removed from plate boundaries. Many intraplate volcanoes are formed by hotspots.

Volcanoes at Mid-Ocean Ridges

Most volcanism on Earth occurs on the ocean floor along mid-ocean ridges, a type of divergent plate boundary (see Chapter 2). These interplate volcanoes are also the least observed and famous, since most of them are located under 3,000-4,500 m (10,000-15,000 ft) of ocean and the eruptions are slow, gentle, and oozing. One exception is the interplate volcanoes of Iceland. The diverging and thinning oceanic plates allow hot mantle rock to rise, releasing pressure and causing decompression melting. Ultramafic mantle rock, consisting largely of peridotite, partially melts and generates magma that is basaltic. Because of this, almost all volcanoes on the ocean floor are basaltic. In fact, most oceanic lithosphere is basaltic near the surface, with phaneritic gabbro and ultramafic peridotite underneath.

When basaltic lava erupts underwater it emerges in small explosions and/or forms pillow-shaped structures called pillow basalts. These seafloor eruptions enable entire underwater ecosystems to thrive in the deep ocean around mid-ocean ridges. This ecosystem exists around tall vents emitting black, hot mineral-rich water called deep-sea hydrothermal vents, also known as black smokers.

Without sunlight to support photosynthesis, these organisms instead utilize a process called chemosynthesis. Certain bacteria are able to turn hydrogen sulfide (H2S), a gas that smells like rotten eggs, into life-supporting nutrients and water. Larger organisms may eat these bacteria or absorb nutrients and water produced by bacteria living symbiotically inside their bodies. The three videos show some of the ecosystems found around deep-sea hydrothermal vents.

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Volcanoes at Subduction Zones

Distribution of volcanoes on the planet. Click here for an

of volcano distributions.

The second most commonly found location for volcanism is adjacent to subduction zones, a type of convergent plate boundary (see Chapter 2). The process of subduction expels water from hydrated minerals in the descending slab, which causes flux melting in the overlying mantle rock. Because subduction volcanism occurs in a volcanic arc, the thickened crust promotes partial melting and magma differentiation. These evolve the mafic magma from the mantle into more silica-rich magma. The Ring of Fire surrounding the Pacific Ocean, for example, is dominated by subduction-generated eruptions of mostly silica-rich lava; the volcanoes and plutons consist largely of intermediate-to-felsic rock such as andesite, rhyolite, pumice, and tuff.

Volcanoes at Continental Rifts

Some volcanoes are created at continental rifts, where crustal thinning is caused by diverging lithospheric plates, such as the East African Rift Basin in Africa. Volcanism caused by crustal thinning without continental rifting is found in the Basin and Range Province in North America. In this location, volcanic activity is produced by rising magma that stretches the overlying crust (see figure). Lower crust or upper mantle material rises through the thinned crust, releases pressure, and undergoes decompression-induced partial melting. This magma is less dense than the surrounding rock and continues to rise through the crust to the surface, erupting as basaltic lava. These eruptions usually result in flood basalts, cinder cones, and basaltic lava flows (see video). Relatively young cinder cones of basaltic lava can be found in south-central Utah, in the Black Rock Desert Volcanic Field, which is part of the zone of Basin and Range crustal extension. These Utah cinder cones and lava flows started erupting around 6 million years ago, with the last eruption occurring 720 years ago.

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Hotspots

Hotspots are the main source of intraplate volcanism. Hotspots occur when lithospheric plates glide over a hot mantle plume, an ascending column of solid heated rock originating from deep within the mantle. The mantle plume generates melts as material rises, with the magma rising even more. When the ascending magma reaches the lithospheric crust, it spreads out into a mushroom-shaped head that is tens to hundreds of kilometers across.

Since most mantle plumes are located beneath the oceanic lithosphere, the early stages of intraplate volcanism typically take place underwater. Over time, basaltic volcanoes may build up from the sea floor into islands, such as the Hawaiian Islands. Where a hotspot is found under a continental plate, contact with the hot mafic magma may cause the overlying felsic rock to melt and mix with the mafic material below, forming intermediate magma. Or the felsic magma may continue to rise, and cool into a granitic batholith or erupt as a felsic volcano. The Yellowstone caldera is an example of hotspot volcanism that resulted in an explosive eruption.

A zone of actively erupting volcanism connected to a chain of extinct volcanoes indicates intraplate volcanism located over a hotspot. These volcanic chains are created by the overriding oceanic plate slowly moving over a hotspot mantle plume. These chains are seen on the seafloor and continents and include volcanoes that have been inactive for millions of years. The Hawaiian Islands on the Pacific Oceanic plate are the active end of a long volcanic chain that extends from the northwest Pacific Ocean to the Emperor Seamounts, all the way to the to the subduction zone beneath the Kamchatka Peninsula. The overriding North American continental plate moved across a mantle plume hotspot for several million years, creating a chain of volcanic calderas that extends from Southwestern Idaho to the presently active Yellowstone caldera in Wyoming.

Two three-minute videos (below)illustrates hotspot volcanoes.

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4.5.2 Volcano Features and Types

There are several different types of volcanoes based on their shape, eruption style, magmatic composition, and other aspects.

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The figure shows the main features of a typical stratovolcano: 1) magma chamber, 2) upper layers of lithosphere, 3) the conduitor narrow pipe through which the lava erupts, 4) the base or edge of the volcano, 5) a sill of magma between layers of the volcano, 6) a diapir or feeder tube to the sill, 7) layers of tephra (ash) from previous eruptions, 8 & 9) layers of lava erupting from the vent and flowing down the sides of the volcano, 10) the crater at the top of the volcano, 11) layers of lava and tephra on (12), a parasitic cone. A parasitic cone is a small volcano located on the flank of a larger volcano such as Shastina on Mount Shasta. Kilauea sitting on the flank of Mauna Loa is not considered a parasitic cone because it has its own separate magma chamber, 13) the vents of the parasite and the main volcano, 14) the rim of the crater, 15) clouds of ash blown into the sky by the eruption; this settles back onto the volcano and surrounding land.

The largest craters are called calderas, such as the Crater Lake Caldera in Oregon. Many volcanic features are produced by viscosity, a basic property of a lava. Viscosity is the resistance to flowing by a fluid. Low viscosity magma flows easily more like syrup, the basaltic volcanism that occurs in Hawaii on shield volcanoes. High viscosity means a thick and sticky magma, typically felsic or intermediate, that flows slowly, similar to toothpaste.

Shield Volcano

The largest volcanoes are shieldvolcanoes. They are characterized by broad low-angle flanks, small vents at the top, and mafic magma chambers. The name comes from the side view, which resembles a medieval warrior’s shield. They are typically associated with hotspots, mid-ocean ridges, or continental rifts with rising upper mantle material. The low-angle flanks are built up slowly from numerous low-viscosity basaltic lava flows that spread out over long distances. The basaltic lava erupts effusively, meaning the eruptions are small, localized, and predictable.

Typically, shield volcano eruptions are not much of a hazard to human life—although non-explosive eruptions of Kilauea (Hawaii) in 2018 produced uncharacteristically large lavas that damaged roads and structures. Mauna Loa (see USGS page) and Kilauea (see USGS page) in Hawaii are examples of shield volcanoes. Shield volcanoes are also found in Iceland, the Galapagos Islands, Northern California, Oregon, and the East African Rift.

The largest volcanic edifice in the Solar System is Olympus Mons on Mars. This (possibly extinct) shield volcano covers an area the size of the state of Arizona. This may indicate the volcano erupted over a hotspot for millions of years, which means Mars had little, if any, plate tectonic activity.

Basaltic lava forms special landforms based on magma temperature, composition, and content of dissolved gases and water vapor. The two main types of basaltic volcanic rock have Hawaiian names—pahoehoe and aa. Pahoehoe might come from low-viscosity lava that flows easily into ropey strands.

Aa(sometimes spelled a’a or ʻaʻā and pronounced “ah-ah”) is more viscous and has a crumbly blocky appearance. The exact details of what forms the two types of flows are still up for debate. Felsic lavas have lower temperatures and more silica, and thus are higher viscosity. These also form aa-style flows.

Low-viscosity, fast-flowing basaltic lava tends to harden on the outside into a tube and continue to flow internally. Once lava flow subsides, the empty outer shell may be left as a lava tube. Lava tubes, with or without collapsed roofs, make famous caves in Hawaii, Northern California, the Columbia River Basalt Plateau of Washington and Oregon, El Malpais National Monument in New Mexico, and Craters of the Moon National Monument in Idaho.

Fissures are cracks that commonly originate from shield-style eruptions. Lava emerging from fissures is typically mafic and very fluid. The 2018 Kiluaea eruption included fissures associated with the lava flows. Some fissures are caused by the volcanic seismic activity rather than lava flows. Some fissures are influenced by plate tectonics, such as the common fissures located parallel to the divergent boundary in Iceland.

Cooling lava can contract into columns with semi-hexagonal cross sections called columnar jointing. This feature forms the famous Devils Tower in Wyoming, possibly an ancient volcanic vent from which the surrounding layers of lava and ash have been removed by erosion. Another well-known exposed example of columnar jointing is the Giant’s Causeway in Ireland.

Stratovolcano

A stratovolcano, also called a composite cone volcano, has steep flanks, a symmetrical cone shape, distinct crater, and rises prominently above the surrounding landscape. The term composite refers to the alternating layers of pyroclastic fragments like ash and bombs, and solidified lava flows of varying composition. Examples include Mount Rainier in Washington state and Mount Fuji in Japan.

Stratovolcanoes usually have felsic to intermediate magma chambers, but can even produce mafic lavas. Stratovolcanoes have viscous lava flows and domes, punctuated by explosive eruptions. This produces volcanoes with steep flanks.

Lava Domes

Lavadomes are accumulations of silica-rich volcanic rock, such as rhyolite and obsidian. Too viscous to flow easily, the felsic lava tends to pile up near the vent in blocky masses. Lava domes often form in a vent within the collapsed crater of a stratovolcano, and grow by internal expansion. As the dome expands, the outer surface cools, hardens, and shatters, and spills loose fragments down the sides. Mount Saint Helens has a good example of a lava dome inside of a collapsed stratovolcano crater. Examples of stand-alone lava domes are Chaiten in Chile and Mammoth Mountain in California.

Caldera

Calderas are steep-walled, basin-shaped depressions formed by the collapse of a volcanic edifice into an empty magma chamber. Calderas are generally very large, with diameters of up to 25 km (15.5 mi). The term caldera specifically refers to a volcanic vent; however, it is frequently used to describe a volcano type. Caldera volcanoes are typically formed by eruptions of high-viscosity felsic lava having high volatiles content.

Crater Lake, Yellowstone, and the Long Valley Caldera are good examples of this type of volcanism. The caldera at Crater Lake National Park in Oregon was created about 6,800 years ago when Mount Mazama, a composite volcano, erupted in a huge explosive blast. The volcano ejected large amounts of volcanic ash and rapidly drained the magma chamber, causing the top to collapse into a large depression that later filled with water. Wizard Island in the middle of the lake is a later resurgent lava dome that formed within the caldera basin.

The Yellowstone volcanic system erupted three times in the recent geologic past—2.1, 1.3, and 0.64 million years ago—leaving behind three caldera basins. Each eruption created large rhyolite lava flows as well as pyroclastic flows that solidified into tuff formations. These extra-large eruptions rapidly emptied the magma chamber, causing the roof to collapse and form a caldera. The youngest of the three calderas contains most of Yellowstone National Park, as well as two resurgent lava domes. The calderas are difficult to see today due to the amount of time since their eruptions and subsequent erosion and glaciation.

Yellowstone volcanism started about 17-million years ago as a hotspot under the North American lithospheric plate near the Oregon/Nevada border. As the plate moved to the southwest over the stationary hotspot, it left behind a track of past volcanic activities. Idaho’s Snake River Plain was created from volcanism that produced a series of calderas and lava flows. The plate eventually arrived at its current location in northwestern Wyoming, where hotspot volcanism formed the Yellowstone calderas.

The Long Valley Caldera near Mammoth, California, is the result of a large volcanic eruption that occurred 760,000 years ago. The explosive eruption dumped enormous amounts of ash across the United States, in a manner similar to the Yellowstone eruptions. The Bishop Tuff deposit near Bishop, California, is made of ash from this eruption. The current caldera basin is 17 km by 32 km (10 mi by 20 mi), large enough to contain the town of Mammoth Lakes, major ski resort, airport, major highway, resurgent dome, and several hot springs.

Cinder Cone

Cinder cones are small volcanoes with steep sides, and made of pyroclastic fragments that have been ejected from a pronounced central vent. The small fragments are called cinders and the largest are volcanicbombs. The eruptions are usually short-lived events, typically consisting of mafic lavas with a high content of volatiles. Hot lava is ejected into the air, cooling and solidifying into fragments that accumulate on the flank of the volcano. Cinder cones are found throughout western North America.

A recent and striking example of a cinder cone is the eruption near the village of Parícutin, Mexico that started in 1943. The cinder cone started explosively shooting cinders out of the vent in the middle of a farmer’s field. The volcanism quickly built up the cone to a height of over 90 m (300 ft) within a week, and 365 m (1,200 ft) within the first 8 months. After the initial explosive eruption of gases and cinders, basaltic lava poured out from the base of the cone. This is a common order of events for cinder cones: violent eruption, cone and crater formation, low-viscosity lava flow from the base. The cinder cone is not strong enough to support a column of lava rising to the top of the crater, so the lava breaks through and emerges near the bottom of the volcano. During nine years of eruption activity, the ashfall covered about 260 km2 (100 mi2) and destroyed the nearby town of San Juan.

Flood Basalts

A rare volcanic eruption type, unobserved in modern times, is the flood basalt. Flood basalts are some of the largest and lowest viscosity types of eruptions known. They are not known from any eruption in human history, so the exact mechanisms of eruption are still mysterious. Some famous examples include the Columbia River Flood Basalts in Washington, Oregon, and Idaho, the Deccan Traps, which cover about 1/3 of the country of India, and the Siberian Traps, which may have been involved in the Earth’s largest mass extinction (see chapter 8).

Carbonatites

Arguably the most unusual volcanic activity are carbonatite eruptions. Only one actively erupting carbonatite volcano exists on Earth today, Ol Doinyo Lengai, in the East African Rift Zone of Tanzania. While all other volcanism on Earth originates from silicate-based magma, carbonatites are a product of carbonate-based magma and produce volcanic rocks containing greater than 50% carbonate minerals. Carbonatite lavas are very low viscosity and relatively cold for lava. The erupting lava is black, and solidifies to brown/grey rock that eventually turns white. These rocks are occasionally found in the geologic record and require special study to distinguish them from metamorphic marbles (see Chapter 6). They are mostly associated with continental rifting.

Igneous rock types and related volcano types. Mid-ocean ridges and shield volcanoes represent more mafic compositions, and strato (composite) volcanoes generally represent a more intermediate or felsic composition and a convergent plate tectonic boundary. Note that there are exceptions to this generalized layout of volcano types and igneous rock composition.

4.5.3 Volcanic Hazards and Monitoring

While the most obvious volcanic hazard is lava, the dangers posed by volcanoes go far beyond lava flows. For example, on May 18, 1980, Mount Saint Helens (Washington, United States) erupted with an explosion and landslide that removed the upper 400 m (1,300 ft) of the mountain. The initial explosion was immediately followed by a lateral blast, which produced a pyroclastic flow that covered nearly 600 km2 (230 mi2) of forest with hot ash and debris. The pyroclastic flow moved at speeds of 80-130 kph (50-80 mph), flattening trees and ejecting clouds of ash into the air. The USGS video provides an account of this explosive eruption that killed 57 people.

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In 79 AD, Mount Vesuvius, located near Naples, Italy, violently erupted sending a pyroclastic flow over the Roman countryside, including the cities of Herculaneum and Pompeii. The buried towns were discovered in an archeological expedition in the 18th century. Pompeii famously contains the remains (casts) of people suffocated by ash and covered by 10 feet (3 m) of ash, pumice lapilli, and collapsed roofs.

Pyroclastic flows

The most dangerous volcanic hazard are pyroclastic flows (video). These flows are a mix of lava blocks, pumice, ash, and hot gases between 200°C-700°C (400°F-1,300°F). The turbulent cloud of ash and gas races down the steep flanks at high speeds up to 193 kph (120 mph) into the valleys around composite volcanoes. Most explosive, silica-rich, high viscosity magma volcanoes such as composite cones usually have pyroclastic flows. The rock tuffand welded tuffis often formed from these pyroclastic flows.

There are numerous examples of deadly pyroclastic flows. In 2014, the Mount Ontake pyroclastic flow in Japan killed 47 people. The flow was caused by magma heating groundwater into steam, which then rapidly ejected with ash and volcanic bombs. Some were killed by inhalation of toxic gases and hot ash, while others were struck by volcanic bombs. Two short videos below document eye-witness video of pyroclastic flows. In the early 1990s, Mount Unzen erupted several times with pyroclastic flows. The pyroclastic flow shown in this famous short videokilled 41 people. In 1902, on the Caribbean Island Martinique, Mount Pelee erupted with a violent pyroclastic flow that destroyed the entire town of St. Pierre and killing 28,000 people in moments.

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Landslides and Landslide-Generated Tsunamis

The steep and unstable flanks of a volcano can lead to slope failure and dangerous landslides. These landslides can be triggered by magma movement, explosive eruptions, large earthquakes, and/or heavy rainfall. During the 1980 Mount St. Helens eruption, the entire north flank of the volcano collapsed and released a huge landslide that moved at speeds of 160-290 kph (100-180 mph).

If enough landslide material reaches the ocean, it may cause a tsunami. In 1792, a landslide caused by the Mount Unzen eruption reached the Ariaka Sea, generating a tsunami that killed 15,000 people (see USGS page). When Mount Krakatau in Indonesia erupted in 1883, it generated ocean waves that towered 40 m (131 ft) above sea level. The tsunami killed 36,000 people and destroyed 165 villages.

Tephra

Volcanoes, especially composite volcanoes, eject large amounts of tephra(ejected rock materials), most notably ash(tephra fragments less than 0.08 inches [2 mm]). Larger tephra is heavier and falls closer to the vent. Larger blocks and bombs pose hazards to those close to the eruption such as at the 2014 Mount Ontake disaster in Japan discussed earlier.

Hot ash poses an immediate danger to people, animals, plants, machines, roads, and buildings located close to the eruption. Ash is fine grained (< 2mm) and can travel airborne long distances away from the eruption site. Heavy accumulations of ash can cause buildings to collapse. In people, it may cause respiratory issues like silicosis. Ash is destructive to aircraft and automobile engines, which can disrupt transportation and shipping services. In 2010, the Eyjafjallajökull volcano in Iceland emitted a large ash cloud into the upper atmosphere, causing the largest air-travel disruption in northern Europe since World War II. No one was injured, but the service disruption was estimated to have cost the world economy billions of dollars.

Volcanic Gases

As magma rises to the surface the confining pressure decreases, and allows dissolved gases to escape into the atmosphere. Even volcanoes that are not actively erupting may emit hazardous gases, such as carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), and hydrogen halides (HF, HCl, or HBr).

Carbon dioxide tends to sink and accumulate in depressions and basins. In volcanic areas known to emit carbon dioxide, low-lying areas may trap hazardous concentrations of this colorless and odorless gas. The Mammoth Mountain Ski Resort in California, is located within the Long Valley Caldera, is one such area of carbon dioxide-producing volcanism. In 2006, three ski patrol members died of suffocation caused by carbon dioxide after falling into a snow depression near a fumarole (info).

In rare cases, volcanism may create a sudden emission of gases without warning. Limnic eruptions (limne is Greek for lake), occur in crater lakes associated with active volcanism. The water in these lakes is supercharged with high concentrations of dissolved gases. If the water is physically jolted by a landslide or earthquake, it may trigger an immediate and massive release of gases out of solution. An analogous example would be what happens to vigorously shaken bottle of carbonated soda when the cap is opened. An infamous limnic eruption occurred in 1986 at Lake Nyos, Cameroon. Almost 2,000 people were killed by a massive release of carbon dioxide.

Lahars

Lahar is an Indonesian word and is used to describe a volcanic mudflow that forms from rapidly melting snow or glaciers. Lahars are slurries resembling wet concrete, and consist of water, ash, rock fragments, and other debris. These mudflows flow down the flanks of volcanoes or mountains covered with freshly-erupted ash and on steep slopes can reach speeds of up to 80 kph (50 mph).

Several major cities, including Tacoma, are located on prehistoric lahar flows that extend for many kilometers across the flood plains surrounding Mount Rainier in Washington (see map). A map of Mount Baker in Oregon shows a similar potential hazard for lahar flows (see map). A tragic scenario played out recently, in 1985, when a lahar from the Nevado del Ruiz volcano in Colombia buried the town of Armero and killed an estimated 23,000 people.

Monitoring

Geologists use various instruments to detect changes or indications that an eruption is imminent. The three videos show different types of volcanic monitoring used to predict eruptions 1) earthquake activity; 2) increases in gas emission; and 3) changes in land surface orientation and elevation.

One video shows how monitoring earthquake frequency, especially special vibrational earthquakes called harmonic tremors, can detect magma movement and possible eruption. Another video shows how gas monitoring may be used to predict an eruption. A rapid increase of gas emission may indicate magma that is actively rising to surface and releasing dissolved gases out of solution, and that an eruption is imminent. The last video shows how a GPS unit and tiltmeter can detect land surface changes, indicating the magma is moving underneath it.

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Take this quiz to check your comprehension of this section.

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Summary

Igneous rock is divided into two major groups: intrusive rock that solidifies from underground magma, and extrusive rock formed from lava that erupts and cools on the surface. Magma is generated from mantle material at several plate tectonics situations by three types of melting: decompression melting, flux melting, or heat-induced melting. Magma composition is determined by differences in the melting temperatures of the mineral components (Bowen’s Reaction Series). The processes affecting magma composition include partial melting, magmatic differentiation, assimilation, and collision. Volcanoes come in a wide variety of shapes and sizes, and are classified by a multiple factors, including magma composition, and plate tectonic activity. Because volcanism presents serious hazards to human civilization, geologists carefully monitor volcanic activity to mitigate or avoid the dangers it presents.

Take this quiz to check your comprehension of this Chapter.

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5 Weathering, Erosion, and Sedimentary Rocks

5 Weathering, Erosion, and Sedimentary Rocks

KEY CONCEPTS

By the end of this chapter, students will be able to:

Describe how water is an integral part of all sedimentary rock formation

Explain how chemical and mechanical weathering turn bedrock into sediment

Differentiate the two main categories of sedimentary rocks : clastic rock formed from pieces of weathered bedrock; and chemical rock that precipitates out of solution by organic or inorganic means

Explain the importance of sedimentary structures and analysis of depositional environments, and how they provide insight into the Earth’s history

Sedimentary rock and the processes that create it, which include weathering, erosion, and lithification, are an integral part of understanding Earth Science. This is because the majority of the Earth’s surface is made up of sedimentary rocks and their common predecessor, sediments. Even though sedimentary rocks can form in drastically different ways, their origin and creation have one thing in common, water.

5.1 The Unique Properties of Water

Water plays a role in the formation of most sedimentary rock. It is one of the main agents involved in creating the minerals in chemical sedimentary rock. It also is a weathering and erosion agent, producing the grains that become detrital sedimentary rock. Several special properties make water an especially unique substance, and integral to the production of sediments and sedimentary rock.

The water molecule consists of two hydrogen atoms covalently bonded to one oxygen atom arranged in a specific and important geometry. The two hydrogen atoms are separated by an angle of about 105 degrees, and both are located to one side of the oxygen atom. This atomic arrangement, with the positively charged hydrogens on one side and negatively charged oxygen on the other side, gives the water molecule a property called polarity. Resembling a battery or a magnet, the molecule’s positive-negative architecture leads to a whole suite of unique properties.

Polarity allows water molecules to stick to other substances. This is called adhesion. Water is also attracted to itself, a property called cohesion, which leads to water’s most common form in the air, a droplet. Cohesion is responsible for creating surface tension, which various insects use to walk on water by distributing their weight across the surface.

The fact that water is attracted to itself leads to another important property, one that is extremely rare in the natural world—the liquid form is denser than the solid form. The polarity of water creates a special type of weak bonding called hydrogen bonds. Hydrogen bonds allow the molecules in liquid water to sit close together. Water is densest at 4°C and is less dense above and below that temperature. As water solidifies into ice, the molecules must move apart in order to fit into the crystal lattice, causing water to expand and become less dense as it freezes. Because of this, ice floats and water at 4oC sinks, which keeps the oceans liquid and prevents them from freezing solid from the bottom up. This unique property of water keeps Earth, the water planet, habitable.

Even more critical for supporting life, water remains liquid over a very large range of temperatures, which is also a result of cohesion. Hydrogen bonding allows liquid water can absorb high amounts of energy before turning into vapor or gas. The wide range across which water remains a liquid, 0°C-100°C (32°F-212°F), is rarely exhibited in other substances. Without this high boiling point, liquid water as we know it would be constricted to narrow temperature zones on Earth, instead water is found from pole to pole. Further, water is the only substance that exists in all three phases, solid, liquid, and gas in Earth’s surface environments.

Water is a universal solvent, meaning it dissolves more substances than any other commonly found, naturally occurring liquid. The water molecules use polarity and hydrogen bonds to pry ions away from the crystal lattice. Water is such a powerful solvent, it can dissolve even the strongest rocks and minerals given enough time.

Take this quiz to check your comprehension of this section.

An interactive H5P element has been excluded from this version of the text. You can view it online here:
https://slcc.pressbooks.pub/introgeology/?p=3139#h5p-32

5.2 Weathering and Erosion

Bedrock refers to the solid rock that makes up the Earth’s outer crust. Weathering is a process that turns bedrock into smaller particles, called sediment. Mechanical weathering includes pressure expansion, frost wedging, root wedging, and salt expansion. Chemical weathering includes carbonic acid and hydrolysis, dissolution, and oxidation.

Erosion is a mechanical process, usually driven by water, wind, gravity, or ice, which transports sediment (and soil) from the place of weathering. Liquid water is the main agent of erosion. Gravity and mass wasting processes (see Chapter 10, Mass Wasting) move rocks and sediment to new locations. Gravity and ice, in the form of glaciers (see Chapter 14, Glaciers), move large rock fragments as well as fine sediment.

Erosion resistance is important in the creation of distinctive geological features. This is well-demonstrated in the cliffs of the Grand Canyon. The cliffs are made of rock left standing after less resistant materials have weathered and eroded away. Rocks with different levels of erosion resistance also create the unique-looking features called hoodoos in Bryce Canyon National Park and Goblin Valley State Park in Utah.

5.2.1 Mechanical Weathering

Mechanical weathering physically breaks bedrock into smaller pieces. The usual agents of mechanical weathering are pressure, temperature, freezing/thawing cycle of water, plant or animal activity, and salt evaporation.

Pressure Expansion

Bedrock buried deep within the Earth is under high pressure and temperature. When uplift and erosion brings bedrock to the surface, its temperature drops slowly, while its pressure drops immediately. The sudden pressure drop causes the rock to rapidly expand and crack; this is called pressure expansion. Sheeting or exfoliation is when the rock surface spalls off in layers. Spheroidal weathering is a type of exfoliation that produces rounded features and is caused when chemical weathering moves along joints in the bedrock.

Frost Wedging

Frost wedging, also called ice wedging, uses the power of expanding ice to break apart rocks. Water works its way into various cracks, voids, and crevices. As the water freezes, it expands with great force, exploiting any weaknesses. When ice melts, the liquid water moves further into the widened spaces. Repeated cycles of freezing and melting eventually pry the rocks apart. The cycles can occur daily when fluctuations of temperature between day and night go from freezing to melting.

Root Wedging

Like frost wedging, root wedging happens when plant roots work themselves into cracks, prying the bedrock apart as they grow. Occasionally these roots may become fossilized. Rhizolith is the term for these roots preserved in the rock record. Tunneling organisms such as earthworms, termites, and ants are biological agents that induce weathering similar to root wedging.

Salt Expansion

Salt expansion, which works similarly to frost wedging, occurs in areas of high evaporation or near-marine environments. Evaporation causes salts to precipitate out of solution and grow and expand into cracks in rock. Salt expansion is one of the causes of tafoni, a series of holes in a rock. Tafonis, cracks, and holes are weak points that become susceptible to increased weathering. Another phenomena that occurs when salt water evaporates can leave behind a square imprint preserved in a soft sediment, called a hopper crystal.

5.2.2 Chemical Weathering

Chemical weathering is the dominate weathering process in warm, humid environments. It happens when water, oxygen, and other reactants chemically degrade the mineral components of bedrock and turn them into water-soluble ions which can then be transported by water. Higher temperatures accelerate chemical weathering rates.

Chemical and mechanical weathering work hand-in-hand via a fundamental concept called surface-area-to-volume ratio. Chemical weathering only occurs on rock surfaces because water and reactants cannot penetrate solid rock. Mechanical weathering penetrates bedrock, breaking large rocks into smaller pieces and creating new rock surfaces. This exposes more surface area to chemical weathering, enhancing its effects. In other words, higher surface-area-to-volume ratios produce higher rates of overall weathering.

Carbonic Acid and Hydrolysis

Carbonic acid (H2CO3) forms when carbon dioxide, the fifth-most abundant gas in the atmosphere, dissolves in water. This happens naturally in clouds, which is why precipitation is normally slightly acidic. Carbonic acid is an important agent in two chemical weathering reactions, hydrolysis and dissolution.

Hydrolysis occurs via two types of reactions. In one reaction, water molecules ionize into positively charged H+1 and OH−1 ions and replace mineral cations in the crystal lattice. In another type of hydrolysis, carbonic acid molecules react directly with minerals, especially those containing silicon and aluminum (i.e. Feldspars), to form molecules of clay minerals.

Hydrolysis is the main process that breaks down silicate rock and creates clay minerals. The following is a hydrolysis reaction that occurs when silica-rich feldspar encounters carbonic acid to produce water-soluble clay and other ions:

feldspar + carbonic acid (in water) → clay + metal cations (Fe++, Mg++, Ca++, Na+, etc.) + bicarbonate anions (HCO3-1) + silica (SiO2)

Clay minerals are platy silicates or phyllosilicates (see Chapter 3, Minerals) similar to micas, and are the main components of very fine-grained sediment. The dissolved substances may later precipitate into chemical sedimentary rocks like evaporite and limestone, as well as amorphous silica or chert nodules.

Dissolution

Dissolution is a hydrolysis reaction that dissolves minerals in bedrock and leaves the ions in solution, usually in water. Some evaporites and carbonates, like salt and calcite, are more prone to this reaction; however, all minerals can be dissolved. Non-acidic water, having a neutral pH of 7, will dissolve any mineral, although it may happen very slowly. Water with higher levels of acid, naturally or man-made, dissolves rocks at a higher rate. Liquid water is normally slightly acidic due to the presence of carbonic acid and free H+ ions. Natural rainwater can be highly acidic, with pH levels as low as 2. Dissolution can be enhanced by a biological agent, such as when organisms like lichen and bacteria release organic acids onto the rocks they are attached to. Regions with high humidity (airborne moisture) and precipitation experience more dissolution due to greater contact time between rocks and water.

The Goldich Dissolution Series shows chemical weathering rates are associated to crystallization rankings in the Bowen’s Reaction Series (see Chapter 4, Igneous Rock and Volcanic Processes). Minerals at the top of the Bowen series crystallize under high temperatures and pressures, and chemically weather at a faster rate than minerals ranked at the bottom. Quartz, a felsic mineral that crystallizes at 700°C, is very resistant to chemical weathering. High crystallization-point mafic minerals, such as olivine and pyroxene (1,250°C), weather relatively rapidly and more completely. Olivine and pyroxene are rarely found as end products of weathering because they tend to break down into elemental ions.

Dissolution is also noteworthy for the special geological features it creates. In places with abundant carbonate bedrock, dissolution weathering can produce a karst topography characterized by sinkholes or caves (see Chapter 10, Mass Wasting).

Timpanogos Cave National Monument in Northern Utah is a well-known dissolution feature. The figure shows a cave formation created from dissolution followed by precipitation—groundwater saturated with calcite seeped into the cavern, where evaporation caused the dissolved minerals to precipitate out.

Oxidation

Oxidation, the chemical reaction that causes rust in metallic iron, occurs geologically when iron atoms in a mineral bond with oxygen. Any minerals containing iron can be oxidized. The resultant iron oxides may permeate a rock if it is rich in iron minerals. Oxides may also form a coating that covers rocks and grains of sediment, or lines rock cavities and fractures. If the oxides are more susceptible to weathering than the original bedrock, they may create void spaces inside the rock mass or hollows on exposed surfaces.

Three commonly found minerals are produced by iron-oxidation reactions: red or grey hematite, brown goethite (pronounced “GUR-tite”), and yellow limonite. These iron oxides coat and bind mineral grains together into sedimentary rocks in a process called cementation, and often give these rocks a dominant color. They color the rock layers of the Colorado Plateau, as well as Zion, Arches, and Grand Canyon National Parks. These oxides can permeate a rock that is rich in iron-bearing minerals or can be a coating that forms in cavities or fractures. When the minerals replacing existing minerals in bedrock are resistant to weathering, iron concretions may occur in the rock. When bedrock is replaced by weaker oxides, this process commonly results in void spaces and weakness throughout the rock mass and often leaves hollows on exposed rock surfaces.

5.2.3 Erosion

Erosion is a mechanical process, usually driven by water, gravity, (see Chapter 10), wind, or ice (see Chapter 14) that removes sediment from the place of weathering. Liquid water is the main agent of erosion.

Erosionresistance is important in the creation of distinctive geological features. This is well demonstrated in the cliffs of the Grand Canyon. The cliffs are made of rock left standing after less resistant materials have weathered and eroded away. Rocks with different levels erosion resistant also create the unique-looking features called hoodoos in Bryce Canyon National Park and Goblin Valley State Park in Utah.

5.2.4. Soil

Soil is a combination of air, water, minerals, and organic matter that forms at the transition between biosphere and geosphere. Soil is made when weathering breaks down bedrock and turns it into sediment. If erosion does not remove the sediment significantly, organisms can access the mineral content of the sediments. These organisms turn minerals, water, and atmospheric gases into organic substances that contribute to the soil.

Soil is an important reservoir for organic components necessary for plants, animals, and microorganisms to live. The organic component of soil, called humus, is a rich source of bioavailable nitrogen. Nitrogen is the most common element in the atmosphere, but it exists in a form most life forms are unable to use. Special bacteria found only in soil provide most nitrogen compounds that are usable, bioavailable, by life forms.

These nitrogen-fixing bacteria absorb nitrogen from the atmosphere and convert it into nitrogen compounds. These compounds are absorbed by plants and used to make DNA, amino acids, and enzymes. Animals obtain bioavailable nitrogen by eating plants, and this is the source of most of the nitrogen used by life. That nitrogen is an essential component of proteins and DNA. Soils range from poor to rich, depending on the amount of humus they contain. Soil productivity is determined by water and nutrient content. Freshly created volcanic soils, called andisols, and clay-rich soils that hold nutrients and water are examples of productive soils.

The nature of the soil, meaning its characteristics, is determined primarily by five components: 1) the mineralogy of the parent material; 2) topography, 3) weathering, 4) climate, and 5) the organisms that inhabit the soil. For example, soil tends to erode more rapidly on steep slopes so soil layers in these areas may be thinner than in flood plains, where it tends to accumulate. The quantity and chemistry of organic matter of soil affects how much and what varieties of life it can sustain. Temperature and precipitation, two major weathering agents, are dependent on climate. Fungi and bacteria contribute organic matter and the ability of soil to sustain life, interacting with plant roots to exchange nitrogen and other nutrients.

In well-formed soils, there is a discernable arrangement of distinct layers called soil horizons. These soil horizons can be seen in road cuts that expose the layers at the edge of the cut. Soil horizons make up the soil profile. Each soil horizon reflects climate, topography, and other soil-development factors, as well as its organic material and mineral sediment composition. The horizons are assigned names and letters. Differences in naming schemes depend on the area, soil type or research topic. The figure shows a simplified soil profile that uses commonly designated names and letters.

O Horizon: The top horizon is a thin layer of predominantly organic material, such as leaves, twigs, and other plant parts that are actively decaying into humus.

A Horizon: The next layer, called topsoil, consists of humus mixed with mineral sediment. As precipitation soaks down through this layer, it leaches out soluble chemicals. In wet climates with heavy precipitation this leaching out produces a separate layer called horizon E, the leaching or eluviation zone.

B Horizon: Also called subsoil, this layer consists of sediment mixed with humus removed from the upper layers. The subsoil is where mineral sediment is chemically weathered. The amount of organic material and degree of weathering decrease with depth. The upper subsoil zone, called regolith, is a porous mixture of humus and highly weathered sediment. In the lower zone, saprolite, scant organic material is mixed with largely unaltered parent rock.

C Horizon: This is substratum and is a zone of mechanical weathering. Here, bedrock fragments are physically broken but not chemically altered. This layer contains no organic material.

R Horizon: The final layer consists of unweathered, parent bedrock and fragments.

The United States governing body for agriculture, the USDA, uses a taxonomic classification to identify soil types, called soil orders. Xoxisols or laterite soils are nutrient-poor soils found in tropical regions. While poorly suited for growing crops, xosisols are home to most of the world’s mineable aluminum ore (bauxite). Ardisol forms in dry climates and can develop layers of hardened calcite, called caliche. Andisols originate from volcanic ash deposits. Alfisols contain silicate clay minerals. These two soil orders are productive for farming due to their high content of mineral nutrients. In general, color can be an important factor in understanding soil conditions. Black soils tend to be anoxic, red oxygen-rich, and green oxygen-poor (i.e. reduced). This is true for many sedimentary rocks as well.

Not only is soil essential to terrestrial life in nature, but also human civilization via agriculture. Careless or uninformed human activity can seriously damage soil’s life-supporting properties. A prime example is the famous Dust Bowl disaster of the 1930s, which affected the midwestern United States. The damage occurred because of large-scale attempts develop prairieland in southern Kansas, Colorado, western Texas, and Oklahoma into farmland. Poor understanding of the region’s geology, ecology, and climate led to farming practices that ruined the soil profile.

The prairie soils and native plants are well adapted to a relatively dry climate. With government encouragement, settlers moved in to homestead the region. They plowed vast areas of prairie into long, straight rows and planted grain. The plowing broke up the stable soil profile and destroyed the natural grasses and plants, which had long roots that anchored the soil layers. The grains they planted had shallower root systems and were plowed up every year, which made the soil prone to erosion. The plowed furrows were aligned in straight rows running downhill, which favored erosion and loss of topsoil.

The local climate does not produce sufficient precipitation to support non-native grain crops, so the farmers drilled wells and over-pumped water from the underground aquifers. The grain crops failed due to lack of water, leaving bare soil that was stripped from the ground by the prairie winds. Particles of midwestern prairie soil were deposited along the east coast and as far away as Europe. Huge dust storms called black blizzards made life unbearable, and the once-hopeful homesteaders left in droves. The setting for John Steinbeck’s famous novel and John Ford’s film, The Grapes of Wrath, is Oklahoma during this time. The lingering question is whether we have learned the lessons of the dust bowl, to avoid creating it again.

Take this quiz to check your comprehension of this section.

An interactive H5P element has been excluded from this version of the text. You can view it online here:
https://slcc.pressbooks.pub/introgeology/?p=3139#h5p-33

5.3 Sedimentary rocks

Sedimentary rock is classified into two main categories: clastic and chemical. Clastic or detrital sedimentary rocks are made from pieces of bedrock, sediment, derived primarily by mechanical weathering. Clastic rocks may also include chemically weathered sediment. Clastic rocks are classified by grain shape, grain size, and sorting. Chemical sedimentary rocks are precipitated from water saturated with dissolved minerals. Chemical rocks are classified mainly by composition of minerals in the rock.

5.3.1 Lithification and Diagenesis

Lithification turns loose sediment grains, created by weathering and transported by erosion, into clastic sedimentary rock via three interconnected steps. Deposition happens when friction and gravity overcome the forces driving sediment transport, allowing sediment to accumulate. Compaction occurs when material continues to accumulate on top of the sediment layer, squeezing the grains together and driving out water. The mechanical compaction is aided by weak attractive forces between the smaller grains of sediment. Groundwater typically carries cementing agents into the sediment. These minerals, such as calcite, amorphous silica, or oxides, may have a different composition than the sediment grains. Cementation is the process of cementing minerals coating the sediment grains and gluing them together into a fused rock.

Diagenesis is an accompanying process to lithification and is a low-temperature form of rock metamorphism (see Chapter 6, Metamorphic Rock). During diagenesis, sediments are chemically altered by heat and pressure. A classic example is aragonite (CaCO3), a form of calcium carbonate that makes up most organic shells. When lithified aragonite undergoes diagenesis, the aragonite reverts to calcite (CaCO3), which has the same chemical formula but a different crystalline structure. In sedimentary rock containing calcite and magnesium (Mg), diagenesis may transform the two minerals into dolomite (CaMg(CO3)2). Diagenesis may also reduce the pore space, or open volume, between sedimentary rock grains. The processes of cementation, compaction, and ultimately lithification occur within the realm of diagenesis, which includes the processes that turn organic material into fossils.

5.3.2 Detrital Sedimentary Rocks (Clastic)

Detrital or clastic sedimentary rocks consist of preexisting sediment pieces that comes from weathered bedrock. Most of this is mechanically weathered sediment, although some clasts may be pieces of chemical rocks. This creates some overlap between the two categories, since clastic sedimentary rocks may include chemical sediments. Detrital or clastic rocks are classified and named based on their grain size.

Grain Size

Detrital rock is classified according to sediment grain size, which is graded from large to small on the Wentworth scale (see figure). Grain size is the average diameter of sediment fragments in sediment or rock. Grain sizes are delineated using a log base 2 scale. For example, the grain sizes in the pebble class are 2.52, 1.26, 0.63, 0.32, 0.16, and 0.08 inches, which correlate respectively to very coarse, coarse, medium, fine, and very fine granules. Large fragments, or clasts, include all grain sizes larger than 2 mm (5/64 in). These include, boulders, cobbles, granules, and gravel. Sand has a grain size between 2 mm and 0.0625 mm, about the lower limit of the naked eye’s resolution. Sediment grains smaller than sand are called silt. Silt is unique; the grains can be felt with a finger or as grit between your teeth, but are too small to see with the naked eye.

Sorting and Rounding

Sorting describes the range of grain sizes within sediment or sedimentary rock. Geologists use the term “well sorted” to describe a narrow range of grain sizes, and “poorly sorted” for a wide range of grain sizes (see figure). It is important to note that soil engineers use similar terms with opposite definitions; well graded sediment consists of a variety of grain sizes, and poorly graded sediment has roughly the same grain sizes.

When reading the story told by rocks, geologists use sorting to interpret erosion or transport processes, as well as deposition energy. For example, wind-blown sands are typically extremely well sorted, while glacial deposits are typically poorly sorted. These characteristics help identify the type of erosion process that occurred. Coarse-grained sediment and poorly sorted rocks are usually found nearer to the source of sediment, while fine sediments are carried farther away. In a rapidly flowing mountain stream you would expect to see boulders and pebbles. In a lake fed by the stream, there should be sand and silt deposits. If you also find large boulders in the lake, this may indicate the involvement of another sediment transport process, such as rockfall caused by ice- or root-wedging.

Rounding is created when angular corners of rock fragments are removed from a piece of sediment due to abrasion during transport. Well-rounded sediment grains are defined as being free of all sharp edges. Very angular sediment retains the sharp corners. Most clast fragments start with some sharp edges due to the bedrock’s crystalline structure, and those points are worn down during transport. More rounded grains imply a longer erosion time or transport distance, or more energetic erosional process. Mineral hardness is also a factor in rounding.

Composition and provenance

Composition describes the mineral components found in sediment or sedimentary rock and may be influenced by local geology, like source rock and hydrology. Other than clay, most sediment components are easily determined by visual inspection (see Chapter 3, Minerals). The most commonly found sediment mineral is quartz because of its low chemical reactivity and high hardness, making it resistant to weathering, and its ubiquitous occurrence in continental bedrock. Other commonly found sediment grains include feldspar and lithic fragments. Lithic fragments are pieces of fine-grained bedrock, and include mud chips, volcanic clasts, or pieces of slate.

Weathering of volcanic rock produces Hawaii’s famous black (basalt) and green (olivine) sand beaches, which are rare elsewhere on Earth. This is because the local rock is composed almost entirely of basalt and provides an abundant source of dark colored clasts loaded with mafic minerals. According to the Goldich Dissolution Series, clasts high in mafic minerals are more easily destroyed compared to clasts composed of felsic minerals like quartz.

Geologists use provenance to discern the original source of sediment or sedimentary rock. Provenance is determined by analyzing mineral composition and types of fossils present, as well as textural features like sorting and rounding. Provenance is important for describing tectonic history, visualizing paleogeographic formations, unraveling an area’s geologic history, or reconstructing past supercontinents.

In quartz sandstone, sometimes called quartz arenite (SiO2), provenance may be determined using a rare, durable clast mineral called zircon (ZrSiO4). Zircon, or zirconium silicate, contains traces of uranium, which can be used for age-dating the source bedrock that contributed sediment to the lithified sandstone rock (see Chapter 7, Geologic Time).

Classification of Clastic Rocks

Clastic rocks are classified according to the grain size of their sediment. Coarse-grained rocks contain clasts with a predominant grain size larger than sand. Typically, smaller sediment grains, collectively called groundmass or matrix, fill in much of the volume between the larger clasts, and hold the clasts together. Conglomerates are rocks containing coarse rounded clasts, and breccias contain angular clasts (see figure). Both conglomerates and breccias are usually poorly sorted.

Medium-grained rocks composed mainly of sand are called sandstone, or sometimes arenite if well sorted. Sediment grains in sandstone can having a wide variety of mineral compositions, roundness, and sorting. Some sandstone names indicate the rock’s mineral composition. Quartz sandstone contains predominantly quartz sediment grains. Arkose is sandstone with significant amounts of feldspar, usually greater than 25%. Sandstone that contains feldspar, which weathers more quickly than quartz, is useful for analyzing the local geologic history. Greywacke is a term with conflicting definitions. Greywacke may refer to sandstone with a muddy matrix, or sandstone with many lithic fragments (small rock pieces).

Fine-grained rocks include mudstone, shale, siltstone, and claystone. Mudstone is a general term for rocks made of sediment grains smaller than sand (less than 2 mm). Rocks that are fissile, meaning they separate into thin sheets, are called shale. Rocks exclusively composed of silt or clay sediment, are called siltstone or claystone, respectively. These last two rock types are rarer than mudstone or shale.

Rock types found as a mixture between the main classifications, may be named using the less-common component as a descriptor. For example, a rock containing some silt but mostly rounded sand and gravel is called silty conglomerate. Sand-rich rock containing minor amounts of clay is called clayey sandstone.

5.3.3. Chemical, Biochemical, and Organic

Chemical sedimentary rocks are formed by processes that do not directly involve mechanical weathering and erosion. Chemical weathering may contribute the dissolved materials in water that ultimately form these rocks. Biochemical and organic sediments are clastic in the sense that they are made from pieces of organic material that is deposited, buried, and lithified; however, they are usually classified as being chemically produced.

Inorganic chemical sedimentary rocks are made of minerals precipitated from ions dissolved in solution, and created without the aid of living organisms. Inorganic chemical sedimentary rocks form in environments where ion concentration, dissolved gasses, temperatures, or pressures are changing, which causes minerals to crystallize.

Biochemical sedimentary rocks are formed from shells and bodies of underwater organisms. The living organisms extract chemical components from the water and use them to build shells and other body parts. The components include aragonite, a mineral similar to and commonly replaced by calcite, and silica.

Organic sedimentary rocks come from organic material that has been deposited and lithified, usually underwater. The source materials are plant and animal remains that are transformed through burial and heat, and end up as coal, oil, and methane (natural gas).

Inorganic chemical

Inorganic chemical sedimentary rocks are formed when minerals precipitate out of an aqueous solution, usually due to water evaporation. The precipitate minerals form various salts known as evaporites. For example, the Bonneville Salt Flats in Utah flood with winter rains and dry out every summer, leaving behind salts such as gypsum and halite. The deposition order of evaporites deposit is opposite to their solubility order, i.e. as water evaporates and increases the mineral concentration in solution, less soluble minerals precipitate out sooner than the highly soluble minerals. The deposition order and saturation percentages are depicted in the table, bearing in mind the process in nature may vary from laboratory derived values.

Mineral sequence	Percent Seawater remaining after evaporation
Calcite	50
Gypsum/anhydrite	20
Halite	10
Various potassium and magnesium salts	5

Table after.

Calcium carbonate–saturated water precipitates porous masses of calcite called tufa. Tufa can form near degassing water and in saline lakes. Waterfalls downstream of springs often precipitate tufa as the turbulent water enhances degassing of carbon dioxide, which makes calcite less soluble and causes it to precipitate. Saline lakes concentrate calcium carbonate from a combination of wave action causing degassing, springs in the lakebed, and evaporation. In salty Mono Lake in California, tufa towers were exposed after water was diverted and lowered the lake levels.

Cave deposits like stalactites and stalagmites are another form of chemical precipitation of calcite, in a form called travertine. Calcite slowly precipitates from water to form the travertine, which often shows banding. This process is similar to the mineral growth on faucets in your home sink or shower that comes from hard (mineral rich) water. Travertine also forms at hot springs such as Mammoth Hot Spring in Yellowstone National Park.

Banded iron formation deposits commonly formed early in Earth’s history, but this type of chemical sedimentary rock is no longer being created. Oxygenation of the atmosphere and oceans caused free iron ions, which are water-soluble, to become oxidized and precipitate out of solution. The iron oxide was deposited, usually in bands alternating with layers of chert.

Chert, another commonly found chemical sedimentary rock, is usually produced from silica (SiO2) precipitated from groundwater. Silica is highly insoluble on the surface of Earth, which is why quartz is so resistant to chemical weathering. Water deep underground is subjected to higher pressures and temperatures, which helps dissolve silica into an aqueous solution. As the groundwater rises toward or emerges at the surface the silica precipitates out, often as a cementing agent or into nodules. For example, the bases of the geysers in Yellowstone National Park are surrounded by silica deposits called geyserite or sinter. The silica is dissolved in water that is thermally heated by a relatively deep magma source. Chert can also form biochemically and is discussed in the Biochemical subsection. Chert has many synonyms, some of which may have gem value such as jasper, flint, onyx, and agate, due to subtle differences in colors, striping, etc., but chert is the more general term used by geologists for the entire group.

Oolites are among the few limestone forms created by an inorganic chemical process, similar to what happens in evaporite deposition. When water is oversaturated with calcite, the mineral precipitates out around a nucleus, a sand grain or shell fragment, and forms little spheres called ooids (see figure). As evaporation continues, the ooids continue building concentric layers of calcite as they roll around in gentle currents.

Biochemical

Biochemical sedimentary rocks are not that different from chemical sedimentary rocks; they are also formed from ions dissolved in solution. However, biochemical sedimentary rocks rely on biological processes to extract the dissolved materials out of the water. Most macroscopic marine organisms use dissolved minerals, primarily aragonite (calcium carbonate), to build hard parts such as shells. When organisms die the hard parts settle as sediment, which become buried, compacted and cemented into rock.

This biochemical extraction and secretion is the main process for forming limestone, the most commonly occurring, non-clastic sedimentary rock. Limestone is mostly made of calcite (CaCO3) and sometimes includes dolomite (CaMgCO3), a close relative. Solid calcite reacts with hydrochloric acid by effervescing or fizzing. Dolomite only reacts to hydrochloric acid when ground into a powder, which can be done by scratching the rock surface (see Chapter 3, Minerals).

Limestone occurs in many forms, most of which originate from biological processes. Entire coral reefs and their ecosystems can be preserved in exquisite detail in limestone rock (see figure). Fossiliferouslimestone contains many visible fossils. A type of limestone called coquina originates from beach sands made predominantly of shells that were then lithified. Coquina is composed of loosely-cemented shells and shell fragments. You can find beaches like this in modern tropical environments, such as the Bahamas. Chalk contains high concentrations of shells from a microorganism called a coccolithophore. Micrite, also known as microscopic calcite mud, is a very fine-grained limestone containing microfossils that can only be seen using a microscope.

Biogenetic chert forms on the deep ocean floor, created from biochemical sediment made of microscopic organic shells. This sediment, called ooze, may be calcareous (calcium carbonate based) or siliceous (silica-based) depending on the type of shells deposited. For example, the shells of radiolarians (zooplankton) and diatoms (phytoplankton) are made of silica, so they produce siliceous ooze.

Organic

Under the right conditions, intact pieces of organic material or material derived from organic sources, is preserved in the geologic record. Although not derived from sediment, this lithified organic material is associated with sedimentary strata and created by similar processes—burial, compaction, and diagenesis. C Deposits of these fuels develop in areas where organic material collects in large quantities. Lush swamplands can create conditions conducive to coal formation. Shallow-water, organic material-rich marine sediment can become highly productive petroleum and natural gas deposits. See Chapter 16, Energy and Mineral Resources, for a more in-depth look at these fossil-derived energy sources.

Classification of Chemical Sedimentary Rocks

In contrast to detrital sediment, chemical, biochemical, and organic sedimentary rocks are classified based on mineral composition. Most of these are monomineralic, composed of a single mineral, so the rock name is usually associated with the identifying mineral. Chemical sedimentary rocks consisting of halite are called rock salt. Rocks made of Limestone (calcite) is an exception, having elaborate subclassifications and even two competing classification methods: Folk Classification and Dunham Classification. The Folk Classification deals with rock grains and usually requires a specialized, petrographic microscope. The Dunham Classification is based on rock texture, which is visible to the naked eye or using a hand lens and is easier for field applications. Most carbonate geologists use the Dunham system.

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5.4 Sedimentary Structures

Sedimentary structures are visible textures or arrangements of sediments within a rock. Geologists use these structures to interpret the processes that made the rock and the environment in which it formed. They use uniformitarianism to usually compare sedimentary structures formed in modern environments to lithified counterparts in ancient rocks. Below is a summary discussion of common sedimentary structures that are useful for interpretations in the rock record.

5.4.1. Bedding Planes

The most basic sedimentary structure is bedding planes, the planes that separate the layers or strata in sedimentary and some volcanic rocks. Visible in exposed outcroppings, each bedding plane indicates a change in sediment deposition conditions. This change may be subtle. For example, if a section of underlying sediment firms up, this may be enough to create a form a layer that is dissimilar from the overlying sediment. Each layer is called a bed, or stratum, the most basic unit of stratigraphy, the study of sedimentary layering.

As would be expected, bed thickness can indicate sediment deposition quantity and timing. Technically, a bed is a bedding plane thicker than 1 cm (0.4 in) and the smallest mappable unit. A layer thinner than 1 cm (0.4 in) is called a lamina. Varves are bedding planes created when laminae and beds are deposited in repetitive cycles, typically daily or seasonally. Varves are valuable geologic records of climatic histories, especially those found in lakes and glacial deposits.

5.4.2. Graded Bedding

Gradedbedding refers to a sequence of increasingly coarse- or fine-grained sediment layers. Graded bedding often develops when sediment deposition occurs in an environment of decreasing energy. A Bouma sequence is graded bedding observed in clastic rock called turbidite. Bouma sequence beds are formed by offshore sediment gravity flows, which are underwater flows of sediment. These subsea density flows begin when sediment is stirred up by an energetic process and becomes a dense slurry of mixed grains. The sediment flow courses downward through submarine channels and canyons due to gravity acting on the density difference between the denser slurry and less dense surrounding seawater. As the flow reaches deeper ocean basins it slows down, loses energy, and deposits sediment in a Bouma sequence of coarse grains first, followed by increasingly finer grains (see figure).

5.4.3. Flow Regime and Bedforms

In fluid systems, such as moving water or wind, sand is the most easily transported and deposited sediment grain. Smaller particles like silt and clay are less movable by fluid systems because the tiny grains are chemically attracted to each other and stick to the underlying sediment. Under higher flow rates, the fine silt and clay sediment tends to stay in place and the larger sand grains get picked up and moved.

Bedforms are sedimentary structures created by fluid systems working on sandy sediment. Grain size, flow velocity, and flow regime or pattern interact to produce bedforms having unique, identifiable physical characteristics. Flow regimes are divided into upper and lower regimes, which are further divided into uppermost, upper, lower, and lowermost parts. The table below shows bedforms and their associated flow regimes. For example, the dunes bedform is created in the upper part of the lower flow regime.

Flow Regime (part)	Bedform	Description
Lower (lowest)	Plane bed	Lower plane bed, flat laminations
Lower (lower)	Ripples	Small (with respect to flow) inclined layers dipping downflow
Lower (upper)	Dunes	Larger inclined cross beds, ±ripples, dipping downflow
Upper (lower)	Plane bed	Flat layers, can include lined-up grains (parting lineations)
Upper (upper)	Antidunes	Hard to preserve reverse dunes dipping shallowly upflow
Upper (uppermost)	Chutes/pools (rare)	Erosional, not really a bedform; rarely found preserved

Plane Beds

Plane beds created in the lower flow regime are like bedding planes, on a smaller scale. The flat, parallel layers form as sandy sediment piles and move on top of layers below. Even non-flowing fluid systems, such as lakes, can produce sediment plane beds. Plane beds in the upper flow regime are created by fast-flowing fluids. They may look identi

cal to lower-flow-regime beds; however, they typically show parting lineations, slight alignments of grains in rows and swaths, caused by high sediment transport rates that only occur in upper flow regimes.

Ripples

Ripples are known by several names: ripple marks, ripple cross beds, or ripple cross laminations. The ridges or undulations in the bed are created as sediment grains pile up on top of the plane bed. With the exception of dunes, the scale of these beds is typically measured in centimeters. Occasionally, large flows like glacial lake outbursts, can produce ripples as tall as 20 m (66 ft).

First scientifically described by Hertha Ayrton, ripple shapes are determined by flow type and can be straight-crested, sinuous, or complex. Asymmetrical ripples form in a unidirectional flow. Symmetrical ripples are the result of an oscillating back-and-forth flow typical of intertidal swash zones. Climbing ripples are created from high sedimentation rates and appear as overlapping layers of ripple shapes (see figure).

Dunes

Dunes are very large and prominent versions of ripples, and typical examples of large cross bedding. Cross bedding happens when ripples or dunes pile atop one another, interrupting, and/or cutting into the underlying layers. Desert sand dunes are probably the first image conjured up by this category of bedform.

British geologist Agnold (1941) considered only Barchan and linear Seif dunes as the only true dune forms. Other workers have recognized transverse and star dunes as well as parabolic and linear dunes anchored by plants that are common in coastal areas as other types of dunes.

Dunes are the most common sedimentary structure found within channelized flows of air or water. The biggest difference between river dunes and air-formed (desert) dunes is the depth of fluid system. Since the atmosphere’s depth is immense when compared to a river channel, desert dunes are much taller than those found in rivers. Some famous air-formed dune landscapes include the Sahara Desert, Death Valley, and the Gobi Desert.

As airflow moves sediment along, the grains accumulate on the dune’s windward surface (facing the wind). The angle of the windward side is typically shallower than the leeward (downwind) side, which has grains falling down over it. This difference in slopes can be seen in a bed cross-section and indicates the direction of the flow in the past. There are typically two styles of dune beds: the more common trough cross beds with curved windward surfaces, and rarer planar cross beds with flat windward surfaces.

In tidal locations with strong in-and-out flows, dunes can develop in opposite directions. This produces a feature called herringbone cross bedding.

Another dune formation variant occurs when very strong, hurricane-strength, winds agitate parts of the usually undisturbed seafloor. These beds are called hummocky cross stratification and have a 3D architecture of hills and valleys, with inclined and declined layering that matches the dune shapes.

Antidunes

Antidunes are so named because they share similar characteristics with dunes, but are formed by a different, opposing process. While dunes form in lower flow regimes, antidunes come from fast-flowing upper flow regimes. In certain conditions of high flow rates, sediment accumulates upstream of a subtle dip instead of traveling downstream (see figure). Antidunes form in phase with the flow; in rivers they are marked by rapids in the current. Antidunes are rarely preserved in the rock record because the high flow rates needed to produce the beds also accelerate erosion.

5.4.4. Bioturbation

Bioturbation is the result of organisms burrowing through soft sediment, which disrupts the bedding layers. These tunnels are backfilled and eventually preserved when the sediment becomes rock. Bioturbation happens most commonly in shallow, marine environments, and can be used to indicate water depth.

5.4.5. Mudcracks

Mudcracks occur in clay-rich sediment that is submerged underwater and later dries out. Water fills voids in the clay’s crystalline structure, causing the sediment grains to swell. When this waterlogged sediment begins to dry out, the clay grains shrink. The sediment layer forms deep polygonal cracks with tapered openings toward the surface, which can be seen in profile. The cracks fill with new sediment and become visible veins running through the lithified rock. These dried-out clay beds are a major source of mud chips, small fragments of mud or shale, which commonly become inclusions in sandstone and conglomerate. What makes this sedimentary structure so important to geologists, is they only form in certain depositional environments—such as tidal flats that form underwater and are later exposed to air. Syneresis cracks are similar in appearance to mudcracks but much rarer; they are formed when subaqueous (underwater) clay sediment shrinks.

5.4.6. Sole Marks

Sole marks are small features typically found in river deposits. They form at the base of a bed, the sole, and on top of the underlying bed. They can indicate several things about the deposition conditions, such as flow direction or stratigraphic up-direction (see Geopetal Structures section). Flute casts or scour marks are grooves carved out by the forces of fluid flow and sediment loads. The upstream part of the flow creates steep grooves and downstream the grooves are shallower. The grooves subsequently become filled by overlying sediment, creating a cast of the original hollow.

Formed similarly to flute casts but with a more regular and aligned shape, groove casts are produced by larger clasts or debris carried along in the water that scrape across the sediment layer. Tool marks come from objects like sticks carried in the fluid downstream or embossed into the sediment layer, leaving a depression that later fills with new sediment.

Load casts, an example of soft-sediment deformation, are small indentations made by an overlying layer of coarse sediment grains or clasts intruding into a softer, finer-grained sediment layer.

5.4.7. Raindrop Impressions

Like their name implies, raindrop impressions are small pits or bumps found in soft sediment. While they are generally believed to be created by rainfall, they may be caused by other agents such as escaping gas bubbles.

5.4.8. Imbrication

Imbrication is a stack of large and usually flat clasts—cobbles, gravels, mud chips, etc.—that are aligned in the direction of fluid flow. The clasts may be stacked in rows, with their edges dipping down and flat surfaces aligned to face the flow (see figure). Or their flat surfaces may be parallel to the layer and long axes aligned with flow. Imbrications are useful for analyzing paleocurrents, or currents found in the geologic past, especially in alluvial deposits.

5.4.9. Geopetal Structures

Geopetal structures, also called up-direction indicators, are used to identify which way was up when the sedimentary rock layers were originally formed. This is especially important in places where the rock layers have been deformed, tilted, or overturned. Well preserved mudcracks, sole marks, and raindrop impressions can be used to determine up direction. Other useful geopetal structures include:

Vugs: Small voids in the rock that usually become filled during diagenesis. If the void is partially filled or filled in stages, it serves as a permanent record of a level bubble, frozen in time.

Cross bedding – In places where ripples or dunes pile on top of one another, where one cross bed interrupts and/or cuts another below, this shows a cross-cutting relationship that indicates up direction.

Ripples, dunes: Sometimes the ripples are preserved well enough to differentiate between the crests (top) and troughs (bottom).

Fossils: Body fossils in life position, meaning the body parts are not scattered or broken, and trace fossils like footprints (see figure) can provide an up direction. Intact fossilized coral reefs are excellent up indicators because of their large size and easily distinguishable top and bottom. Index fossils, such as ammonites, can be used to age date strata and determine up direction based on relative rock ages.

Vesicles – Lava flows eliminate gas upwards. An increase of vesicles toward the top of the flow indicates up.

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5.5 Depositional Environments

The ultimate goal of many stratigraphy studies is to understand the original depositional environment. Knowing where and how a particular sedimentary rock was formed can help geologists paint a picture of past environments—such as a mountain glacier, gentle floodplain, dry desert, or deep-sea ocean floor. The study of depositional environments is a complex endeavor; the table shows a simplified version of what to look for in the rock record.

Location	Sediment	Common Rock Types	Typical Fossils	Sedimentary Structures
Abyssal	very fine muds and oozes, diatomaceous Earth	chert	diatoms	few
Submarine fan	gradedBouma sequences, alternating sand/mud	clastic rocks	rare	channels, fan shape
Continental slope	mud, possible sand, countourites	shale , siltstone , limestone	rare	swaths
Lower shoreface	laminated sand	sandstone	bioturbation	hummocky cross beds
Upper shoreface	planar sand	sandstone	bioturbation	plane beds , cross beds
Littoral (beach)	very well sorted sand	sandstone	bioturbation	few
Tidal Flat	mud and sand with channels	shale , mudstone , siltstone	bioturbation	mudcracks , symmetric ripples
Reef	lime mud with coral	limestone	many, commonly coral	few
Lagoon	laminated mud	shale	many, bioturbation	laminations
Delta	channelized sand with mud, ±swamp	clastic rocks	many to few	cross beds
Fluvial ( river )	sand and mud, can have larger sediments	sandstone , conglomerate	bone beds (rare)	cross beds , channels, asymmetric ripples
Alluvial	mud to boulders, poorly sorted	clastic rocks	rare	channels, mud cracks
Lacustrine (lake)	fine-grained laminations	shale	invertebrates, rare (deep) bone beds	laminations
Paludal (swamp)	plant material	coal	plant debris	rare
Aeolian ( dunes )	very well sorted sand and silt	sandstone	rare	cross beds (large)
Glacial	mud to boulders, poorly sorted	conglomerate ( tillite )	striations, drop stones

5.5.1. Marine

Marinedepositional environments are completely and constantly submerged in seawater. Their depositional characteristics are largely dependent on the depth of water with two notable exceptions, submarine fans and turbidites.

Abyssal

Abyssal sedimentary rocks form on the abyssal plain. The plain encompasses relatively flat ocean floor with some minor topographical features, called abyssal hills. These small seafloor mounts range 100 m to 20 km in diameter, and are possibly created by extension. Most abyssal plains do not experience significant fluid movement, so sedimentary rock formed there are very fine grained.

There are three categories of abyssal sediment. Calcareous oozes consist of calcite-rich plankton shells that have fallen to the ocean floor. An example of this type of sediment is chalk. Siliceous oozes are also made of plankton debris, but these organisms build their shells using silica or hydrated silica. In some cases such as with diatomaceous earth, sediment is deposited below the calcite compensation depth, a depth where calcite solubility increases. Any calcite-based shells are dissolved, leaving only silica-based shells. Chert is another common rock formed from these types of sediment. These two types of abyssal sediment are also classified as biochemical in origin. (see BIOCHEMICAL section).

The third sediment type is pelagic clay. Very fine-grained clay particles, typically brown or red, descend through the water column very slowly. Pelagic clay deposition occurs in areas of remote open ocean, where there is little plankton accumulation.

Two notable exceptions to the fine-grained nature of abyssal sediment are submarine fan and turbidite deposits. Submarine fans occur offshore at the base of large river systems. They are initiated during times of low sea level, as strong river currents carve submarine canyons into the continental shelf. When sea levels rise, sediment accumulates on the shelf typically forming large, fan-shaped floodplains called deltas. Periodically, the sediment is disturbed creating dense slurries that flush down the underwater canyons in large gravity-induced events called turbidites. The submarine fan is formed by a network of turbidites that deposit their sediment loads as the slope decreases, much like what happens above-water at alluvial fans and deltas. This sudden flushing transports coarser sediment to the ocean floor where they are otherwise uncommon. Turbidites are also the typical origin of graded Bouma sequences. (see Chapter 5, Weathering, Erosion, and Sedimentary Rock).

Continental Slope

Continental slope deposits are not common in the rock record. The most notable type of continental slope deposits are contourites. Contourites form on the slope between the continental shelf and deep ocean floor. Deep-water ocean currents deposit sediment into smooth drifts of various architectures, sometimes interwoven with turbidites.

Lower shoreface

The lower shoreface lies below the normal depth of wave agitation, so the sediment is not subject to daily winnowing and deposition. These sediment layers are typically finely laminated, and may contain hummocky cross-stratification. Lower shoreface beds are affected by larger waves, such those as generated by hurricanes and other large storms.

Upper shoreface

The upper shoreface contains sediments within the zone of normal wave action, but still submerged below the beach environment. These sediments usually consist of very well sorted, fine sand. The main sedimentary structure is planar bedding consistent with the lower part of the upper flow regime, but it can also contain cross bedding created by longshore currents.

5.5.2. Transitional coastline environments

Transitional environments, more often called shoreline or coastline environments, are zones of complex interactions caused by ocean water hitting land. The sediment preservation potential is very high in these environments because deposition often occurs on the continental shelf and underwater. Shoreline environments are an important source of hydrocarbon deposits (petroleum, natural gas).

The study of shoreline depositional environments is called sequence stratigraphy. Sequence stratigraphy examines depositional changes and 3D architectures associated with rising and falling sea levels, which is the main force at work in shoreline deposits. These sea-level fluctuations come from the daily tides, as well as climate changes and plate tectonics. A steady rise in sea level relative to the shoreline is called transgression. Regression is the opposite, a relative drop in sea level. Some common components of shoreline environments are littoral zones, tidal flats, reefs, lagoons, and deltas. For a more in-depth look at these environments, see Chapter 12, Coastlines.

Littoral

The littoral zone, better known as the beach, consists of highly weathered, homogeneous, well-sorted sand grains made mostly of quartz. There are black sand and other types of sand beaches, but they tend to be unique exceptions rather than the rule. Because beach sands, past or present, are so highly evolved, the amount grain weathering can be discerned using the minerals zircon, tourmaline, and rutile. This tool is called the ZTR (zircon, tourmaline, rutile) index. The ZTR index is higher in more weathered beaches, because these relatively rare and weather-resistant minerals become concentrated in older beaches. In some beaches, the ZTR index is so high the sand can be harvested as an economically viable source of these minerals. The beach environment has no sedimentary structures, due to the constant bombardment of wave energy delivered by surf action. Beach sediment is moved around via multiple processes. Some beaches with high sediment supplies develop dunes nearby.

Tidal Flats

Tidal flats, or mud flats, are sedimentary environments that are regularly flooded and drained by ocean tides. Tidal flats have large areas of fine-grained sediment but may also contain coarser sands. Tidal flat deposits typically contain gradational sediments and may include multi-directional ripple marks. Mudcracks are also commonly seen due to the sediment being regularly exposed to air during low tides; the combination of mudcracks and ripple marks is distinctive to tidal flats.

Tidal water carries in sediment, sometimes focusing the flow through a narrow opening called a tidal inlet. Tidal channels, creek channels influenced by tides, can also focus tidally-induced flow. Areas of higher flow like inlets and tidal channels feature coarser grain sizes and larger ripples, which in some cases can develop into dunes.

Reefs

Reefs, which most people would immediately associate with tropical coral reefs found in the oceans, are not only made by living things. Natural buildups of sand or rock can also create reefs, similar to barrier islands. Geologically speaking, a reef is any topographically-elevated feature on the continental shelf, located oceanward of and separate from the beach. The term reef can also be applied to terrestrial (atop the continental crust) features. Capitol Reef National Park in Utah contains a topographic barrier, a reef, called the Waterpocket Fold.

Most reefs, now and in the geologic past, originate from the biological processes of living organisms. The growth habits of coral reefs provide geologists important information about the past. The hard structures in coral reefs are built by soft-bodied marine organisms, which continually add new material and enlarge the reef over time. Under certain conditions, when the land beneath a reef is subsiding, the coral reef may grow around and through existing sediment, holding the sediment in place, and thus preserving the record of environmental and geological condition around it.

Sediment found in coral reefs is typically fine-grained, mostly carbonate, and tends to deposit between the intact coral skeletons. Water with high levels of silt or clay particles can inhibit the reef growth because coral organisms require sunlight to thrive; they host symbiotic algae called zooxanthellae that provide the coral with nourishment via photosynthesis. Inorganic reef structures have much more variable compositions. Reefs have a big impact on sediment deposition in lagoon environments since they are natural storm breaks, wave and storm buffers, which allows fine grains to settle and accumulate.

Reefs are found around shorelines and islands; coral reefs are particularly common in tropical locations. Reefs are also found around features known as seamounts, which is the base of an ocean island left standing underwater after the upper part is eroded away by waves. Examples include the Emperor Seamounts, formed millions of years ago over the Hawaiian Hotspot. Reefs live and grow along the upper edge of these flat-topped seamounts. If the reef builds up above sea level and completely encircles the top of the seamount, it is called a coral-ringed atoll. If the reef is submerged, due to erosion, subsidence, or sea level rise, the seamount–reef structure is called a guyot.

Lagoon

Lagoons are small bodies of seawater located inland from the shore or isolated by another geographic feature, such as a reef or barrier island. Because they are protected from the action of tides, currents, and waves, lagoon environments typically have very fine grained sediments. Lagoons, as well as estuaries, are ecosystems with high biological productivity. Rocks from these environments often includes bioturbation marks or coal deposits. Around lagoons where evaporation exceeds water inflow, salt flats, also known as sabkhas, and sand dune fields may develop at or above the high tide line.

Deltas

Deltas form where rivers enter lakes or oceans and are of three basic shapes: river-dominated deltas, wave-dominated deltas, and tide-dominated deltas. The name delta comes from the Greek letter Δ (delta, uppercase), which resembles the triangular shape of the Nile River delta. The velocity of water flow is dependent on riverbed slope or gradient, which becomes shallower as the river descends from the mountains. At the point where a river enters an ocean or lake, its slope angle drops to zero degrees (0°). The flow velocity quickly drops as well, and sediment is deposited, from coarse clasts, to fine sand, and mud to form the delta. As one part of the delta becomes overwhelmed by sediment, the slow-moving flow gets diverted back and forth, over and over, and forms a spread out network of smaller distributary channels.

Deltas are organized by the dominant process that controls their shape: tide-dominated, wave-dominated, or river-dominated. Wave-dominated deltas generally have smooth coastlines and beach-ridges on the land that represent previous shorelines. The Nile River delta is a wave-dominated type. (see figure).

The Mississippi River delta is a river-dominated delta. shaped by levees along the river and its distributaries that confine the flow forming a shape called a birdfoot delta. Other times the tides or the waves can be a bigger factor, and can reshape the delta in various ways.

A tide-dominated delta is dominated by tidal currents. During flood stages when rivers have lots of water available, it develops distributaries that are separated by sand bars and sand ridges. The tidal delta of the Ganges River is the largest delta in the world.

5.5.3. Terrestrial

Terrestrialdepositional environments are diverse. Water is a major factor in these environments, in liquid or frozen states, or even when it is lacking (arid conditions).

Fluvial

Fluvial (river) systems are formed by water flowing in channels over the land. They generally come in two main varieties: meandering or braided. In meandering streams, the flow carries sediment grains via a single channel that wanders back and forth across the floodplain. The floodplain sediment away from the channel is mostly fine grained material that only gets deposited during floods.

Braidedfluvial systems generally contain coarser sediment grains, and form a complicated series of intertwined channels that flow around gravel and sand bars (see Chapter 11, Water).

Alluvial

A distinctive characteristic of alluvial systems is the intermittent flow of water. Alluvial deposits are common in arid places with little soil development. Lithified alluvial beds are the primary basin-filling rock found throughout the Basin and Range region of the western United States. The most distinctive alluvial sedimentary deposit is the alluvial fan, a large cone of sediment formed by streams flowing out of dry mountain valleys into a wider and more open dry area. Alluvial sediments are typically poorly sorted and coarse grained, and often found near playa lakes or aeolian deposits (see Chapter 13, Deserts).

Lacustrine

Lake systems and deposits, called lacustrine, form via processes somewhat similar to marine deposits, but on a much smaller scale. Lacustrine deposits are found in lakes in a wide variety of locations. Lake Baikal in southeast Siberia (Russia) is in a tectonic basin. Crater Lake (Oregon) sits in a volcanic caldera. The Great Lakes (northern United States) came from glacially carved and deposited sediment. Ancient Lake Bonneville (Utah) formed in a pluvial setting that during a climate that was relatively wetter and cooler than that of modern Utah. Oxbow lakes, named for their curved shape, originated in fluvial floodplains. Lacustrine sediment tends to be very fine grained and thinly laminated, with only minor contributions from wind-blown, current, and tidal deposits. When lakes dry out or evaporation outpaces precipitation, playas form. Playa deposits resemble those of normal lake deposits but contain more evaporite minerals. Certain tidal flats can have playa-type deposits as well.

Paludal

Paludal systems include bogs, marshes, swamps, or other wetlands, and usually contain lots of organic matter. Paludal systems typically develop in coastal environments, but are common occur in humid, low-lying, low-latitude, warm zones with large volumes of flowing water. A characteristic paludal deposit is a peat bog, a deposit rich in organic matter that can be converted into coal when lithified. Paludal environments may be associated with tidal, deltaic, lacustrine, and/or fluvial deposition.

Aeolian

Aeolian, sometimes spelled eolian or œolian, are deposits of windblown sediments. Since wind has a much lower carrying capacity than water, aeolian deposits typically consists of clast sizes from fine dust to sand. Fine silt and clay can cross very long distances, even entire oceans suspended in air.

With sufficient sediment influx, aeolian systems can potentially form large dunes in dry or wet conditions. The figure shows dune features and various types. British geologist Ralph A. Bagnold (1896-1990) considered only Barchan and linear Seif dunes as the only true dune forms. Other scientists recognize transverse, star, parabolic, and linear dune types. Parabolic and linear dunes grow from sand anchored by plants and are common in coastal areas.

Compacted layers of wind-blown sediment is known as loess. Loess commonly starts as finely ground up rock flour created by glaciers. Such deposits cover thousands of square miles in the Midwestern United States. Loess may also form in desert regions (see Chapter 13). Silt for the Loess Plateau in China came from the Gobi Desert in China and Mongolia.

Glacial

Glacial sedimentation is very diverse, and generally consists of the most poorly-sorted sediment deposits found in nature. The main clast type is called diamictite, which literally means two sizes, referring to the unsorted mix of large and small rock fragments found in glacial deposits. Many glacial tills, glacially derived diamictites, include very finely-pulverized rock flour along with giant erratic boulders. The surfaces of larger clasts typically have striations from the rubbing, scraping, and polishing of surfaces by abrasion during the movement of glacial ice. Glacial systems are so large and produce so much sediment, they frequently create multiple, individualized depositional environments, such as fluvial, deltaic, lacustrine, pluvial, alluvial, and/or aeolian (see Chapter 14, Glaciers).

5.5.4. Facies

In addition to mineral composition and lithification process, geologists also classify sedimentary rock by its depositional characteristics, collectively called facies or lithofacies. Sedimentary facies consist of physical, chemical, and/or biological properties, including relative changes in these properties in adjacent beds of the same layer or geological age. Geologists analyze sedimentary rock facies to interpret the original deposition environment, as well as disruptive geological events that may have occurred after the rock layers were established.

It boggles the imagination to think of all the sedimentary deposition environments working next to each other, at the same time, in any particular region on Earth. The resulting sediment beds develop characteristics reflecting contemporaneous conditions at the time of deposition, which later may become preserved into the rock record. For example in the Grand Canyon, rock strata of the same geologic age includes many different depositional environments: beach sand, tidal flat silt, offshore mud, and farther offshore limestone. In other words, each sedimentary or stratigraphic facies presents recognizable characteristics that reflect specific, and different, depositional environments that were present at the same time.

Facies may also reflect depositional changes in the same location over time. During periods of rising sea level, called marine transgression, the shoreline moves inland as seawater covers what was originally dry land and creates new offshore depositional environments. When these sediment beds turn into sedimentary rock, the vertical stratigraphy sequence reveals beach lithofacies buried by offshore lithofacies.

Biological facies are remnants (coal, diatomaceous earth) or evidence (fossils) of living organisms. Index fossils, fossilized life forms specific to a particular environment and/or geologic time period, are an example of biological facies. The horizontal assemblage and vertical distribution of fossils are particularly useful for studying species evolution because transgression, deposition, burial, and compaction processes happen over a considerable geologic time range.

Fossil assemblages that show evolutionary changes greatly enhance our interpretation of Earth’s ancient history by illustrating the correlation between stratigraphic sequence and geologic time scale. During the Middle Cambrian period (see Chapter 7, Geologic Time), regions around the Grand Canyon experienced marine transgression in a southeasterly direction (relative to current maps). This shift of the shoreline is reflected in the Tapeats Sandstone beach facies, Bright Angle Shale near-offshore mud facies, and Muav Limestone far-offshore facies. Marine organisms had plenty of time to evolve and adapt to their slowly changing environment; these changes are reflected in the biological facies, which show older life forms in the western regions of the canyon and younger life forms in the east.

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Summary

Sedimentary rocks are grouped into two main categories: clastic (detrital) and chemical. Clastic (detrital) rocks are made of mineral clasts or sediment that lithifies into solid material. Sediment is produced by the mechanical or chemical weathering of bedrock and transported away from the source via erosion. Sediment that is deposited, buried, compacted, and sometimes cemented becomes clastic rock. Clastic rocks are classified by grain size; for example sandstone is made of sand-sized particles. Chemical sedimentary rocks comes from minerals precipitated out an aqueous solution and is classified according to mineral composition. The chemical sedimentary rock limestone is made of calcium carbonate. Sedimentary structures have textures and shapes that give insight on depositional histories. Depositional environments depend mainly on fluid transport systems and encompass a wide variety of underwater and above ground conditions. Geologists analyze depositional conditions, sedimentary structures, and rock records to interpret the paleogeographic history of a region.

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References

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Normark, W.R., 1978, Fan valleys, channels, and depositional lobes on modern submarine fans: characters for recognition of sandy turbidite environments: AAPG Bull., v. 62, no. 6, p. 912–931.

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Stanistreet, I.G., and McCarthy, T.S., 1993, The Okavango Fan and the classification of subaerial fan systems: Sediment. Geol., v. 85, no. 1, p. 115–133.

Stow, D.A.V., Faugères, J.-C., Viana, A., and Gonthier, E., 1998, Fossil contourites: a critical review: Sediment. Geol., v. 115, no. 1–4, p. 3–31.

Stow, D.A.V., and Piper, D.J.W., 1984, Deep-water fine-grained sediments: facies models: Geological Society, London, Special Publications, v. 15, no. 1, p. 611–646.

Udden, J.A., 1914, Mechanical composition of clastic sediments: Geol. Soc. Am. Bull., v. 25, no. 1, p. 655–744.

Wentworth, C.K., 1922, A scale of grade and class terms for clastic sediments: J. Geol., v. 30, no. 5, p. 377–392.

Yin, D., Peakall, J., Parsons, D., Chen, Z., Averill, H.M., Wignall, P., and Best, J., 2016, Bedform genesis in bedrock substrates: Insights into formative processes from a new experimental approach and the importance of suspension-dominated abrasion: Geomorphology, v. 255, p. 26–38.

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6 Metamorphic Rocks

6 Metamorphic Rocks

Contributing Author: Dr. Peter Davis, Pacific Lutheran University

KEY CONCEPTS

Describe the temperature and pressure conditions of the metamorphic environment

Identify and describe the three principal metamorphic agents

Describe what recrystallization is and how it affects mineral crystals

Explain what foliation is and how it results from directed pressure and recrystallization

Explain the relationships among slate, phyllite, schist, and gneiss in terms of metamorphic grade

Define index mineral

Explain how metamorphic facies relate to plate tectonic processes

Describe what a contact aureole is and how contact metamorphism affects surrounding rock

Describe the role of hydrothermal metamorphism in forming mineral deposits and ore bodies

Metamorphic rocks, meta- meaning change and –morphos meaning form, is one of the three rock categories in the rock cycle (see Chapter 1). Metamorphic rock material has been changed by temperature, pressure, and/or fluids. The rock cycle shows that both igneous and sedimentary rocks can become metamorphic rocks. And metamorphic rocks themselves can be re-metamorphosed. Because metamorphism is caused by plate tectonic motion, metamorphic rock provides geologists with a history book of how past tectonic processes shaped our planet.

6.1 Metamorphic Processes

Metamorphism occurs when solid rock changes in composition and/or texture without the mineral crystals melting, which is how igneous rock is generated. Metamorphic source rocks, the rocks that experience the metamorphism, are called the parent rock or protolith, from proto– meaning first, and lithos- meaning rock. Most metamorphic processes take place deep underground, inside the earth’s crust. During metamorphism, protolith chemistry is mildly changed by increased temperature (heat), a type of pressure called confining pressure, and/or chemically reactive fluids. Rock texture is changed by heat, confining pressure, and a type of pressure called directed stress.

6.1.1 Temperature (Heat)

Temperature measures a substance’s energy—an increase in temperature represents an increase in energy. Temperature changes affect the chemical equilibrium or cation balance in minerals. At high temperatures atoms may vibrate so vigorously they jump from one position to another within the crystal lattice, which remains intact. In other words, this atom swapping can happen while the rock is still solid.

The temperatures of metamorphic rock lies in between surficial processes (as in sedimentary rock) and magma in the rock cycle. Heat-driven metamorphism begins at temperatures as cold as 200˚C, and can continue to occur at temperatures as high as 700°C-1,100°C. Higher temperatures would create magma, and thus, would no longer be a metamorphic process. Temperature increases with increasing depth in the Earth along a geothermal gradient (see Chapter 4) and metamorphic rock records these depth-related temperature changes.

6.1.2 Pressure

Pressure is the force exerted over a unit area on a material. Like heat, pressure can affect the chemical equilibrium of minerals in a rock. The pressure that affects metamorphic rocks can be grouped into confining pressure and directed stress. Stress is a scientific term indicating a force. Strain is the result of this stress, including metamorphic changes within minerals.

Confining Pressure

Pressure exerted on rocks under the surface is due to the simple fact that rocks lie on top of one another. When pressure is exerted from rocks above, it is balanced from below and sides, and is called confiningor lithostatic pressure. Confining pressure has equal pressure on all sides (see figure) and is responsible for causing chemical reactions to occur just like heat. These chemical reactions will cause new minerals to form.

Confining pressure is measured in bars and ranges from 1 bar at sea level to around 10,000 bars at the base of the crust. For metamorphic rocks, pressures range from a relatively low-pressure of 3,000 bars around 50,000 bars, which occurs around 15-35 kilometers below the surface.

Directed Stress

Directed stress, also called differential or tectonic stress, is an unequal balance of forces on a rock in one or more directions (see previous figure). Directed stresses are generated by the movement of lithospheric plates. Stress indicates a type of force acting on rock. Strain describes the resultant processes caused by stress and includes metamorphic changes in the minerals. In contrast to confining pressure, directed stress occurs at much lower pressures and does not generate chemical reactions that change mineral composition and atomic structure. Instead, directed stress modifies the parent rock at a mechanical level, changing the arrangement, size, and/or shape of the mineral crystals. These crystalline changes create identifying textures, which is shown in the figure below comparing the phaneritic texture of igneous granite with the foliated texture of metamorphic gneiss.

Directed stresses produce rock textures in many ways. Crystals are rotated, changing their orientation in space. Crystals can get fractured, reducing their grain size. Conversely, they may grow larger as atoms migrate. Crystal shapes also become deformed. These mechanical changes occur via recrystallization, which is when minerals dissolve from an area of rock experiencing high stress and precipitate or regrow in a location having lower stress. For example, recrystallization increases grain size much like adjacent soap bubbles coalesce to form larger ones. Recrystallization rearranges mineral crystals without fracturing the rock structure, deforming the rock like silly putty; these changes provide important clues to understanding the creation and movement of deep underground rock faults.

6.1.3 Fluids

A third metamorphic agent is chemically reactive fluids that are expelled by crystallizing magma and created by metamorphic reactions. These reactive fluids are made of mostly water (H2O) and carbon dioxide (CO2), and smaller amounts of potassium (K), sodium (Na), iron (Fe), magnesium (Mg), calcium (Ca), and aluminum (Al). These fluids react with minerals in the protolith, changing its chemical equilibrium and mineral composition, in a process similar to the reactions driven by heat and pressure. In addition to using elements found in the protolith, the chemical reaction may incorporate substances contributed by the fluids to create new minerals. In general, this style of metamorphism, in which fluids play an important role, is called hydrothermalmetamorphism or hydrothermal alteration. Water actively participates in chemical reactions and allows extra mobility of the components in hydrothermal alteration.

Fluids-activated metamorphism is frequently involved in creating economically important mineral deposits that are located next to igneous intrusions or magma bodies. For example, the mining districts in the Cottonwood Canyons and Mineral Basin of northern Utah produce valuable ores such as argentite (silver sulfide), galena (lead sulfide), and chalcopyrite (copper iron sulfide), as well as the native element gold. These mineral deposits were created from the interaction between a granitic intrusion called the Little Cottonwood Stock and country rock consisting of mostly limestone and dolostone. Hot, circulating fluids expelled by the crystallizing granite reacted with and dissolved the surrounding limestone and dolostone, precipitating out new minerals created by the chemical reaction. Hydrothermal alternation of mafic mantle rock, such as olivine and basalt, creates the metamorphic rock serpentinite, a member of the serpentine subgroup of minerals. This metamorphic process happens at mid-ocean spreading centers where newly formed oceanic crust interacts with seawater.

<img class="wp-image-2545" title="By University of Washington; NOAA/OAR/OER. (NOAA Photo Library: expl2366) [CC BY 2.0 or Public domain], via Wikimedia Commons” src=”https://slcc.pressbooks.pub/app/uploads/sites/35/2021/09/BlackSmoker-233×300.jpg” alt=”There is a large build up of minerals around the vent” width=”304″ height=”392″> Black smoker hydrothermal vent with a colony of giant (6’+) tube worms.Some hydrothermal alterations remove elements from the parent rock rather than deposit them. This happens when seawater circulates down through fractures in the fresh, still-hot basalt, reacting with and removing mineral ions from it. The dissolved minerals are usually ions that do not fit snugly in the silicate crystal structure, such as copper. The mineral-laden water emerges from the sea floor via hydrothermal vents called black smokers, named after the dark-colored precipitates produced when the hot vent water meets cold seawater. (see Chapter 4, Igneous Rock and Volcanic Processes) Ancient black smokers were an important source of copper ore for the inhabitants of Cyprus (Cypriots) as early as 4,000 BCE, and later by the Romans.

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6.2 Metamorphic textures

Metamorphictexture is the description of the shape and orientation of mineral grains in a metamorphic rock. Metamorphic rock textures are foliated, non-foliated, or lineated are described below.

6.2.1 Foliation and Lineation

Foliation is a term used that describes minerals lined up in planes. Certain minerals, most notably the mica group, are mostly thin and planar by default. Foliated rocks typically appear as if the minerals are stacked like pages of a book, thus the use of the term ‘folia’, like a leaf. Other minerals, with hornblende being a good example, are longer in one direction, linear like a pencil or a needle, rather than a planar-shaped book. These linear objects can also be aligned within a rock. This is referred to as a lineation. Linear crystals, such as hornblende, tourmaline, or stretched quartz grains, can be arranged as part of a foliation, a lineation, or foliation/lineation together. If they lie on a plane with mica, but with no common or preferred direction, this is foliation. If the minerals line up and point in a common direction, but with no planar fabric, this is lineation. When minerals lie on a plane AND point in a common direction; this is both foliation and lineation.

Foliatedmetamorphic rocks are named based on the style of their foliations. Each rock name has a specific texture that defines and distinguishes it, with their descriptions listed below.

Slateis a fine-grained metamorphic rock that exhibits a foliation called slaty cleavagethat is the flat orientation of the small platy crystals of mica and chlorite forming perpendicular to the direction of stress. The minerals in slate are too small to see with the unaided eye. The thin layers in slate may resemble sedimentary bedding, but they are a result of directed stress and may lie at angles to the original strata. In fact, original sedimentary layering may be partially or completely obscured by the foliation. Thin slabs of slate are often used as a building material for roofs and tiles.

<img class="wp-image-3169" title="By Uta Baumfelder at de.wikipedia (Own work) [Public domain], via Wikimedia Commons” src=”https://slcc.pressbooks.pub/app/uploads/sites/35/2021/12/06.2-Ehemaliger_Schiefertagebau_am_Brand-300×225.jpg” alt=”Rock breaking along flat even planes.” width=”383″ height=”287″> Slate mine in Germany cleavage.

Phylliteis a foliated metamorphic rock in which platy minerals have grown larger and the surface of the foliation shows a sheen from light reflecting from the grains, perhaps even a wavy appearance, called crenulations. Similar to phyllite but with even larger grains is the foliated metamorphic rock schist, which has large platy grains visible as individual crystals. Common minerals are muscovite, biotite, and porphyroblasts of garnets. A porphyroblast is a large crystal of a particular mineral surrounded by small grains. Schistosityis a textural description of foliation created by the parallel alignment of platy visible grains. Some schists are named for their minerals such as mica schist (mostly micas), garnet schist (mica schist with garnets), and staurolite schist (mica schists with staurolite).

<img class="wp-image-3179" title="By No machine-readable author provided. Siim assumed (based on copyright claims). [GFDL or CC-BY-SA-3.0], via Wikimedia Commons” src=”https://slcc.pressbooks.pub/app/uploads/sites/35/2021/12/06.2_Gneiss-300×181.jpg” alt=”Alternating bands of light and dark minerals.” width=”354″ height=”213″> Gneiss

Gneissic banding is a metamorphic foliationin which visible silicate minerals separate into dark and light bands or lineations. These grains tend to be coarse and often folded. A rock with this texture is called gneiss. Since gneisses form at the highest temperatures and pressures, some partial melting may occur. This partially melted rock is a transition between metamorphic and igneous rocks called a migmatite.

Migmatites appear as dark and light banded gneiss that may be swirled or twisted some since some minerals started to melt. Thin accumulations of light colored rock layers can occur in a darker rock that are parallel to each other, or even cut across the gneissic foliation. The lighter colored layers are interpreted to be the result of the separation of a felsic igneous melt from the adjacent highly metamorphosed darker layers, or injection of a felsic melt from some distance away.

6.2.2 Non-foliated

Non-foliatedtextures do not have lineations, foliations, or other alignments of mineral grains. Non-foliated metamorphic rocks are typically composed of just one mineral, and therefore, usually show the effects of metamorphism with recrystallization in which crystals grow together, but with no preferred direction. The two most common examples of non-foliated rocks are quartzite and marble. Quartziteis a metamorphic rock from the protolith sandstone. In quartzites, the quartz grains from the original sandstone are enlarged and interlocked by recrystallization. A defining characteristic for distinguishing quartzite from sandstone is that when broken with a rock hammer, the quartz crystals break across the grains. In a sandstone, only a thin mineral cement holds the grains together, meaning that a broken piece of sandstone will leave the grains intact. Because most sandstones are rich in quartz, and quartz is a mechanically and chemically durable substance, quartzite is very hard and resistant to weathering.

Marbleis metamorphosed limestone (or dolostone) composed of calcite (or dolomite). Recrystallization typically generates larger interlocking crystals of calcite or dolomite. Marble and quartzite often look similar, but these minerals are considerably softer than quartz. Another way to distinguish marble from quartzite is with a drop of dilute hydrochloric acid. Marble will effervesce (fizz) if it is made of calcite.

A third non-foliated rock is hornfelsidentified by its dense, fine grained, hard, blocky or splintery texture composed of several silicate minerals. Crystals in hornfels grow smaller with metamorphism, and become so small that specialized study is required to identify them. These are common around intrusive igneous bodies and are hard to identify. The protolith of hornfels can be even harder to distinguish, which can be anything from mudstone to basalt.

<img class="wp-image-3191" title="By Manishwiki15 (Own work) [CC BY-SA 3.0], via Wikimedia Commons” src=”https://slcc.pressbooks.pub/app/uploads/sites/35/2021/12/Sample_of_Quartzite-e1493780752118-300×210.jpg” alt=”Interlocking quartz grains in a quartzite.” width=”413″ height=”289″> Macro view of quartzite. Note the interconnectedness of the grains.<img class="wp-image-3192" title="By Wilson44691 (Own work) [Public domain], via Wikimedia Commons” src=”https://slcc.pressbooks.pub/app/uploads/sites/35/2021/12/640px-CoralPinkSandDunesSand-300×225.jpg” alt=”Undeformed quartz grains do not interlock.” width=”420″ height=”315″> Unmetamorphosed, unconsolidated sand grains have space between the grains.

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6.3 Metamorphic Grade

Metamorphicgrade refers to the range of metamorphic change a rock undergoes, progressing from low (little metamorphic change) grade to high (significant metamorphic change) grade. Low-grade metamorphism begins at temperatures and pressures just above sedimentary rock conditions. The sequence slate→phyllite→schist→gneiss illustrates an increasing metamorphic grade.

Geologists use index minerals that form at certain temperatures and pressures to identify metamorphic grade. These index minerals also provide important clues to a rock’s sedimentary protolith and the metamorphic conditions that created it. Chlorite, muscovite, biotite, garnet, and staurolite are index minerals representing a respective sequence of low-to-high grade rock. The figure shows a phase diagram of three index minerals—sillimanite, kyanite, and andalusite—with the same chemical formula (Al2SiO5) but having different crystal structures (polymorphism) created by different pressure and temperature conditions.

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Some metamorphic rocks are named based on the highest grade of index mineral present. Chlorite schist includes the low-grade index mineral chlorite. Muscovite schist contains the slightly higher grade muscovite, indicating a greater degree of metamorphism. Garnet schist includes the high grade index mineral garnet, and indicating it has experienced much higher pressures and temperatures than chlorite.

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6.4 Metamorphic Environments

As with igneous processes, metamorphic rocks form at different zones of pressure (depth) and temperature as shown on the pressure-temperature (P-T) diagram. The term faciesis an objective description of a rock. In metamorphic rocks facies are groups of minerals called mineral assemblages. The names of metamorphic facieson the pressure-temperature diagram reflect minerals and mineral assemblages that are stable at these pressures and temperatures and provide information about the metamorphic processes that have affected the rocks. This is useful when interpreting the history of a metamorphic rock.

In the late 1800s, British geologist George Barrow mapped zones of index minerals in different metamorphic zones of an area that underwent regional metamorphism. Barrow outlined a progression of index minerals, named the Barrovian Sequence, that represents increasing metamorphic grade: chlorite (slates and phyllites) -> biotite (phyllites and schists) -> garnet (schists) -> staurolite (schists) -> kyanite (schists) -> sillimanite (schists and gneisses).

The first of the Barrovian sequence has a mineral group that is commonly found in the metamorphic greenschist facies. Greenschist rocks form under relatively low pressure and temperatures and represent the fringes of regional metamorphism. The “green” part of the name is derived from green minerals like chlorite, serpentine, and epidote, and the “schist” part is applied due to the presence of platy minerals such as muscovite.

Many different styles of metamorphic facies are recognized, tied to different geologic and tectonic processes. Recognizing these facies is the most direct way to interpret the metamorphic history of a rock. A simplified list of major metamorphic facies is given below.

6.4.1 Burial Metamorphism

Burial metamorphismoccurs when rocks are deeply buried, at depths of more than 2000 meters (1.24 miles). Burial metamorphism commonly occurs in sedimentary basins, where rocks are buried deeply by overlying sediments. As an extension of diagenesis, a process that occurs during lithification (Chapter 5), burial metamorphism can cause clay minerals, such as smectite, in shales to change to another clay mineral illite. Or it can cause quartz sandstone to metamorphose into the quartzite such the Big Cottonwood Formation in the Wasatch Range of Utah. This formation was deposited as ancient near-shore sands in the late Proterozoic (see Chapter 7), deeply buried and metamorphosed to quartzite, folded, and later exposed at the surface in the Wasatch Range today. Increase of temperature with depth in combination with an increase of confining pressure produces low-grade metamorphic rocks with a mineral assemblages indicative of a zeolite facies.

6.4.2 Contact Metamorphism

Contact metamorphism occurs in rock exposed to high temperature and low pressure, as might happen when hot magma intrudes into or lava flows over pre-existing protolith. This combination of high temperature and low pressure produces numerous metamorphic facies. The lowest pressure conditions produce hornfels facies, while higher pressure creates greenschist, amphibolite, or granulite facies.

As with all metamorphic rock, the parent rock texture and chemistry are major factors in determining the final outcome of the metamorphic process, including what index minerals are present. Fine-grained shale and basalt, which happen to be chemically similar, characteristically recrystallize to produce hornfels. Sandstone (silica) surrounding an igneous intrusion becomes quartzite via contact metamorphism, and limestone (carbonate) becomes marble.

<img class="wp-image-3201" title="By Random Tree (Own work) [CC0], via Wikimedia Commons” src=”https://slcc.pressbooks.pub/app/uploads/sites/35/2021/12/Metamorphic_Aureole_in_the_Henry_Mountains-300×225.jpg” alt=”Altered rock adjacent to an igneous intrusion.” width=”185″ height=”139″> Contact metamorphism in outcrop.When contact metamorphism occurs deeper in the Earth, metamorphism can be seen as rings of facies around the intrusion, resulting in aureoles. These differences in metamorphism appear as distinct bands surrounding the intrusion, as can be seen around the Alta Stock in Little Cottonwood Canyon, Utah. The Alta Stock is a granite intrusion surrounded first by rings of the index minerals amphibole (tremolite) and olivine (forsterite), with a ring of talc (dolostone) located further away.

6.4.3 Regional Metamorphism

Regional metamorphism occurs when parent rock is subjected to increased temperature and pressure over a large area, and is often located in mountain ranges created by converging continental crustal plates. This is the setting for the Barrovian sequence of rock facies, with the lowest grade of metamorphism occurring on the flanks of the mountains and highest grade near the core of the mountain range, closest to the convergent boundary.

An example of an old regional metamorphic environment is visible in the northern Appalachian Mountains while driving east from New York state through Vermont and into New Hampshire. Along this route the degree of metamorphism gradually increases from sedimentary parent rock, to low-grade metamorphic rock, then higher-grade metamorphic rock, and eventually the igneous core. The rock sequence is sedimentary rock, slate, phyllite, schist, gneiss, migmatite, and granite. In fact, New Hampshire is nicknamed the Granite State. The reverse sequence can be seen heading east, from eastern New Hampshire to the coast.

6.4.4 Subduction Zone Metamorphism

Subduction zone metamorphism is a type of regional metamorphism that occurs when a slab of oceanic crust is subducted under continental crust (see Chapter 2). Because rock is a good insulator, the temperature of the descending oceanic slab increases slowly relative to the more rapidly increasing pressure, creating a metamorphic environment of high pressure and low temperature. Glaucophane, which has a distinctive blue color, is an index mineral found in blueschist facies (see metamorphic facies diagram). The California Coast Range near San Francisco has blueschist–facies rocks created by subduction-zone metamorphism, which include rocks made of blueschist, greenstone, and red chert. Greenstone, which is metamorphized basalt, gets its color from the index mineral chlorite.

6.4.5 Fault Metamorphism

There are a range of metamorphic rocks made along faults. Near the surface, rocks are involved in repeated brittle faulting produce a material called rock flour,which is rock ground up to the particle size of flour used for food.At lower depths,faulting create cataclastites, chaotically-crushed mixes of rock material with little internal texture. At depths below cataclasites, where strain becomes ductile, mylonites are formed. Mylonitesare metamorphic rocks created by dynamic recrystallization through directed shear forces, generally resulting in a reduction of grain size. When larger, stronger crystals (like feldspar, quartz, garnet) embedded in a metamorphic matrix are sheared into an asymmetrical eye-shaped crystal, an augenis formed.

6.4.6 Shock Metamorphism

<img class="wp-image-3212" title="By Glen A. Izett [Public domain], via Wikimedia Commons” src=”https://slcc.pressbooks.pub/app/uploads/sites/35/2021/12/820qtz-300×253.jpg” alt=”A small grain of sand showing a prismatic inside with lines across it.” width=”218″ height=”184″> Shock lamellae in a quartz grain.

Shock(also known as impact) metamorphismis metamorphism resulting from meteor or other bolide impacts, or from a similar high-pressure shock event. Shock metamorphism is the result of very high pressures (and higher, but less extreme temperatures) delivered relatively rapidly. Shock metamorphism produces planar deformation features, tektites, shatter cones, and quartz polymorphs. Shock metamorphism produces planar deformation features (shock laminae), which are narrow planes of glassy material with distinct orientations found in silicate mineral grains. Shocked quartz has planar deformation features.

Shatter cones are cone-shaped pieces of rock created by dynamic branching fractures caused by impacts. While not strictly a metamorphic structure, they are common around shock metamorphism. Their diameter can range from microscopic to several meters. Fine-grained rocks with shatter cones show a distinctive horsetail pattern.

Shock metamorphism can also produce index minerals, though they are typically only found via microscopic analysis. The quartz polymorphs coesite and stishovite are indicative of impact metamorphism. As discussed in chapter 3, polymorphs are minerals with the same composition but different crystal structures. Intense pressure (> 10 GPa) and moderate to high temperatures (700-1200 °C) are required to form these minerals.

Shock metamorphism can also produce glass. Tektites are gravel-size glass grains ejected during an impact event. They resemble volcanic glass but, unlike volcanic glass, tektites contain no water or phenocrysts, and have a different bulk and isotopic chemistry. Tektites contain partially melted inclusions of shocked mineral grains. Although all are melt glasses, tektites are also chemically distinct from trinitite, which is produced from thermonuclear detonations, and fulgurites, which are produced by lightning strikes. All geologic glasses not derived from volcanoes can be called with the general term pseudotachylytes, a name which can also be applied to glasses created by faulting. The term pseudo in this context means ‘false’ or ‘in the appearance of’, a volcanic rock called tachylite because the material observed looks like a volcanic rock, but is produced by significant shear heating.

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Summary

Metamorphism is the process that changes existing rocks (called protoliths) into new rocks with new minerals and new textures. Increases in temperature and pressure are the main causes of metamorphism, with fluids adding important mobilization of materials. The primary way metamorphic rocks are identified is with texture. Foliated textures come from platy minerals forming planes in a rock, while non-foliated metamorphic rocks have no internal fabric. Grade describes the amount of metamorphism in a rock, and facies are a set of minerals that can help guide an observer to an interpretation of the metamorphic history of a rock. Different tectonic or geologic environments cause metamorphism, including collisions, subduction, faulting, and even impacts from space.

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References

Bucher, K., and Grapes, R., 2011, Petrogenesis of metamorphic rocks: Springer, 341 p.

Jeong, I.-K., Heffner, R.H., Graf, M.J., and Billinge, S.J.L., 2003, Lattice dynamics and correlated atomic motion from the atomic pair distribution function: Phys. Rev. B Condens. Matter, v. 67, no. 10, p. 104301.

Marshak, S., 2009, Essentials of Geology, 3rd or 4th Edition:

Proctor, B.P., McAleer, R., Kunk, M.J., and Wintsch, R.P., 2013, Post-Taconic tilting and Acadian structural overprint of the classic Barrovian metamorphic gradient in Dutchess County, New York: Am. J. Sci., v. 313, no. 7, p. 649–682.

Timeline of Art History, 2007, Reference Reviews, v. 21, no. 8, p. 45–45.

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7 Geologic Time

KEY CONCEPTS

Explain the difference between relative time and numeric time

Describe the five principles of stratigraphy

Apply relative dating principles to a block diagram and interpret the sequence of geologic events

Define an isotope, and explain alpha decay, beta decay, and electron capture as mechanisms of radioactive decay

Describe how radioisotopic dating is accomplished and list the four key isotopes used

Explain how carbon-14 forms in the atmosphere and how it is used in dating recent events

Explain how scientists know the numeric age of the Earth and other events in Earth history

Explain how sedimentary sequences can be dated using radioisotopes and other techniques

Define a fossil and describe types of fossils preservation

Outline how natural selection takes place as a mechanism of evolution

Describe stratigraphic correlation

List the eons, eras, and periods of the geologic time scale and explain the purpose behind the divisions

Explain the relationship between time units and corresponding rock units—chronostratigraphy versus lithostratigraphy

The geologic time scale and basic outline of Earth’s history were worked out long before we had any scientific means of assigning numerical age units, like years, to events of Earth history. Working out Earth’s history depended on realizing some key principles of relative time. Nicolas Steno (1638-1686) introduced basic principles of stratigraphy, the study of layered rocks, in 1669. William Smith (1769-1839), working with the strata of English coal mines, noticed that strata and their sequence were consistent throughout the region. Eventually he produced the first national geologic map of Britain,becoming known as “the Father of English Geology.” Nineteenth-century scientists developed a relative time scale using Steno’s principles, with names derived from the characteristics of the rocks in those areas. The figure of this geologic time scale shows the names of the units and subunits. Using this time scale, geologists can place all events of Earth history in order without ever knowing their numerical ages. The specific events within Earth history are discussed in Chapter 8.

7.1 Relative Dating

Relative dating is the process of determining if one rock or geologic event is older or younger than another, without knowing their specific ages—i.e., how many years ago the object was formed. The principles of relative time are simple, even obvious now, but were not generally accepted by scholars until the scientific revolution of the 17th and 18th centuries. James Hutton (see Chapter 1) realized geologic processes are slow and his ideas on uniformitarianism (i.e., “the present is the key to the past”) provided a basis for interpreting rocks of the Earth using scientific principles.

7.1.1 Relative Dating Principles

Stratigraphyis the study of layered sedimentary rocks. This section discusses principles of relative time used in all of geology, but are especially useful in stratigraphy.

Principle of Superposition: In an otherwise undisturbed sequence of sedimentary strata, or rock layers, the layers on the bottom are the oldest and layers above them are younger.

Principle of Original Horizontality: Layers of rocks deposited from above, such as sediments and lava flows, are originally laid down horizontally. The exception to this principle is at the margins of basins, where the strata can slope slightly downward into the basin.

Principle of Lateral Continuity: Within the depositional basin, strata are continuous in all directions until they thin out at the edge of that basin. Of course, all strata eventually end, either by hitting a geographic barrier, such as a ridge, or when the depositional process extends too far from its source, either a sediment source or a volcano. Strata that are cut by a canyon later remain continuous on either side of the canyon.

Principle of Cross-Cutting Relationships: Deformation events like folds, faults and igneous intrusions that cut across rocks are younger than the rocks they cut across.

Principle of Inclusions: When one rock formation contains pieces or inclusions of another rock, the included rock is older than the host rock.

Principle of Fossil Succession:Evolution has produced a succession of unique fossils that correlate to the units of the geologic time scale. Assemblages of fossils contained in strata are unique to the time they lived, and can be used to correlate rocks of the same age across a wide geographic distribution. Assemblages of fossils refers to groups of several unique fossils occurring together.

7.1.2 Grand Canyon Example

The Grand Canyon of Arizona illustrates the stratigraphic principles. The photo shows layers of rock on top of one another in order, from the oldest at the bottom to the youngest at the top, based on the principle of superposition. The predominant white layer just below the canyon rim is the Coconino Sandstone. This layer is laterally continuous, even though the intervening canyon separates its outcrops. The rock layers exhibit the principle of lateral continuity, as they are found on both sides of the Grand Canyon which has been carved by the Colorado River.

The diagram called “Grand Canyon’s Three Sets of Rocks” shows a cross-section of the rocks exposed on the walls of the Grand Canyon, illustrating the principle of cross-cutting relationships, superposition, and original horizontality. In the lowest parts of the Grand Canyon are the oldest sedimentary formations, with igneous and metamorphic rocks at the bottom. The principle of cross-cutting relationships shows the sequence of these events. The metamorphic schist (#16) is the oldest rock formation and the cross-cutting granite intrusion (#17) is younger. As seen in the figure, the other layers on the walls of the Grand Canyon are numbered in reverse order with #15 being the oldest and #1 the youngest. This illustrates the principle of superposition. The Grand Canyon region lies in Colorado Plateau, which is characterized by horizontal or nearly horizontal strata, which follows the principle of original horizontality. These rock strata have been barely disturbed from their original deposition, except by a broad regional uplift.

The photo of the Grand Canyon here show strata that were originally deposited in a flat layer on top of older igneous and metamorphic “basement” rocks, per the original horizontality principle. Because the formation of the basement rocks and the deposition of the overlying strata is not continuous but broken by events of metamorphism, intrusion, and erosion, the contact between the strata and the older basement is termed an unconformity. An unconformity represents a period during which deposition did not occur or erosion removed rock that had been deposited, so there are no rocks that represent events of Earth history during that span of time at that place. Unconformities appear in cross sections and stratigraphic columns as wavy lines between formations. Unconformities are discussed in the next section.

7.1.3 Unconformities

There are three types of unconformities, nonconformity, disconformity, and angular unconformity. A nonconformity occurs when sedimentary rock is deposited on top of igneous and metamorphic rocks as is the case with the contact between the strata and basement rocks at the bottom of the Grand Canyon.

The strata in the Grand Canyon represent alternating marine transgressions and regressions where sea level rose and fell over millions of years. When the sea level was high marine strata formed. When sea-level fell, the land was exposed to erosion creating an unconformity. In the Grand Canyon cross-section, this erosion is shown as heavy wavy lines between the various numbered strata. This is a type of unconformity called a disconformity, where either non-deposition or erosion took place. In other words, layers of rock that could have been present, are absent. The time that could have been represented by such layers is instead represented by the disconformity. Disconformities are unconformities that occur between parallel layers of strata indicating either a period of no deposition or erosion.

The Phanerozoic strata in most of the Grand Canyon are horizontal. However, near the bottom horizontal strata overlie tilted strata. This is known as the Great Unconformity and is an example of an angular unconformity. The lower strata were tilted by tectonic processes that disturbed their original horizontality and caused the strata to be eroded. Later, horizontal strata were deposited on top of the tilted strata creating the angular unconformity.

Here are three graphical illustrations of the three types of unconformity.

Disconformity, where is a break or stratigraphic absence between strata in an otherwise parallel sequence of strata.

Nonconformity, where sedimentary strata are deposited on crystalline (igneous or metamorphic) rocks.

Angular unconformity, where sedimentary strata are deposited on a terrain developed on sedimentary strata that have been deformed by tilting, folding, and/or faulting. so that they are no longer horizontal.

7.1.3 Applying Relative Dating Principles

In the block diagram, the sequence of geological events can be determined by using the relative-dating principles and known properties of igneous, sedimentary, metamorphic rock (see Chapter 4, Chapter 5, and Chapter 6). The sequence begins with the folded metamorphic gneiss on the bottom. Next, the gneiss is cut and displaced by the fault labeled A. Both the gneiss and fault A are cut by the igneous granitic intrusion called batholith B; its irregular outline suggests it is an igneous granitic intrusion emplaced as magma into the gneiss. Since batholith B cuts both the gneiss and fault A, batholith B is younger than the other two rock formations. Next, the gneiss, fault A, and batholith B were eroded forming a nonconformity as shown with the wavy line. This unconformity was actually an ancient landscape surface on which sedimentary rock C was subsequently deposited perhaps by a marine transgression. Next, igneous basaltic dike D cut through all rocks except sedimentary rock E. This shows that there is a disconformity between sedimentary rocks C and E. The top of dike D is level with the top of layer C, which establishes that erosion flattened the landscape prior to the deposition of layer E, creating a disconformity between rocks D and E. Fault F cuts across all of the older rocks B, C and E, producing a fault scarp, which is the low ridge on the upper-left side of the diagram. The final events affecting this area are current erosion processes working on the land surface, rounding off the edge of the fault scarp, and producing the modern landscape at the top of the diagram.

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7.2 Absolute Dating

Relative time allows scientists to tell the story of Earth events, but does not provide specific numeric ages, and thus, the rate at which geologic processes operate. Based on Hutton’s principle of uniformitarianism (see Chapter 1), early geologists surmised geological processes work slowly and the Earth is very old. Relative dating principles was how scientists interpreted Earth history until the end of the 19th Century. Because science advances as technology advances, the discovery of radioactivity in the late 1800s provided scientists with a new scientific tool called radioisotopic dating. Using this new technology, they could assign specific time units, in this case years, to mineral grains within a rock. These numerical values are not dependent on comparisons with other rocks such as with relative dating, so this dating method is called absolute dating. There are several types of absolute dating discussed in this section but radioisotopic dating is the most common and therefore is the focus on this section.

7.2.1 Radioactive Decay

All elements on the Periodic Table of Elements (see Chapter 3) contain isotopes. An isotope is an atom of an element with a different number of neutrons. For example, hydrogen (H) always has 1 proton in its nucleus (the atomic number), but the number of neutrons can vary among the isotopes (0, 1, 2). Recall that the number of neutrons added to the atomic number gives the atomic mass. When hydrogen has 1 proton and 0 neutrons it is sometimes called protium (1H), when hydrogen has 1 proton and 1 neutron it is called deuterium (2H), and when hydrogen has 1 proton and 2 neutrons it is called tritium (3H).

Many elements have both stable and unstable isotopes. For the hydrogen example, 1H and 2H are stable, but 3H is unstable. Unstable isotopes, called radioactiveisotopes, spontaneously decay over time releasing subatomic particles or energy in a process called radioactive decay. When this occurs, an unstable isotope becomes a more stable isotope of another element. For example, carbon-14 (14C) decays to nitrogen-14 (14N).

The radioactive decay of any individual atom is a completely unpredictable and random event. However, some rock specimens have an enormous number of radioactive isotopes, perhaps trillions of atoms, and this large group of radioactive isotopes does have a predictable pattern of radioactive decay. The radioactive decay of half of the radioactive isotopes in this group takes a specific amount of time. The time it takes for half of the atoms in a substance to decay is called the half-life. In other words, the half-life of an isotope is the amount of time it takes for half of a group of unstable isotopes to decay to a stable isotope. The half-life is constant and measurable for a given radioactive isotope, so it can be used to calculate the age of a rock. For example, the half-life uranium-238 (238U) is 4.5 billion years and the half-life of 14C is 5,730 years.

The principles behind this dating method require two key assumptions. First, the mineral grains containing the isotope formed at the same time as the rock, such as minerals in an igneous rock that crystallized from magma. Second, the mineral crystals remain a closed system, meaning they are not subsequently altered by elements moving in or out of them.

These requirements place some constraints on the kinds of rock suitable for dating, with igneous rock being the best. Metamorphic rocks are crystalline, but the processes of metamorphism may reset the clock and derived ages may represent a smear of different metamorphic events rather than the age of original crystallization. Detrital sedimentary rocks contain clasts from separate parent rocks from unknown locations and derived ages are thus meaningless. However, sedimentary rocks with precipitated minerals, such as evaporites, may contain elements suitable for radioisotopic dating. Igneous pyroclastic layers and lava flows within a sedimentary sequence can be used to date the sequence. Cross-cutting igneous rocks and sills can be used to bracket the ages of affected, older sedimentary rocks. The resistant mineral zircon, found as clasts in many ancient sedimentary rocks, has been successfully used for establishing very old dates, including the age of Earth’s oldest known rocks. Knowing that zircon minerals in metamorphosed sediments came from older rocks that are no longer available for study, scientists can date zircon to establish the age of the pre-metamorphic source rocks.

There are several ways radioactive atoms decay. We will consider three of them here—alpha decay, beta decay, and electron capture. Alpha decay is when an alpha particle, which consists of two protons and two neutrons, is emitted from the nucleus of an atom. This also happens to be the nucleus of a helium atom; helium gas may get trapped in the crystal lattice of a mineral in which alpha decay has taken place. When an atom loses two protons from its nucleus, lowering its atomic number, it is transformed into an element that is two atomic numbers lower on the Periodic Table of the Elements.

The loss of four particles, in this case two neutrons and two protons, also lowers the mass of the atom by four. For example alpha decay takes place in the unstable isotope 238U, which has an atomic number of 92 (92 protons) and mass number of 238 (total of all protons and neutrons). When 238U spontaneously emits an alpha particle, it becomes thorium-234 (234Th). The radioactive decay product of an element is called its daughter isotope and the original element is called the parent isotope. In this case, 238U is the parent isotope and 234Th is the daughter isotope. The half-life of 238U is 4.5 billion years, i.e., the time it takes for half of the parent isotope atoms to decay into the daughter isotope. This isotope of uranium, 238U, can be used for absolute dating the oldest materials found on Earth, and even meteorites and materials from the earliest events in our solar system.

B

Beta decay is when a neutron in its nucleus splits into an electron and a proton. The electron is emitted from the nucleus as a beta ray. The new proton increases the element’s atomic number by one, forming a new element with the same atomic mass as the parent isotope. For example, 234Th is unstable and undergoes beta decay to form protactinium-234 (234Pa), which also undergoes beta decay to form uranium-234 (234U). Notice these are all isotopes of different elements but they have the same atomic mass of 234. The decay process of radioactive elements like uranium keeps producing radioactive parents and daughters until a stable, or non-radioactive, daughter is formed. Such a series is called a decay chain. The decay chain of the radioactive parent isotope 238U progresses through a series of alpha (red arrows on the adjacent figure) and beta decays (blue arrows), until it forms the stable daughter isotope, lead-206 (206Pb).

Electron capture is when a proton in the nucleus captures an electron from one of the electron shells and becomes a neutron. This produces one of two different effects: 1) an electron jumps in to fill the missing spot of the departed electron and emits an X-ray, or 2) in what is called the Auger process, another electron is released and changes the atom into an ion. The atomic number is reduced by one and mass number remains the same. An example of an element that decays by electron capture is potassium-40 (40K). Radioactive 40K makes up a tiny percentage (0.012%) of naturally occurring potassium, most of which not radioactive. 40K decays to argon-40 (40Ar) with a half-life of 1.25 billion years, so it is very useful for dating geological events. Below is a table of some of the more commonly-used radioactive dating isotopes and their half-lives.

Elements	Parent symbol	Daughter symbol	Half-life
Uranium-238/Lead-206	238U	206Pb	4.5 billion years
Uranium-235/Lead-207	235U	207Pb	704 million years
Potassium-40/Argon-40	40K	40Ar	1.25 billion years
Rubidium-87/Strontium-87	87Rb	87Sr	48.8 billion years
Carbon-14/Nitrogen-14	14C	14N	5,730 years

Some common isotopes used for radioisotopic dating.

7.2.2 Radioisotopic Dating

For a given a sample of rock, how is the dating procedure carried out? The parent and daughter isotopes are separated out of the mineral using chemical extraction. In the case of uranium, 238U and 235U isotopes are separated out together, as are the 206Pb and 207Pb with an instrument called a mass spectrometer.

Here is a simple example of age calculation using the daughter-to-parent ratio of isotopes. When the mineral initially forms, it consists of 0% daughter and 100% parent isotope, so the daughter-to-parent ratio (D/P) is 0. After one half-life, half the parent has decayed so there is 50% daughter and 50% parent, a 50/50 ratio, with D/P = 1. After two half-lives, there is 75% daughter and 25% parent (75/25 ratio) and D/P = 3. This can be further calculated for a series of half-lives as shown in the table. The table does not show more than 10 half-lives because after about 10 half-lives, the amount of remaining parent is so small it becomes too difficult to accurately measure via chemical analysis. Modern applications of this method have achieved remarkable accuracies of plus or minus two million years in 2.5 billion years (that’s ±0.055%). Applying the uranium/lead technique in any given sample analysis provides two separate clocks running at the same time, 238U and 235U. The existence of these two clocks in the same sample gives a cross-check between the two. Many geological samples contain multiple parent/daughter pairs, so cross-checking the clocks confirms that radioisotopic dating is highly reliable.

Half-lives (#)	Parent present (%)	Daughter present (%)	Daughter/ Parent ratio	Parent/ Daughter ratio
Start the clock	100	0	0	infinite
1	50	50	1	1
2	25	75	3	0.33
3	12.5	87.5	7	0.143
4	6.25	93.75	15	0.0667
5	3.125	96.875	31	0.0325
10	0.098	99.9	1023	0.00098

Ratio of parent to daughter in terms of half-life.

Another radioisotopic dating method involves carbon and is useful for dating archaeologically important samples containing organic substances like wood or bone. Radiocarbon dating, also called carbon dating, uses the unstable isotope carbon-14 (14C) and the stable isotope carbon-12 (12C). Carbon-14 is constantly being created in the atmosphere by the interaction of cosmic particles with atmospheric nitrogen-14 (14N). Cosmic particles such as neutrons strike the nitrogen nucleus, kicking out a proton but leaving the neutron in the nucleus. The collision reduces the atomic number by one, changing it from seven to six, changing the nitrogen into carbon with the same mass number of 14. The 14C quickly bonds with oxygen (O) in the atmosphere to form carbon dioxide (14CO2) that mixes with other atmospheric carbon dioxide (12CO2) and this mix of gases is incorporated into living matter. While an organism is alive, the ratio of 14C/12C in its body doesn’t really change since CO2 is constantly exchanged with the atmosphere. However, when it dies, the radiocarbon clock starts ticking as the 14C decays back to 14N by beta decay, which has a half-life of 5,730 years. The radiocarbon dating technique is thus useful for 57,300 years or so, about 10 half-lives back.

Radiocarbon dating relies on daughter-to-parent ratios derived from a known quantity of parent 14C. Early applications of carbon dating assumed the production and concentration of 14C in the atmosphere remained fairly constant for the last 50,000 years. However, it is now known that the amount of parent 14C levels in the atmosphere has varied. Comparisons of carbon ages with tree-ring data and other data for known events have allowed reliable calibration of the radiocarbon dating method. Taking into account carbon-14 baseline levels must be calibrated against other reliable dating methods, carbon dating has been shown to be a reliable method for dating archaeological specimens and very recent geologic events.

7.2.3 Age of the Earth

The work of Hutton and other scientists gained attention after the Renaissance (see Chapter 1), spurring exploration into the idea of an ancient Earth. In the late 19th century William Thompson, a.k.a. Lord Kelvin, applied his knowledge of physics to develop the assumption that the Earth started as a hot molten sphere. He estimated the Earth is 98 million years old, but because of uncertainties in his calculations stated the age as a range of between 20 and 400 million years. This animation illustrates how Kelvin calculated this range and why his numbers were so far off, which has to do with unequal heat transfer within the Earth. It has also been pointed out that Kelvin failed to consider pliability and convection in the Earth’s mantle as a heat transfer mechanism. Kelvin’s estimate for Earth’s age was considered plausible but not without challenge, and the discovery of radioactivity provided a more accurate method for determining ancient ages.

In the 1950’s, Clair Patterson (1922–1995) thought he could determine the age of the Earth using radioactive isotopes from meteorites, which he considered to be early solar system remnants that were present at the time Earth was forming. Patterson analyzed meteorite samples for uranium and lead using a mass spectrometer. He used the uranium/lead dating technique in determining the age of the Earth to be 4.55 billion years, give or take about 70 million (± 1.5%). The current estimate for the age of the Earth is 4.54 billion years, give or take 50 million (± 1.1%). It is remarkable that Patterson, who was still a graduate student at the University of Chicago, came up with a result that has been little altered in over 60 years, even as technology has improved dating methods.

7.2.4 Dating Geological Events

Radioactiveisotopes of elements that are common in mineral crystals are useful for radioisotopic dating. The uranium/lead method, with its two cross-checking clocks, is most often used with crystals of the mineral zircon (ZrSiO4) where uranium can substitute for zirconium in the crystal lattice. Zircon is resistant to weathering which makes it useful for dating geological events in ancient rocks. During metamorphic events, zircon crystals may form multiple crystal layers, with each layer recording the isotopic age of an event, thus tracing the progress of the several metamorphic events.

Geologists have used zircon grains to do some amazing studies that illustrate how scientific conclusions can change with technological advancements. Zircon crystals from Western Australia that formed when the crust first differentiated from the mantle 4.4 billion years ago have been determined to be the oldest known rocks. The zircon grains were incorporated into metasedimentary host rocks, sedimentary rocks showing signs of having undergone partial metamorphism. The host rocks were not very old but the embedded zircon grains were created 4.4 billion years ago, and survived the subsequent processes of weathering, erosion, deposition, and metamorphism. From other properties of the zircon crystals, researchers concluded that not only were continental rocks exposed above sea level, but also that conditions on the early Earth were cool enough for liquid water to exist on the surface. The presence of liquid water allowed the processes of weathering and erosion to take place. Researchers at UCLA studied 4.1 billion-year-old zircon crystals and found carbon in the zircon crystals that may be biogenic in origin, meaning that life may have existed on Earth much earlier than previously thought.

Igneous rocks best suited for radioisotopic dating because their primary minerals provide dates of crystallization from magma. Metamorphic processes tend to reset the clocks and smear the igneous rock’s original date. Detrital sedimentary rocks are less useful because they are made of minerals derived from multiple parent sources with potentially many dates. However, scientists can use igneous events to date sedimentary sequences. For example, if sedimentary strata are between a lava flow and volcanic ash bed with radioisotopic dates of 54 million years and 50 million years, then geologists know the sedimentary strata and its fossils formed between 54 and 50 million years ago. Another example would be a 65 million year old volcanic dike that cut across sedimentary strata. This provides an upper limit age on the sedimentary strata, so this strata would be older than 65 million years. Potassium is common in evaporite sediments and has been used for potassium/argon dating. Primary sedimentary minerals containing radioactive isotopes like 40K, has provided dates for important geologic events.

7.2.5 Other Absolute Dating Techniques

Luminescence (aka Thermoluminescence): Radioisotopic dating is not the only way scientists determine numeric ages. Luminescence dating measures the time elapsed since some silicate minerals, such as coarse-sediments of silicate minerals, were last exposed to light or heat at the surface of Earth. All buried sediments are exposed to radiation from normal background radiation from the decay process described above. Some of these electrons get trapped in the crystal lattice of silicate minerals like quartz. When exposed at the surface, ultraviolet radiation and heat from the Sun releases these electrons, but when the minerals are buried just a few inches below the surface, the electrons get trapped again. Samples of coarse sediments collected just a few feet below the surface are analyzed by stimulating them with light in a lab. This stimulation releases the trapped electrons as a photon of light which is called luminescence. The amount luminescence released indicates how long the sediment has been buried. Luminescence dating is only useful for dating young sediments that are less than 1 million years old. In Utah, luminescence dating is used to determine when coarse-grained sediment layers were buried near a fault. This is one technique used to determine the recurrence interval of large earthquakes on faults like the Wasatch Fault that primarily cut coarse-grained material and lack buried organic soils for radiocarbon dating.

Fission Track: Fission track dating relies on damage to the crystal lattice produced when unstable 238U decays to the daughter product 234Th and releases an alpha particle. These two decay products move in opposite directions from each other through the crystal lattice leaving a visible track of damage. This is common in uranium-bearing mineral grains such as apatite. The tracks are large and can be visually counted under an optical microscope. The number of tracks correspond to the age of the grains. Fission track dating works from about 100,000 to 2 billion (1 × 105 to 2 × 109) years ago. Fission track dating has also been used as a second clock to confirm dates obtained by other methods.

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7.3 Fossils and Evolution

Fossils are any evidence of past life preserved in rocks. They may be actual remains of body parts (rare), impressions of soft body parts, casts and molds of body parts (more common), body parts replaced by mineral (common) or evidence of animal behavior such as footprints and burrows. The body parts of living organisms range from the hard bones and shells of animals, soft cellulose of plants, soft bodies of jellyfish, down to single cells of bacteria and algae. Which body parts can be preserved? The vast majority of life today consists soft-bodied and/or single celled organisms, and will not likely be preserved in the geologic record except under unusual conditions. The best environment for preservation is the ocean, yet marine processes can dissolve hard parts and scavenging can reduce or eliminate remains. Thus, even under ideal conditions in the ocean, the likelihood of preservation is quite limited. For terrestrial life, the possibility of remains being buried and preserved is even more limited. In other words, the fossil record is incomplete and records only a small percentage of life that existed. Although incomplete, fossil records are used for stratigraphic correlation, using the Principle of Faunal Succession, and provide a method used for establishing the age of a formation on the Geologic Time Scale.

7.3.1 Types of Preservation

Remnants or impressions of hard parts, such as a marine clam shell or dinosaur bone, are the most common types of fossils. The original material has almost always been replaced with new minerals that preserve much of the shape of the original shell, bone, or cell. The common types of fossil preservation are actual preservation, permineralization, molds and casts, carbonization, and trace fossils.

Actual preservationis a rare form of fossilization where the original materials or hard parts of the organism are preserved. Preservation of soft-tissue is very rare since these organic materials easily disappear because of bacterial decay. Examples of actual preservation are unaltered biological materials like insects in amber or original minerals like mother-of-pearl on the interior of a shell. Another example is mammoth skin and hair preserved in post-glacial deposits in the Arctic regions. Rare mummification has left fragments of soft tissue, skin, and sometimes even blood vessels of dinosaurs, from which proteins have been isolated and evidence for DNA fragments have been discovered.

Permineralization occurs when an organism is buried, and then elements in groundwater completely impregnate all spaces within the body, even cells. Soft body structures can be preserved in great detail, but stronger materials like bone and teeth are the most likely to be preserved. Petrified wood is an example of detailed cellulose structures in the wood being preserved. The University of California Berkeley website has more information on permineralization.

Molds and casts form when the original material of the organism dissolves and leaves a cavity in the surrounding rock. The shape of this cavity is an external mold. If the mold is subsequently filled with sediments or a mineral precipitate, the organism’s external shape is preserved as a cast. Sometimes internal cavities of organisms, such internal casts of clams, snails, and even skulls are preserved as internal casts showing details of soft structures. If the chemistry is right, and burial is rapid, mineral nodules form around soft structures preserving the three-dimensional detail. This is called authigenic mineralization.

Carbonization occurs when the organic tissues of an organism are compressed, the volatiles are driven out, and everything but the carbon disappears leaving a carbon silhouette of the original organism. Leaf and fern fossils are examples of carbonization.

Trace fossils are indirect evidence left behind by an organism, such as burrows and footprints, as it lived its life. Ichnology is specifically the study of prehistoric animal tracks. Dinosaur tracks testify to their presence and movement over an area, and even provide information about their size, gait, speed, and behavior. Burrows dug by tunneling organisms tell of their presence and mode of life. Other trace fossils include fossilized feces called coprolites and stomach stones called gastroliths that provide information about diet and habitat.

7.3.2 Evolution

Evolution has created a variety of ancient fossils that are important to stratigraphic correlation. (see chapter 7 and Chapter 5) This section is a brief discussion of the process of evolution. The British naturalist Charles Darwin (1809-1882) recognized that life forms evolve into progeny life forms. He proposed natural selection—which operated on organisms living under environmental conditions that posed challenges to survival—was the mechanism driving the process of evolution forward.

The basic classification unit of life is the species: a population of organisms that exhibit shared characteristics and are capable of reproducing fertile offspring. For a species to survive, each individual within a particular population is faced with challenges posed by the environment and must survive them long enough to reproduce. Within the natural variations present in the population, there may be individuals possessing characteristics that give them some advantage in facing the environmental challenges. These individuals are more likely to reproduce and pass these favored characteristics on to successive generations. If sufficient individuals in a population fail to surmount the challenges of the environment and the population cannot produce enough viable offspring, the species becomes extinct. The average lifespan of a species in the fossil record is around a million years. That life still exists on Earth shows the role and importance of evolution as a natural process in meeting the continual challenges posed by our dynamic Earth. If the inheritance of certain distinctive characteristics is sufficiently favored over time, populations may become genetically isolated from one another, eventually resulting in the evolution of separate species. This genetic isolation may also be caused by a geographic barrier, such as an island surrounded by ocean. This theory of evolution by natural selection was elaborated by Darwin in his book On the Origin of Species (see Chapter 1). Since Darwin’s original ideas, technology has provided many tools and mechanisms to study how evolution and speciation take place and this arsenal of tools is growing. Evolution is well beyond the hypothesis stage and is a well-established theory of modern science.

Variation within populations occurs by the natural mixing of genes through sexual reproduction or from naturally occurring mutations. Some of this genetic variation can introduce advantageous characteristics that increase the individual’s chances of survival. While some species in the fossil record show little morphological change over time, others show gradual or punctuated changes, within which intermediate forms can be seen.

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7.4 Correlation

Correlation is the process of establishing which sedimentary strata are of the same age but geographically separate. Correlation can be determined by using magnetic polarity reversals (Chapter 2), rock types, unique rock sequences, or index fossils. There are four main types of correlation: stratigraphic, lithostratigraphic, chronostratigraphic, and biostratigraphic.

7.4.1 Stratigraphic Correlation

Stratigraphic correlation is the process of establishing which sedimentary strata are the same age at distant geographical areas by means of their stratigraphic relationship. Geologists construct geologic histories of areas by mapping and making stratigraphic columns-a detailed description of the strata from bottom to top. An example of stratigraphic relationships and correlation between Canyonlands National Park and Zion National Park in Utah. At Canyonlands, the Navajo Sandstone overlies the Kayenta Formation which overlies the cliff-forming Wingate Formation. In Zion, the Navajo Sandstone overlies the Kayenta formation which overlies the cliff-forming Moenave Formation. Based on the stratigraphic relationship, the Wingate and Moenave Formations correlate. These two formations have unique names because their composition and outcrop pattern is slightly different. Other strata in the Colorado Plateau and their sequence can be recognized and correlated over thousands of square miles.

7.4.2 Lithostratigraphic Correlation

Lithostratigraphic correlation establishes a similar age of strata based on the lithology that is the composition and physical properties of that strata. Lithos is Greek for stone and -logy comes from the Greek word for doctrine or science. Lithostratigraphic correlation can be used to correlate whole formations long distances or can be used to correlate smaller strata within formations to trace their extent and regional depositional environments.

For example, the Navajo Sandstone, which makes up the prominent walls of Zion National Park, is the same Navajo Sandstone in Canyonlands because the lithology of the two are identical even though they are hundreds of miles apart. Extensions of the same Navajo Sandstone formation are found miles away in other parts of southern Utah, including Capitol Reef and Arches National Parks. Further, this same formation is the called the Aztec Sandstone in Nevada and Nugget Sandstone near Salt Lake City because they are lithologically distinct enough to warrant new names.

7.4.3 Chronostratigraphic Correlation

Chronostratigraphic correlation matches rocks of the same age, even though they are made of different lithologies. Different lithologies of sedimentary rocks can form at the same time at different geographic locations because depositional environments vary geographically. For example, at any one time in a marine setting there could be this sequence of depositional environments from beach to deep marine: beach, near shore area, shallow marine lagoon, reef, slope, and deep marine. Each depositional environment will have a unique sedimentary rock formation. On the figure of the Permian Capitan Reef at Guadalupe National Monument in West Texas, the red line shows a chronostratigraphic time line that represents a snapshot in time. Shallow-water marine lagoon/back reef area is light blue, the main Capitan reef is dark blue, and deep-water marine siltstone is yellow. All three of these unique lithologies were forming at the same time in Permian along this red timeline.

7.4.4 Biostratigraphic Correlation

Biostratigraphic correlation uses index fossils to determine strata ages. Index fossils represent assemblages or groups of organisms that were uniquely present during specific intervals of geologic time. Assemblages is referring a group of fossils. Fossils allow geologists to assign a formation to an absolute date range, such as the Jurassic Period (199 to 145 million years ago), rather than a relative time scale. In fact, most of the geologic time ranges are mapped to fossil assemblages. The most useful index fossils come from lifeforms that were geographically widespread and had a species lifespan that was limited to a narrow time interval. In other words, index fossils can be found in many places around the world, but only during a narrow time frame. Some of the best fossils for biostratigraphic correlation are microfossils, most of which came from single-celled organisms. As with microscopic organisms today, they were widely distributed across many environments throughout the world. Some of these microscopic organisms had hard parts, such as exoskeletons or outer shells, making them better candidates for preservation. Foraminifera, single celled organisms with calcareous shells, are an example of an especially useful index fossil for the Cretaceous Period and Cenozoic Era.

Conodonts are another example of microfossils useful for biostratigraphic correlation of the Cambrian through Triassic Periods. Conodonts are tooth-like phosphatic structures of an eel-like multi-celled organism that had no other preservable hard parts. The conodont-bearing creatures lived in shallow marine environments all over the world. Upon death, the phosphatic hard parts were scattered into the rest of the marine sediments. These distinctive tooth-like structures are easily collected and separated from limestone in the laboratory.

Because the conodont creatures were so widely abundant, rapidly evolving, and readily preserved in sediments, their fossils are especially useful for correlating strata, even though knowledge of the actual animal possessing them is sparse. Scientists in the 1960s carried out a fundamental biostratigraphic correlation that tied Triassic conodont zonation into ammonoids, which are extinct ancient cousins of the pearly nautilus. Up to that point ammonoids were the only standard for Triassic correlation, so cross-referencing micro- and macro-index fossils enhanced the reliability of biostratigraphic correlation for either type. That conodont study went on to establish the use of conodonts to internationally correlate Triassicstrata located in Europe, Western North America, and the Arctic Islands of Canada.

7.4.5 Geologic Time Scale

Geologic time has been subdivided into a series of divisions by geologists. Eon is the largest division of time, followed by era, period, epoch, and age. The partitions of the geologic time scale is the same everywhere on Earth; however, rocks may or may not be present at a given location depending on the geologic activity going on during a particular period of time. Thus, we have the concept of time vs. rock, in which time is an unbroken continuum but rocks may be missing and/or unavailable for study. The figure of the geologic time scale, represents time flowing continuously from the beginning of the Earth, with the time units presented in an unbroken sequence. But that does not mean there are rocks available for study for all of these time units.

The geologic time scale was developed during the 19th century using the principles of stratigraphy. The relative order of the time units was determined before geologist had the tools to assign numerical ages to periods and events. Biostratigraphic correlation using fossils to assign era and period names to sedimentary rocks on a worldwide scale. With the expansion of science and technology, some geologists think the influence of humanity on natural processes has become so great they are suggesting a new geologic time period, known as the Anthropocene.

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Chapter summary

Events in Earth history can be placed in sequence using the five principles of relative dating. The geologic time scale was completely worked out in the 19th Century using these principles without knowing any actual numeric ages for the events. The discovery of radioactivity in the late 1800s enabled absolute dating, the assignment of numerical ages to events in the Earth’s history, using decay of unstable radioactive isotopes. Accurately interpreting radioisotopic dating data depends on the type of rock tested and accurate assumptions about isotope baseline values. With a combination of relative and absolute dating, the history of geological events, age of Earth, and a geologic time scale have been determined with considerable accuracy. Stratigraphic correlation is additional tool used for understanding how depositional environments change geographically. Geologic time is vast, providing plenty of time for the evolution of various lifeforms, and some of these have become preserved as fossils that can be used for biostratigraphic correlation. The geologic time scale is continuous, although the rock record may be broken because rocks representing certain time periods may be missing.

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Mosher, L.C., 1968, Triassic conodonts from Western North America and Europe and Their Correlation: J. Paleontol., v. 42, no. 4, p. 895–946.

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Valley, J.W., Peck, W.H., King, E.M., and Wilde, S.A., 2002, A cool early Earth: Geology, v. 30, no. 4, p. 351–354.

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Winchester, S., 2009, The Map That Changed the World: William Smith and the Birth of Modern Geology: HarperCollins.

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8 Earth History

KEY CONCEPTS

By the end of this chapter, students should be able to:

Explain the big-bang theory and origin of the elements

Explain the solar system’s origin and the consequences for Earth.

Describe the turbulent beginning of Earth during the Hadean and Archean Eons

Identify the transition to modern atmosphere, plate tectonics, and evolution that occurred in the Proterozoic Eon

Describe the Paleozoic evolution and extinction of invertebrates with hard parts, fish, amphibians, reptiles, tetrapods, and land plants; and tectonics and sedimentation associated with the supercontinent Pangea

Describe the Mesozoic evolution and extinction of birds, dinosaurs, and mammmals; and tectonics and sedimentation associated with the breakup of Pangea

Describe the Cenozoic evolution of mammals and birds, paleoclimate, and tectonics that shaped the modern world

Entire courses and careers have been based on the wide-ranging topics covering Earth’s history. Throughout the long history of Earth, change has been the norm. Looking back in time, an untrained eye would see many unfamiliar life forms and terrains. The main topics studied in Earth history are paleogeography, paleontology, and paleoecology and paleoclimatology—respectively, past landscapes, past organisms, past ecosystems, and past environments. This chapter will cover briefly the origin of the universe and the 4.6 billion year history of Earth. This Earth history will focus on the major physical and biological events in each Eons and Era.

8.1 Origin of the Universe

The universe appears to have an infinite number of galaxies and solar systems and our solar system occupies a small section of this vast entirety. The origins of the universe and solar system set the context for conceptualizing the Earth’s origin and early history.

8.1.1 Big-Bang Theory

The mysterious details of events prior to and during the origin of the universe are subject to great scientific debate. The prevailing idea about how the universe was created is called the big-bang theory. Although the ideas behind the big-bang theory feel almost mystical, they are supported by Einstein’s theory of general relativity. Other scientific evidence, grounded in empirical observations, supports the big-bang theory.

The big-bang theory proposes the universe was formed from an infinitely dense and hot core of material. The bang in the title suggests there was an explosive, outward expansion of all matter and space that created atoms. Spectroscopy confirms that hydrogen makes up about 74% of all matter in the universe. Since its creation, the universe has been expanding for 13.8 billion years and recent observations suggest the rate of this expansion is increasing.

Spectroscopy

Spectroscopy is the investigation and measurement of spectra produced when materials interacts with or emits electromagnetic radiation. Spectra is the plural for spectrum which is a particular wavelength from the electromagnetic spectrum. Common spectra include the different colors of visible light, X-rays, ultraviolet waves, microwaves, and radio waves. Each beam of light is a unique mixture of wavelengths that combine across the spectrum to make the color we see. The light wavelengths are created or absorbed inside atoms, and each wavelength signature matches a specific element. Even white light from the Sun, which seems like an uninterrupted continuum of wavelengths, has gaps in some wavelengths. The gaps correspond to elements present in the Earth’s atmosphere that act as filters for specific wavelengths. These missing wavelengths were famously observed by Joseph von Fraunhofer (1787–1826) in the early 1800s, but it took decades before scientists were able to relate the missing wavelengths to atmospheric filtering. Spectroscopy shows that the Sun is mostly made of hydrogen and helium. Applying this process to light from distant stars, scientists can calculate the abundance of elements in a specific star and visible universe as a whole. Also, this spectroscopic information can be used as an interstellar speedometer.

Redshift

The Doppler effect is the same process that changes the pitch of the sound of an approaching car or ambulance from high to low as it passes. When an object emits waves, such as light or sound, while moving toward an observer, the wavelengths get compressed. In sound, this results in a shift to a higher pitch. When an object moves away from an observer, the wavelengths are extended, producing a lower pitched sound. The Doppler effect is used on light emitted from stars and galaxies to determine their speed and direction of travel. Scientists, including Vesto Slipher (1875–1969)and Edwin Hubble (1889–1953), examined galaxies both near and far and found that almost all galaxies outside of our galaxy are moving away from each other, and us. Because the light wavelengths of receding objects are extended, visible light is shifted toward the red end of the spectrum, called a redshift. In addition, Hubble noticed that galaxies that were farther away from Earth also had the greater amount of redshift, and thus, the faster they are traveling away from us. The only way to reconcile this information is to deduce the universe is still expanding. Hubble’s observation forms the basis of big-bang theory.

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Cosmic Microwave Background Radiation

Another strong indication of the big-bang is cosmic microwave background radiation. Cosmic radiation was accidentally discovered by Arno Penzias (1933–) and Robert Woodrow Wilson (1936–) when they were trying to eliminate background noise from a communication satellite. They discovered very faint traces of energy or heat that are omnipresent across the universe. This energy was left behind from the big bang, like an echo.

8.1.2 Stellar Evolution

Astronomers think the big bang created lighter elements, mostly hydrogen and smaller amounts of elements helium, lithium, and beryllium. Another process must be responsible for creating the other 90 heavier elements. The current model of stellar evolution explains the origins of these heavier elements.

Birth of a star

Stars start their lives as elements floating in cold, spinning clouds of gas and dust known as nebulas. Gravitational attraction or perhaps a nearby stellar explosion causes the elements to condense and spin into disk shape. In the center of this disk shape a new star is born under the force of gravity. The spinning whirlpool concentrates material in the center, and the increasing gravitational forces collect even more mass. Eventually, the immensely concentrated mass of material reaches a critical point of such intense heat and pressure it initiates fusion.

Fusion

Fusion is not a chemical reaction. Fusion is a nuclear reaction in which two or more nuclei, the centers of atoms, are forced together and combine creating a new larger atom. This reaction gives off a tremendous amount of energy, usually as light and solar radiation. An element such as hydrogen combines or fuses with other hydrogen atoms in the core of a star to become a new element, in this case, helium. Another product of this process is energy, such as solar radiation that leaves the Sun and comes to the Earth as light and heat. Fusion is a steady and predictable process, which is why we call this the main phase of a star’s life. During its main phase, a star turns hydrogen into helium. Since most stars contain plentiful amounts of hydrogen, the main phase may last billions of years, during which their size and energy output remains relatively steady.

The giant phase in a star’s life occurs when the star runs out of hydrogen for fusion. If a star is large enough, it has sufficient heat and pressure to start fusing helium into heavier elements. This style of fusion is more energetic and the higher energy and temperature expand the star to a larger size and brightness. This giant phase is predicted to happen to our Sun in another few billion years, growing the radius of the Sun to Earth’s orbit, which will render life impossible. The mass of a star during its main phase is the primary factor in determining how it will evolve. If the star has enough mass and reaches a point at which the primary fusion element, such as helium, is exhausted, fusion continues using new, heavier elements. This occurs over and over in very large stars, forming progressively heavier elements like carbon and oxygen. Eventually, fusion reaches its limit as it forms iron and nickel. This progression explains the abundance of iron and nickel in rocky objects, like Earth, within the solar system. At this point, any further fusion absorbs energy instead of giving it off, which is the beginning of the end of the star’s life.

Death of a Star

The death of a star can range from spectacular to other-worldly (see figure). Stars like the Sun form a planetary nebula, which comes from the collapse of the star’s outer layers in an event like the implosion of a building. In the tug-of-war between gravity’s inward pull and fusion’s outward push, gravity instantly takes over when fusion ends, with the outer gasses puffing away to form a nebula. More massive stars do this as well but with a more energetic collapse, which starts another type of energy release mixed with element creation known as a supernova. In a supernova, the collapse of the core suddenly halts, creating a massive outward-propagating shock wave. A supernova is the most energetic explosion in the universe short of the big bang. The energy release is so significant the ensuing fusion can make every element up through uranium.

The death of the star can result in the creation of white dwarfs, neutron stars, or black holes. Following their deaths, stars like the Sun turn into white dwarfs.

White dwarfs are hot star embers, formed by packing most of a dying star’s mass into a small and dense object about the size of Earth. Larger stars may explode in a supernova that packs their mass even tighter to become neutron stars. Neutron stars are so dense that protons combine with electrons to form neutrons. The largest stars collapse their mass even further, becoming objects so dense that light cannot escape their gravitational grasp. These are the infamous black holes and the details of the physics of what occurs in them are still up for debate.

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8.2 Origin of the Solar System: The Nebular Hypothesis

Our solar system formed at the same time as our Sun as described in the nebular hypothesis. The nebular hypothesis is the idea that a spinning cloud of dust made of mostly light elements, called a nebula, flattened into a protoplanetary disk, and became a solar system consisting of a star with orbiting planets. The spinning nebula collected the vast majority of material in its center, which is why the sun Accounts for over 99% of the mass in our solar system.

8.2.1 Planet Arrangement and Segregation

As our solar system formed, the nebular cloud of dispersed particles developed distinct temperature zones. Temperatures were very high close to the center, only allowing condensation of metals and silicate minerals with high melting points. Farther from the Sun, the temperatures were lower, allowing the condensation of lighter gaseous molecules such as methane, ammonia, carbon dioxide, and water. This temperature differentiation resulted in the inner four planets of the solar system becoming rocky, and the outer four planets becoming gas giants.

Both rocky and gaseous planets have a similar growth model. Particles of dust, floating in the disc were attracted to each other by static charges and eventually, gravity. As the clumps of dust became bigger, they interacted with each other—colliding, sticking, and forming proto-planets. The planets continued to grow over the course of many thousands or millions of years, as material from the protoplanetary disc was added. Both rocky and gaseous planets started with a solid core. Rocky planets built more rock on that core, while gas planets added gas and ice. Ice giants formed later and on the furthest edges of the disc, accumulating less gas and more ice. That is why the gas-giant planets Jupiter and Saturn are composed of mostly hydrogen and helium gas, more than 90%. The ice giants Uranus and Neptune are composed of mostly methane ices and only about 20% hydrogen and helium gases.

The planetary composition of the gas giants is clearly different from the rocky planets. Their size is also dramatically different for two reasons: First, the original planetary nebula contained more gases and ices than metals and rocks. There was abundant hydrogen, carbon, oxygen, nitrogen, and less silicon and iron, giving the outer planets more building material. Second, the stronger gravitational pull of these giant planets allowed them to collect large quantities of hydrogen and helium, which could not be collected by weaker gravity of the smaller planets.

Jupiter’s massive gravity further shaped the solar system and growth of the inner rocky planets. As the nebula started to coalesce into planets, Jupiter’s gravity accelerated the movement of nearby materials, generating destructive collisions rather than constructively gluing material together. These collisions created the asteroid belt, an unfinished planet, located between Mars and Jupiter. This asteroid belt is the source of most meteorites that currently impact the Earth. Study of asteroids and meteorites help geologist to determine the age of Earth and the composition of its core, mantle, and crust. Jupiter’s gravity may also explain Mars’ smaller mass, with the larger planet consuming material as it migrated from the inner to outer edge of the solar system.

Pluto and planet definition

The outermost part of the solar system is known as the Kuiper belt, which is a scattering of rocky and icy bodies. Beyond that is the Oort cloud, a zone filled with small and dispersed ice traces. These two locations are where most comets form and continue to orbit, and objects found here have relatively irregular orbits compared to the rest of the solar system. Pluto, formerly the ninth planet, is located in this region of space. The XXVIth General Assembly of the International Astronomical Union (IAU) stripped Pluto of planetary status in 2006 because scientists discovered an object more massive than Pluto, which they named Eris. The IAU decided against including Eris as a planet, and therefore, excluded Pluto as well. The IAU narrowed the definition of a planet to three criteria: 1) enough mass to have gravitational forces that force it to be rounded, 2) not massive enough to create fusion, and 3) large enough to be in a cleared orbit, free of other planetesimals that should have been incorporated at the time the planet formed. Pluto passed the first two parts of the definition, but not the third. Pluto and Eris are currently classified as dwarf planets.

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8.3 Hadean Eon

Geoscientists use the geological time scale to assign relative age names to events and rocks, separating major events in Earth’s history based on significant changes as recorded in rocks and fossils. This section summarizes the most notable events of each major time interval. For a breakdown on how these time intervals are chosen and organized, see chapter 7.

The Hadean Eon, named after the Greek god and ruler of the underworld Hades, is the oldest eon and dates from 4.5–4.0 billion years ago.

This time represents Earth’s earliest history, during which the planet was characterized by a partially molten surface, volcanism, and asteroid impacts. Several mechanisms made the newly forming Earth incredibly hot: gravitational compression, radioactive decay, and asteroid impacts. Most of this initial heat still exists inside the Earth. The Hadean was originally defined as the birth of the planet occurring 4.0 billion years ago and preceding the existence of many rocks and life forms. However, geologists have dated minerals at 4.4 billion years, with evidence that liquid water was present. There is possibly even evidence of life existing over 4.0 billion years ago. However, the most reliable record for early life, the microfossil record, starts at 3.5 billion years ago.

8.3.1 Origin of Earth’s Crust

As Earth cooled from its molten state, minerals started to crystallize and settle resulting in a separation of minerals based on density and the creation of the crust, mantle, and core. The earliest Earth was chiefly molten material and would have been rounded by gravitational forces so it resembled a ball of lava floating in space. As the outer part of the Earth slowly cooled, the high melting-point minerals (see Bowen’s Reaction Series in Chapter 4) formed solid slabs of early crust. These slabs were probably unstable and easily reabsorbed into the liquid magma until the Earth cooled enough to allow numerous larger fragments to form a thin primitive crust. Scientists generally assume this crust was oceanic and mafic in composition, and littered with impacts, much like the Moon’s current crust. There is still some debate over when plate tectonics started, which would have led to the formation of continental and felsic crust. Regardless of this, as Earth cooled and solidified, less dense felsic minerals floated to the surface of the Earth to form the crust, while the denser mafic and ultramafic materials sank to form the mantle and the highest-density iron and nickel sank into the core. This differentiated the Earth from a homogenous planet into a heterogeneous one with layers of felsic crust, mafic crust, ultramafic mantle, and iron and nickel core.

8.3.2 Origin of the Moon

Several unique features of Earth’s Moon have prompted scientists to develop the current hypothesis about its formation. The Earth and Moon are tidally locked, meaning that as the Moon orbits, one side always faces the Earth and the opposite side is not visible to us. Also and most importantly, the chemical compositions of the Earth and Moon show nearly identical isotope ratiosand volatile content. Apollo missions returned from the Moon with rocks that allowed scientists to conduct very precise comparisons between Moon and Earth rocks. Other bodies in the solar system and meteorites do not share the same degree of similarity and show much higher variability. If the Moon and Earth formed together, this would explain why they are so chemically similar.

Many ideas have been proposed for the origin of the Moon: The Moon could have been captured from another part of the solar system and formed in place together with the Earth, or the Moon could have been ripped out of the early Earth. None of proposed explanations can account for all the evidence. The currently prevailing hypothesis is the giant-impact hypothesis. It proposes a body about half of Earth’s size must have shared at least parts of Earth’s orbit and collided with it, resulting in a violent mixing and scattering of material from both objects. Both bodies would be composed of a combination of materials, with more of the lower density splatter coalescing into the Moon. This may explain why the Earth has a higher density and thicker core than the Moon.

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Computer simulation of the evolution of the Moon (2 minutes).

8.3.3 Origin of Earth’s Water

Explanations for the origin of Earth’s water include volcanic outgassing, comets, and meteorites. The volcanic outgassing hypothesis for the origin of Earth’s water is that it originated from inside the planet, and emerged via tectonic processes as vapor associated with volcanic eruptions. Since all volcanic eruptions contain some water vapor, at times more than 1% of the volume, these alone could have created Earth’s surface water. Another likely source of water was from space. Comets are a mixture of dust and ice, with some or most of that ice being frozen water. Seemingly dry meteors can contain small but measurable amounts of water, usually trapped in their mineral structures. During heavy bombardment periods later in Earth’s history, its cooled surface was pummeled by comets and meteorites, which could be why so much water exists above ground. There isn’t a definitive answer for what process is the source of ocean water. Earth’s water isotopically matches water found in meteorites much better than that of comets. However, it is hard to know if Earth processes could have changed the water’s isotopic signature over the last 4-plus billion years. It is possible that all three sources contributed to the origin of Earth’s water.

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8.4 Archean Eon

The ArcheanEon, which lasted from 4.0–2.5 billion years ago, is named after the Greek word for beginning. This eon represents the beginning of the rock record. Although there is current evidence that rocks and minerals existed during the Hadean Eon, the Archean has a much more robust rock and fossil record.

8.4.1 Late Heavy Bombardment

Objects were chaotically flying around at the start of the solar system, building the planets and moons. There is evidence that after the planets formed, about 4.1–3.8 billion years ago, a second large spike of asteroid and comet impacted the Earth and Moon in an event called late heavy bombardment. Meteorites and comets in stable or semi-stable orbits became unstable and started impacting objects throughout the solar system. In addition, this event is called the lunar cataclysm because most of the Moons craters are from this event. During late heavy bombardment, the Earth, Moon, and all planets in the solar system were pummeled by material from the asteroid and Kuiper belts. Evidence of this bombardment was found within samples collected from the Moon.

It is universally accepted that the solar system experienced extensive asteroid and comet bombardment at its start; however, some other process must have caused the second increase in impacts hundreds of millions of years later. A leading theory blames gravitational resonance between Jupiter and Saturn for disturbing orbits within the asteroid and Kuiper belts based on a similar process observed in the Eta Corvi star system.

8.4.2 Origin of the Continents

In order for plate tectonics to work as it does currently, it necessarily must have continents. However, the easiest way to create continental material is via assimilation and differentiation of existing continents (see Chapter 4). This chicken-and-egg quandary over how continents were made in the first place is not easily answered because of the great age of continental material and how much evidence has been lost during tectonics and erosion. While the timing and specific processes are still debated, volcanic action must have brought the first continental material to the Earth’s surface during the Hadean, 4.4 billion years ago. This model does not solve the problem of continentformation, since magmatic differentiation seems to need thicker crust. Nevertheless, the continents formed by some incremental process during the early history of Earth. The best idea is that density differences allowed lighter felsic materials to float upward and heavier ultramafic materials and metallic iron to sink. These density differences led to the layering of the Earth, the layers that are now detected by seismic studies. Early protocontinents accumulated felsic materials as developing plate–tectonic processes brought lighter material from the mantle to the surface.

The first solid evidence of modern plate tectonics is found at the end of the Archean, indicating at least some continental lithosphere must have been in place. This evidence does not necessarily mark the starting point of plate tectonics; remnants of earlier tectonic activity could have been erased by the rock cycle.

The stable interiors of the current continents are called cratons and were mostly formed in the Archean Eon. A craton has two main parts: the shield, which is crystalline basement rock near the surface, and the platform made of sedimentary rocks covering the shield. Most cratons have remained relatively unchanged with most tectonic activity having occurred around cratons instead of within them. Whether they were created by plate tectonics or another process, Archean continents gave rise to the Proterozoic continents that now dominate our planet.

The general guideline as to what constitutes a continent and differentiates oceanic from continental crust is under some debate. At passive margins, continental crust grades into oceanic crust at passive margins, making a distinction difficult. Even island-arc and hot-spot material can seem more closely related to continental crust than oceanic. Continents usually have a craton in the middle with felsic igneous rocks. There is evidence that submerged masses like Zealandia, that includes present-day New Zealand, would be considered a continent. Continental crust that does not contain a craton is called a continental fragment, such as the island of Madagascar off the east coast of Africa.

8.4.3 First Life on Earth

Life most likely started during the late Hadean or early Archean Eons. The earliest evidence of life are chemical signatures, microscopic filaments, and microbial mats. Carbon found in 4.1 billion year old zircon grains have a chemical signature suggesting an organic origin. Other evidence of early life are 3.8–4.3 billion-year-old microscopic filaments from a hydrothermal vent deposit in Quebec, Canada. While the chemical and microscopic filaments evidence is not as robust as fossils, there is significant fossil evidence for life at 3.5 billion years ago. These first well-preserved fossils are photosynthetic microbial mats, called stromatolites, found in Australia.

Although the origin of life on Earth is unknown, hypotheses include a chemical origin in the early atmosphere and ocean, deep-sea hydrothermal vents, and delivery to Earth by comets or other objects. One hypothesis is that life arose from the chemical environment of the Earth’s early atmosphere and oceans, which was very different than today. The oxygen-free atmosphere produced a reducing environment with abundant methane, carbon dioxide, sulfur, and nitrogen compounds. This is what the atmosphere is like on other bodies in the solar system. In the famous Miller-Urey experiment, researchers simulated early Earth’s atmosphere and lightning within a sealed vessel. After igniting sparks within the vessel, they discovered the formation of amino acids, the fundamental building blocks of proteins. In 1977, when scientists discovered an isolated ecosystem around hydrothermal vents on a deep-sea mid-ocean ridge (see Chapter 4), it opened the door for another explanation of the origin of life. The hydrothermal vents have a unique ecosystem of critters with chemosynthesis as the foundation of the food chain instead of photosynthesis. The ecosystem is deriving its energy from hot chemical-rich waters pouring out of underground towers. This suggests that life could have started on the deep ocean floor and derived energy from the heat from the Earth’s interior via chemosynthesis. Scientists have since expanded the search for life to more unconventional places, like Jupiter’s icy moon Europa.

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Animation of the original Miller-Urey 1959 experiment that simulated the early atmosphere and created amino acids from simple elements and compounds.

Another possibility is that life or its building blocks came to Earth from space, carried aboard comets or other objects. Amino acids, for example, have been found within comets and meteorites. This intriguing possibility also implies a high likelihood of life existing elsewhere in the cosmos.

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8.5 Proterozoic Eon

The ProterozoicEon, meaning “earlier life,” comes after the Archean Eon and ranges from 2.5 billion to 541 million years old. During this time, most of the central parts of the continents had formed and plate tectonic processes had started. Photosynthesis by microbial organisms, such as single-celled cyanobacteria, had been slowly adding oxygen to the oceans. As cyanobacteria evolved into multicellular organisms, they completely transformed the oceans and later the atmosphere by adding massive amounts of free oxygen gas (O2) and initiated what is called the Great Oxygenation Event (GOE). This drastic environmental change decimated the anaerobic bacteria, which could not survive in the presence of free oxygen. On the other hand, aerobic organisms could thrive in ways they could not earlier.

An oxygenated world also changed the chemistry of the planet in significant ways. For example, iron remained in solution in the non-oxygenated environment of the earlier Archean Eon. In chemistry, this is known as a reducing environment. Once the environment was oxygenated, iron combined with free oxygen to form solid precipitates of iron oxide, such as the mineral hematite or magnetite. These precipitates accumulated into large mineral deposits with red chert known as banded-iron formations, which are dated at about 2 billion years.

The formation of iron oxide minerals and red chert (see figure) in the oceans lasted a long time and prevented oxygen levels from increasing significantly, since precipitation took the oxygen out of the water and deposited it into the rock strata. As oxygen continued to be produced and mineral precipitation leveled off, dissolved oxygen gas eventually saturated the oceans and started bubbling out into the atmosphere. Oxygenation of the atmosphere is the single biggest event that distinguishes the Archean and Proterozoic environments. In addition to changing mineral and ocean chemistry, the GOE is also tabbed as triggering Earth’s first glaciation event around 2.1 billion years ago, the Huron Glaciation. Free oxygen reacted with methane in the atmosphere to produce carbon dioxide. Carbon dioxide and methane are called greenhouse gases because they trap heat within the Earth’s atmosphere, like the insulated glass of a greenhouse. Methane is a more effective insulator than carbon dioxide, so as the proportion of carbon dioxide in the atmosphere increased, the greenhouse effect decreased, and the planet cooled.

8.5.1 Rodinia

By the Proterozoic Eon, lithospheric plates had formed and were moving according to plate tectonic forces that were similar to current times. As the moving plates collided, the ocean basins closed to form a supercontinent called Rodinia. The supercontinent formed about 1 billion years ago and broke up about 750 to 600 million years ago, at the end of the Proterozoic. One of the resulting fragments was a continental mass called Laurentia that would later become North America. Geologists have reconstructed Rodinia by matching and aligning ancient mountain chains, assembling the pieces like a jigsaw puzzle, and using paleomagnetics to orient to magnetic north.

The disagreements over these complex reconstructions is exemplified by geologists proposing at least six different models for the breakup of Rodinia to create Australia, Antarctica, parts of China, the Tarim craton north of the Himalaya, Siberia, or the Kalahari craton of eastern Africa. This breakup created lots of shallow-water, biologically favorable environments that fostered the evolutionary breakthroughs marking the start of the next eon, the Phanerozoic.

8.5.2 Life Evolves

Early life in the Archean and earlier is poorly documented in the fossil record. Based on chemical evidence and evolutionary theory, scientists propose this life would have been single-celled photosynthetic organisms, such as the cyanobacteria that created stromatolites. Cyanobacteria produced free oxygen in the atmosphere through photosynthesis. Cyanobacteria, archaea, and bacteria are prokaryotes—primitive organisms made of single cells that lack cell nuclei and other organelles.

A large evolutionary step occurred during the Proterozoic Eon with the appearance of eukaryotes around 2.1 to 1.6 billion years ago. Eukaryotic cells are more complex, having nuclei and organelles. The nuclear DNA is capable of more complex replication and regulation than that of prokaryotic cells. The organelles include mitochondria for producing energy and chloroplasts for photosynthesis. The eukaryote branch in the tree of life gave rise to fungi, plants, and animals.

Another important event in Earth’s biological history occurred about 1.2 billion years ago when eukaryotes invented sexual reproduction. Sharing genetic material from two reproducing individuals, male and female, greatly increased genetic variability in their offspring. This genetic mixing accelerated evolutionary change, contributing to more complexity among individual organisms and within ecosystems (see Chapter 7).

Proterozoic land surfaces were barren of plants and animals and geologic processes actively shaped the environment differently because land surfaces were not protected by leafy and woody vegetation. For example, rain and rivers would have caused erosion at much higher rates on land surfaces devoid of plants. This resulted in thick accumulations of pure quartz sandstone from the Proterozoic Eon such as the extensive quartzite formations in the core of the Uinta Mountains in Utah.

Fauna during the Ediacaran Period, 635.5 to 541 million years ago are known as the Ediacaran fauna, and offer a first glimpse at the diversity of ecosystems that evolved near the end of the Proterozoic. These soft-bodied organisms were among the first multicellular life forms and probably were similar to jellyfish or worm-like. Ediacaran fauna did not have hard parts like shells and were not well preserved in the rock records. However, studies suggest they were widespread in the Earth’s oceans. Scientists still debate how many species were evolutionary dead-ends that became extinct and how many were ancestors of modern groupings. The transition of soft-bodied Ediacaran life to life forms with hard body parts occurred at the end of the Proterozoic and beginning of the Phanerozoic Eons. This evolutionary explosion of biological diversity made a dramatic difference in scientists’ ability to understand the history of life on Earth.

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8.6 Phanerozoic Eon: Paleozoic Era

The Phanerozoic Eon is the most recent, 541 million years ago to today, and means “visible life” because the Phanerozoic rock record is marked by an abundance of fossils. Phanerozoic organisms had hard body parts like claws, scales, shells, and bones that were more easily preserved as fossils. Rocks from the older Precambrian time are less commonly found and rarely include fossils because these organisms had soft body parts. Phanerozoic rocks are younger, more common, and contain the majority of extant fossils. The study of rocks from this eon yields much greater detail. The Phanerozoic is subdivided into three eras, from oldest to youngest they are Paleozoic (“ancient life”), Mesozoic (“middle life”), and Cenozoic (“recent life”) and the remaining three chapter headings are on these three important eras.

Life in the early PaleozoicEra was dominated by marine organisms but by the middle of the era plants and animals evolved to live and reproduce on land. Fish evolved jaws and fins evolved into jointed limbs. The development of lungs allowed animals to emerge from the sea and become the first air-breathing tetrapods (four-legged animals) such as amphibians. From amphibians evolved reptiles with the amniotic egg. From reptiles evolved an early ancestor to birds and mammals and their scales became feathers and fur. Near the end of the Paleozoic Era, the Carboniferous Period had some of the most extensive forests in Earth’s history. Their fossilized remains became the coal that powered the industrial revolution

8.6.1 Paleozoic Tectonics and Paleogeography

During the Paleozoic Era, sea-levels rose and fell four times. With each sea-level rise, the majority of North America was covered by a shallow tropical ocean. Evidence of these submersions are the abundant marine sedimentary rocks such as limestone with fossils corals and ooids. Extensive sea-level falls are documented by widespread unconformities. Today, the midcontinent has extensive marine sedimentary rocks from the Paleozoic and western North America has thick layers of marine limestone on block faulted mountain ranges such as Mt. Timpanogos near Provo, Utah.

The assembly of supercontinent Pangea, sometimes spelled Pangaea, was completed by the late Paleozoic Era. The name Pangea was originally coined by Alfred Wegener and means “all land.” Pangea is the when all of the major continents were grouped together as one by a series of tectonic events including subduction island-arc accretion, and continental collisions, and ocean-basin closures. In North America, these tectonic events occurred on the east coast and are known as the Taconic, Acadian, Caledonian, and Alleghanian orogenies.The Appalachian Mountains are the erosional remnants of these mountain building events in North America. Surrounding Pangea was a global ocean basin known as the Panthalassa. Continued plate movement extended the ocean into Pangea, forming a large bay called the Tethys Sea that eventually divided the land mass into two smaller supercontinents, Laurasia and Gondwana. Laurasia consisted of Laurentia and Eurasia, and Gondwana consisted of the remaining continents of South America, Africa, India, Australia, and Antarctica.

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Animation of plate movement the last 3.3 billion years. Pangea occurs at the 4:40 mark.

While the east coast of North America was tectonically active during the Paleozoic Era, the west coast remained mostly inactive as a passive margin during the early Paleozoic. The western edge of North American continent was near the present-day Nevada-Utah border and was an expansive shallow continental shelf near the paleoequator. However, by the Devonian Period, the Antler orogeny started on the west coast and lasted until the Pennsylvanian Period. The Antler orogeny was a volcanic island arc that was accreted onto western North America with the subduction direction away from North America. This created a mountain range on the west coast of North American called the Antler highlands and was the first part of building the land in the west that would eventually make most of California, Oregon, and Washington states. By the late Paleozoic, the Sonoma orogeny began on the west coast and was another collision of an island arc. The Sonoma orogeny marks the change in subduction direction to be toward North America with a volcanic arc along the entire west coast of North America by late Paleozoic to early Mesozoic Eras.

By the end of the Paleozoic Era, the east coast of North America had a very high mountain range due to continental collision and the creation of Pangea. The west coast of North America had smaller and isolated volcanic highlands associated with island arc accretion. During the Mesozoic Era, the size of the mountains on either side of North America would flip, with the west coast being a more tectonically active plate boundary and the east coast changing into a passive margin after the breakup of Pangea.

8.6.2 Paleozoic Evolution

The beginning of the Paleozoic Era is marked by the first appearance of hard body parts like shells, spikes, teeth, and scales; and the appearance in the rock record of most animal phyla known today. That is, most basic animal body plans appeared in the rock record during the Cambrian Period. This sudden appearance of biological diversity is called the Cambrian Explosion. Scientists debate whether this sudden appearance is more from a rapid evolutionary diversification as a result of a warmer climate following the late Proterozoic glacial environments, better preservation and fossilization of hard parts, or artifacts of a more complete and recent rock record. For example, fauna may have been diverse during the Ediacaran Period, setting the state for the Cambrian Explosion, but they lacked hard body parts and would have left few fossils behind. Regardless, during the Cambrian Period 541–485 million years ago marked the appearance of most animal phyla.

One of the best fossil sites for the Cambrian Explosion was discovered in 1909 by Charles Walcott (1850–1927) in the Burgess Shale in western Canada. The Burgess Shale is a Lagerstätte, a site of exceptional fossil preservation that includes impressions of soft body parts. This discovery allowed scientists to study Cambrian animals in immense detail because soft body parts are not normally preserved and fossilized. Other Lagerstätte sites of similar age in China and Utah have allowed scientist to form a detailed picture of Cambrian biodiversity. The biggest mystery surrounds animals that do not fit existing lineages and are unique to that time. This includes many famous fossilized creatures: the first compound-eyed trilobites; Wiwaxia, a creature covered in spiny plates; Hallucigenia, a walking worm with spikes; Opabinia, a five-eyed arthropod with a grappling claw; and Anomalocaris, the alpha predator of its time, complete with grasping appendages and circular mouth with sharp plates. Most notably appearing during the Cambrian is an important ancestor to humans. A segmented worm called Pikaia is thought to be the earliest ancestor of the Chordata phylum that includes vertebrates, animals with backbones.

By the end of the Cambrian, mollusks, brachiopods, nautiloids, gastropods, graptolites, echinoderms, and trilobites covered the sea floor. Although most animal phyla appeared by the Cambrian, the biodiversity at the family, genus, and species level was low until the Ordovician Period. During the Great Ordovician Biodiversification Event, vertebrates and invertebrates (animals without backbone) became more diverse and complex at family, genus, and species level. The cause of the rapid speciation event is still debated but some likely causes are a combination of warm temperatures, expansive continental shelves near the equator, and more volcanism along the mid-ocean ridges. Some have shown evidence that an asteroid breakup event and consequent heavy meteorite impacts correlate with this diversification event. The additional volcanism added nutrients to ocean water helping support a robust ecosystem. Many life forms and ecosystems that would be recognizable in current times appeared at this time. Mollusks, corals, and arthropods in particular multiplied to dominate the oceans.

One important evolutionary advancement during the Ordovician Period was reef-building organisms, mostly colonial coral. Corals took advantage of the ocean chemistry, using calcite to build large structures that resembled modern reefs like the Great Barrier Reef off the coast of Australia. These reefs housed thriving ecosystems of organisms that swam around, hid in, and crawled over them. Reefs are important to paleontologists because of their preservation potential, massive size, and in-place ecosystems. Few other fossils offer more diversity and complexity than reef assemblages.

According to evidence from glacial deposits, a small ice age caused sea-levels to drop and led to a major mass extinction by the end of the Ordovician. This is the earliest of five mass extinction events documented in the fossil record. During this mass extinction, an unusually large number of species abruptly disappear in the fossil record (see video).

Life bounced back during the Silurian period. The major evolutionary event was the development of the forward pair of gill arches into jaws, allowing fish new feeding strategies and opening up new ecological niches.

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3-minute video describing mass extinctions and how they are defined.

Life bounced back during the Silurian period. The period’s major evolutionary event was the development of jaws from the forward pair of gill arches in bony fishes and sharks. Hinged jaws allowed fish to exploit new food sources and ecological niches. This period also included the start of armored fishes, known as the placoderms. In addition to fish and jaws, Silurian rocks provide the first evidence of terrestrial or land-dwelling plants and animals. The first vascular plant, Cooksonia, had woody tissues, pores for gas exchange, and veins for water and food transport. Insects, spiders, scorpions, and crustaceans began to inhabit moist, freshwater terrestrial environments.

The Devonian Period is called the Age of Fishes due to the rise in plated, jawed, and lobe-finned fishes . The lobe-finned fishes, which were related to the modern lungfish and coelacanth, are important for their eventual evolution into tetrapods, four-limbed vertebrate animals that can walk on land. The first lobe-finned land-walking fish, named Tiktaalik, appeared about 385 million years ago and serves as a transition fossil between fish and early tetrapods. Though Tiktaalik was clearly a fish, it had some tetrapod structures as well. Several fossils from the Devonian are more tetrapod like than fish like but these weren’t fully terrestrial. The first fully terrestrial tetrapod arrived in the Mississippian (early Carboniferous) period. By the Mississippian (early Carboniferous) period, tetrapods had evolved into two main groups, amphibians and amniotes, from a common tetrapod ancestor. The amphibians were able to breathe air and live on land but still needed water to nurture their soft eggs. The first reptile (an amniote) could live and reproduce entirely on land with hard-shelled eggs that wouldn’t dry out.

Land plants had also evolved into the first trees and forests. Toward the end of the Devonian, another mass extinction event occurred. This extinction, while severe, is the least temporally defined, with wide variations in the timing of the event or events. Reef building organisms were the hardest hit, leading to dramatic changes in marine ecosystems.

The next time period, called the Carboniferous (North American geologists have subdivided this into the Mississippian and Pennsylvanian periods), saw the highest levels of oxygen ever known, with forests (e.g., ferns, club mosses) and swamps dominating the landscape . This helped cause the largest arthropods ever, like the millipede Arthropleura, at 2.5 meters (6.4 feet) long! It also saw the rise of a new group of animals, the reptiles. The evolutionary advantage that reptiles have over amphibians is the amniote egg (egg with a protective shell), which allows them to rely on non-aquatic environments for reproduction. This widened the terrestrial reach of reptiles compared to amphibians. This booming life, especially plant life, created cooling temperatures as carbon dioxide was removed from the atmosphere. By the middle Carboniferous, these cooler temperatures led to an ice age (called the Karoo Glaciation) and less-productive forests. The reptiles fared much better than the amphibians, leading to their diversification. This glacial event lasted into the early Permian.

By the Permian, with Pangea assembled, the supercontinent led to a dryer climate, and even more diversification and domination by the reptiles. The groups that developed in this warm climate eventually radiated into dinosaurs. Another group, known as the synapsids, eventually evolved into mammals. Synapsids, including the famous sail-backed Dimetrodonare commonly confused with dinosaurs. Pelycosaurs (of the Pennsylvanian to early Permian like Dimetrodon) are the first group of synapsids that exhibit the beginnings of mammalian characteristics such as well-differentiated dentition: incisors, highly developed canines in lower and upper jaws and cheek teeth, premolars and molars. Starting in the late Permian, a second group of synapsids, called the therapsids (or mammal-like reptiles) evolve, and become the ancestors to mammals.

Permian Mass Extinction

The end of the Paleozoic era is marked by the largest mass extinction in earth history. The Paleozoic era had two smaller mass extinctions, but these were not as large as the Permian Mass Extinction, also known as the Permian-Triassic Extinction Event. It is estimated that up to 96% of marine species and 70% of land-dwelling (terrestrial) vertebrates went extinct. Many famous organisms, like sea scorpions and trilobites, were never seen again in the fossil record. What caused such a widespread extinction event? The exact cause is still debated, though the leading idea relates to extensive volcanism associated with the Siberian Traps, which are one of the largest deposits of flood basalts known on Earth, dating to the time of the extinction event. The eruption size is estimated at over 3 million cubic kilometersthat is approximately 4,000,000 times larger than the famous 1980 Mt. St. Helens eruption in Washington. The unusually large volcanic eruption would have contributed a large amount of toxic gases, aerosols, and greenhouse gasses into the atmosphere. Further, some evidence suggests that the volcanism burned vast coal deposits releasing methane (a greenhouse gas) into the atmosphere. As discussed in Chapter 15, greenhouse gases cause the climate to warm. This extensive addition of greenhouse gases from the Siberian Traps may have caused a runaway greenhouse effect that rapidly changed the climate, acidified the oceans, disrupted food chains, disrupted carbon cycling, and caused the largest mass extinction.

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8.7 Phanerozoic Eon: Mesozoic Era

Following the Permian Mass Extinction, the Mesozoic (“middle life”) was from 252 million years ago to 66 million years ago. As Pangea started to break apart, mammals, birds, and flowering plants developed. The Mesozoic is probably best known as the age of reptiles, most notably, the dinosaurs.

8.7.1 Mesozoic Tectonics and Paleogeography

Pangea started breaking up (in a region that would become eastern Canada and United States) around 210 million years ago in the Late Triassic. Clear evidence for this includes the age of the sediments in the Newark Supergroup rift basins and the Palisades sill of the eastern part of North America and the age of the Atlantic ocean floor. Due to sea-floor spreading, the oldest rocks on the Atlantic’s floor are along the coast of northern Africa and the east coast of North America, while the youngest are along the mid-ocean ridge.

This age pattern shows how the Atlantic Ocean opened as the young Mid-Atlantic Ridge began to create the seafloor. This means the Atlantic ocean started opening and was first formed here. The southern Atlantic opened next, with South America separating from central and southern Africa. Last (happening after the Mesozoic ended) was the northernmost Atlantic, with Greenland and Scandinavia parting ways. The breaking points of each rifted plate margin eventually turned into the passive plate boundaries of the east coast of the Americas today.

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Video of Pangea breaking apart and plates moving to their present locations. By Tanya Atwater.

In western North America, an active plate margin had started with subduction, controlling most of the tectonics of that region in the Mesozoic. Another possible island-arc collision created the Sonoman Orogeny in Nevada during the latest Paleozoic to the Triassic. In the Jurassic, another island-arc collision caused the Nevadan Orogeny, a large Andean-style volcanic arc and thrust belt. The Sevier Orogeny followed in the Cretaceous, which was mainly a volcanic arc to the west and a thin-skinned fold and thrust belt to the east, meaning stacks of shallow faults and folds built up the topography. Many of the structures in the Rocky Mountains today date from this orogeny.

Tectonics had an influence in one more important geographic feature in North America: the Cretaceous Western Interior Foreland Basin, which flooded during high sea levels forming the Cretaceous Interior Seaway. Subduction from the west was the Farallon Plate, an oceanic plate connected to the Pacific Plate (seen today as remnants such as the Juan de Fuca Plate, off the coast of the Pacific Northwest). Subduction was shallow at this time because a very young, hot and less dense portion of the Farallon plate was subducted. This shallow subduction caused a downwarping in the central part of North America. High sea levels due to shallow subduction, and increasing rates of seafloor spreading and subduction, high temperatures, and melted ice also contributed to the high sea levels. These factors allowed a shallow epicontinental seaway that extended from the Gulf of Mexico to the Arctic Ocean to divide North America into two separate land masses, Laramidia to the west and Appalachia to the east, for 25 million years. Many of the coal deposits in Utah and Wyoming formed from swamps along the shores of this seaway. By the end of the Cretaceous, cooling temperatures caused the seaway to regress.

8.7.2 Mesozoic Evolution

The Mesozoic era is dominated by reptiles, and more specifically, the dinosaurs. The Triassic saw devastated ecosystems that took over 30 million years to fully re-emerge after the Permian Mass Extinction. The first appearance of many modern groups of animals that would later flourish occurred at this time. This includes frogs (amphibians), turtles (reptiles), marine ichthyosaurs and plesiosaurs (marine reptiles), mammals, and the archosaurs. The archosaurs (“ruling reptiles”) include ancestral groups that went extinct at the end of the Triassic, as well as the flying pterosaurs, crocodilians, and the dinosaurs. Archosaurs, like the placental mammals after them, occupied all major environments: terrestrial (dinosaurs), in the air (pterosaurs), aquatic (crocodilians) and even fully marine habitats (marine crocodiles). The pterosaurs, the first vertebrate group to take flight, like the dinosaurs and mammals, start small in the Triassic.

At the end of the Triassic, another mass extinction event occurred, the fourth major mass extinction in the geologic record. This was perhaps caused by the Central Atlantic Magmatic Province flood basalt. The end-Triassic extinction made certain lineages go extinct and helped spur the evolution of survivors like mammals, pterosaurs (flying reptiles), ichthyosaurs/plesiosaurs/mosasaurs (marine reptiles), and dinosaurs.

Mammals, as previously mentioned, got their start from a reptilian synapsid ancestor possibly in the late Paleozoic. Mammals stayed small, in mainly nocturnal niches, with insects being their largest prey. The development of warm-blooded circulation and fur may have been a response to this lifestyle.

In the Jurassic, species that were previously common, flourished due to a warmer and more tropical climate. The dinosaurs were relatively small animals in the Triassic period of the Mesozoic, but became truly massive in the Jurassic. Dinosaurs are split into two groups based on their hip structure, i.e. orientation of the pubis and ischium bones in relationship to each other. This is referred to as the “reptile hipped” saurischians and the “bird hipped” ornithischians. This has recently been brought into question by a new idea for dinosaur lineage.

Most of the dinosaurs of the Triassic were saurischians, but all of them were bipedal. The major adaptive advantage dinosaurs had was changes in the hip and ankle bones, tucking the legs under the body for improved locomotion as opposed to the semi-erect gait of crocodiles or the sprawling posture of reptiles. In the Jurassic, limbs (or a lack thereof) were also important to another group of reptiles, leading to the evolution of Eophis, the oldest snake.

There is a paucity of dinosaur fossils from the Early and Middle Jurassic, but by the Late Jurassic they were dominating the planet. The saurischians diversified into the giant herbivorous (plant-eating) long-necked sauropods weighing up to 100 tons and bipedal carnivorous theropods, with the possible exception of the Therizinosaurs. All of the ornithischians (e.g Stegosaurus, Iguanodon, Triceratops, Ankylosaurus, Pachycephhlosaurus) were herbivorous with a strong tendency to have a “turtle-like” beak at the tips of their mouths.

The pterosaurs grew and diversified in the Jurassic, and another notable arial organism developed and thrived in the Jurassic: birds. When Archeopteryxwas found in the Solnhofen Lagerstätte of Germany, a seeming dinosaur-bird hybrid, it started the conversation on the origin of birds. The idea that birds evolved from dinosaurs occurred very early in the history of research into evolution, only a few years after Darwin’s On the Origin of Species. This study used a remarkable fossil of Archeopteryxfrom a transitional animal between dinosaurs and birds. Small meat-eating theropod dinosaurs were likely the branch that became birds due to their similar features. A significant debate still exists over how and when powered flight evolved. Some have stated a running-start model, while others have favored a tree-leaping gliding modelor even a semi-combination: flapping to aid in climbing.

The Cretaceous saw a further diversification, specialization, and domination of the dinosaurs and other fauna. One of the biggest changes on land was the transition to angiosperm-dominated flora. Angiosperms, which are plants with flowers and seeds, had originated in the Cretaceous, switching many plains to grasslands by the end of the Mesozoic. By the end of the period, they had replaced gymnosperms (evergreen trees) and ferns as the dominant plant in the world’s forests. Haplodiploid eusocial insects (bees and ants) are descendants from Jurassic wasp-like ancestors that co-evolved with the flowering plants during this time period. The breakup of Pangea not only shaped our modern world’s geography, but biodiversity at the time as well. Throughout the Mesozoic, animals on the isolated, now separated island continents (formerly parts of Pangea), took strange evolutionary turns. This includes giant titanosaurian sauropods (Argentinosaurus) and theropods (Giganotosaurus) from South America.

K-T Extinction

Similar to the end of the Paleozoic era, the Mesozoic Era ended with the K-Pg Mass Extinction (previously known as the K-T Extinction) 66 million years ago.This extinction event was likely caused by a large bolide(an extraterrestrial impactor such as an asteroid, meteoroid, or comet) that collided with earth. Ninety percent of plankton species, 75% of plant species, and all the dinosaurs went extinct at this time.

One of the strongest pieces of evidence comes from the element iridium. Quite rare on Earth, and more common in meteorites, it has been found all over the world in higher concentrations at a particular layer of rock that formed at the time of the K-T boundary. Soon other scientists started to find evidence to back up the claim. Melted rock spheres, a special type of “shocked” quartz called stishovite, that only is found at impact sites, was found in many places around the world . The huge impact created a strong thermal pulse that could be responsible for global forest fires, strong acid rains, a corresponding abundance of ferns, the first colonizing plants after a forest fire, enough debris thrown into the air to significantly cool temperatures afterward, and a 2-km high tsunami inferred from deposits found from Texas to Alabama.

Still, with all this evidence, one large piece remained missing: the crater where the bolide impacted. It was not until 1991 that the crater was confirmed using petroleum company geophysical data. Even though it is the third largest confirmed crater on Earth at roughly 180 km wide, the Chicxulub Craterwas hard to find due to being partially underwater and partially obscured by the dense forest canopy of the Yucatan Peninsula. Coring of the center of the impact called the peak ring contained granite, indicating the impact was so powerful that it lifted basement sediment from the crust several miles toward the surface. In 2010, an international team of scientists reviewed 20 years of research and blamed the impact for the extinction.

With all of this information, it seems like the case would be closed. However, there are other events at this time which could have partially aided the demise of so many organisms. For example, sea levels are known to be slowly decreasing at the time of the K-T event, which is tied to marine extinctions, though any study on gradual vs. sudden changes in the fossil record is flawed due to the incomplete nature of the fossil record. Another big event at this time was the Deccan Traps flood basaltvolcanism in India. At over 1.3 million cubic kilometers of material, it was certainly a large source of material hazardous to ecosystems at the time, and it has been suggested as at least partially responsible for the extinction. Some have found the impact and eruptions too much of a coincidence, and have even linked the two together.

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8.8 Phanerozoic Eon: Cenozoic Era

The Cenozoic, meaning “new life,” is known as the age of mammals because it is in this era that mammals came to be a dominant and large life form, including human ancestors. Birds, as well, flourished in the open niches left by the dinosaur’s demise. Most of the Cenozoic has been relatively warm, with the main exception being the ice age that started about 2.558 million years ago and (despite recent warming) continues today. Tectonic shifts in the west caused volcanism, but eventually changed the long-standing subduction zone into a transform boundary.

8.8.1 Cenozoic Tectonics and Paleogeography

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Animation of the last 38 million years of movement in western North America. Note, that after the ridge is subducted, convergent turns to transform (with divergent inland).

In the Cenozoic, the plates of the Earth moved into more familiar places, with the biggest change being the closing of the Tethys Sea with collisions such as the Alps, Zagros, and Himalaya, a collision that started about 57 million years ago, and continues today. Maybe the most significant tectonic feature that occurred in the Cenozoic of North America was the conversion of the west coast of California from a convergent boundary subduction zone to a transform boundary. Subduction off the coast of the western United States, which had occurred throughout the Mesozoic, had continued in the Cenozoic. After the Sevier Orogeny in the late Mesozoic, a subsequent orogeny called the Laramide Orogeny, occurred in the early Cenozoic. The Laramide was thick-skinned, different than the Sevier Orogeny. It involved deeper crustal rocks, and produced bulges that would become mountain ranges like the Rockies, Black Hills, Wind River Range, Uinta Mountains, and the San Rafael Swell. Instead of descending directly into the mantle, the subducting plate shallowed out and moved eastward beneath the continental plate affecting the overlying continent hundreds of miles east of the continental margin and building high mountains. This occurred because the subducting plate was so young and near the spreading center and the density of the plate was therefore low and subduction was hindered.

As the mid-ocean ridge itself started to subduct, the relative motion had changed. Subduction caused a relative convergence between the subducting Farallon plate and the North American plate. On the other side of the mid-ocean ridge from the Farallon plate was the Pacific plate, which was moving away from the North American plate. Thus, as the subduction zone consumed the mid-ocean ridge, the relative movement became transform instead of convergent, which went on to become the San Andreas Fault System. As the San Andreas grew, it caused east-west directed extensional forces to spread over the western United States, creating the Basin and Range province. The transform fault switched position over the last 18 million years, twisting the mountains around Los Angeles, and new faults in the southeastern California deserts may become a future San Andreas-style fault. During this switch from subduction to transform, the nearly horizontal Farallon slab began to sink into the mantle. This caused magmatism as the subducting slab sank, allowing asthenosphere material to rise around it. This event is called the Oligocene ignimbrite flare-up, which was one of the most significant periods of volcanism ever, including the largest single confirmed eruption, the 5000 cubic kilometer Fish Canyon Tuff.

8.8.2 Cenozoic Evolution

There are five groups of early mammals in the fossil record, based primarily on fossil teeth, the hardest bone in vertebrate skeletons. For the purpose of this text, the most important group are the Eupantotheres, that diverge into the two main groups of mammals, the marsupials (like Sinodelphys) and placentals or eutherians (like Eomaia) in the Cretaceous and then diversified in the Cenozoic. The marsupials dominated on the isolated island continents of South America and Australia, and many went extinct in South America with the introduction of placental mammals. Some well-known mammal groups have been highly studied with interesting evolutionary stories in the Cenozoic. For example, horses started small with four toes, ended up larger and having just one toe. Cetaceans (marine mammals like whales and dolphins) started on land from small bear-like (mesonychids) creatures in the early Cenozoic and gradually took to water. However, no study of evolution has been more studied than human evolution. Hominids, the name for human-like primates, started in eastern Africa several million years ago.

The first critical event in this story is an environmental change from jungle to more of a savanna, probably caused by changes in Indian Ocean circulation. While bipedalism is known to have evolved before this shift, it is generally believed that our bipedal ancestors (like Australopithecus) had an advantage by covering ground more easily in a more open environment compared to their non-bipedal evolutionary cousins. There is also a growing body of evidence, including the famous “Lucy” fossil of an Australopithecine, that our early ancestors lived in trees. Arboreal animals usually demand a high intelligence to navigate through a three-dimensional world. It is from this lineage that humans evolved, using endurance running as a means to acquire more resources and possibly even hunt. This can explain many uniquely human features, from our long legs, strong achilles, lack of lower gut protection, and our wide range of running efficiencies.

Now that the hands are freed up, the next big step is a large brain. There have been arguments from a switch to more meat eating, cooking with fire, tool use, and even the construct of society itselfto explain this increase in brain size. Regardless of how, it was this increased cognitive power that allowed humans to reign as their ancestors moved out of Africa and explored the world, ultimately entering the Americas through land bridges like the Bering Land Bridge. The details of this worldwide migration and the different branches of the hominid evolutionary tree are very complex, and best reserved for its own course.

Anthropocene and Extinction

Humans have had an influence on the Earth, its ecosystems and climate. Yet, human activity can not explain all of the changes that have occurred in the recent past. The start of the Quaternary period, the last and current period of the Cenozoic, is marked by the start of our current ice age 2.58 million years ago. During this time period, ice sheets advanced and retreated, most likely due to Milankovitch cycles (see ch. 15). Also at this time, various cold-adapted megafauna emerged (like giant sloths, saber-tooth cats, and woolly mammoths), and most of them went extinct as the Earth warmed from the most recent glacial maximum. A long-standing debate is over the cause of these and other extinctions. Is climate warming to blame, or were they caused by humans? Certainly, we know of recent human extinctions of animals like the dodo or passenger pigeon. Can we connect modern extinctions to extinctions in the recent past? If so, there are several ideas as to how this happened. Possibly the most widely accepted and oldest is the hunting/overkill hypothesis. The idea behind this hypothesis is that humans hunted large herbivores for food, then carnivores could not find food, and human arrival times in locations has been shown to be tied to increased extinction rates in many cases.

Modern human impact on the environment and the Earth as a whole is unquestioned. In fact, many scientists are starting to suggest that the rise of human civilization ended and/or replaced the Holocene epoch and defines a new geologic time interval: the Anthropocene. Evidence for this change includes extinctions, increased tritium (hydrogen with two neutrons) due to nuclear testing, rising pollutants like carbon dioxide, more than 200 never-before seen mineral species that have occurred only in this epoch, materials such as plastic and metals which will be long lasting “fossils” in the geologic record, and large amounts of earthen material moved. The biggest scientific debate with this topic is the starting point. Some say that humans’ invention of agriculture would be recognized in geologic strata and that should be the starting point, around 12,000 years ago. Others link the start of the industrial revolution and the subsequent addition of vast amounts of carbon dioxide in the atmosphere. Either way, the idea is that alien geologists visiting Earth in the distant future would easily recognize the impact of humans on the Earth as the beginning of a new geologic period.

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Summary

The changes that have occurred since the inception of Earth are vast and significant. From the oxygenation of the atmosphere, the progression of life forms, the assembly and deconstruction of several supercontinents, to the extinction of more life forms than exist today, having a general understanding of these changes can put present change into a more rounded perspective.

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References

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9 Crustal Deformation and Earthquakes

KEY CONCEPTS

By the end of this chapter, students should be able to:

Differentiate between stress and strain Identify the three major types of stress Differentiate between brittle, ductile, and elastic deformation Describe the geological map symbol used for strike and dip of strata Name and describe different fold types Differentiate the three major fault types and describe their associated movements Explain how elastic rebound relates to earthquakes

Differentiate between stress and strain

Identify the three major types of stress

Differentiate between brittle, ductile, and elastic deformation

Describe the geological map symbol used for strike and dip of strata

Name and describe different fold types

Differentiate the three major fault types and describe their associated movements

Explain how elastic rebound relates to earthquakes

Describe different seismic wave types and how they are measured

Explain how humans can induce seismicity

Describe how seismographs work to record earthquake waves

From seismograph records, locate the epicenter of an earthquake

Explain the difference between earthquake magnitude and intensity

List earthquake factors that determine ground shaking and destruction

Identify secondary earthquake hazards

Describe notable historical earthquakes

Name and describe different fold types Differentiate the three major fault types and describe their associated movements Explain how elastic rebound relates to earthquakes Describe different seismic wave types and how they are measured Explain how humans can induce seismicity Describe how seismographs work to record earthquake waves From seismograph records, locate the epicenter of an earthquake Explain the difference between earthquake magnitude and intensity List earthquake factors that determine ground shaking and destruction Identify secondary earthquake hazards Describe notable historical earthquakes

Crustal deformation occurs when applied forces exceed the internal strength of rocks, physically changing their shapes. These forces are called stress, and the physical changes they create are called strain. Forces involved in tectonic processes as well as gravity and igneous pluton emplacement produce strains in rocks that include folds, fractures, and faults. When rock experiences large amounts of shear stress and breaks with rapid, brittle deformation, energy is released in the form of seismic waves, commonly known as an earthquake.

9.1 Stress and Strain

Stressis the force exerted per unit area and strainis the physical change that results in response to that force. When applied stress is greater than the internal strength of rock, strain results in the form of deformation of the rock caused by the stress. Strain in rocks can be represented as a change in rock volume and/or rock shape, as well as fracturing the rock. There are three types of stress: tensional, compressional, and shear. Tensional stress involves forces pulling in opposite directions, which results in strain that stretches and thins rock. Compressional stress involves forces pushing together, and compressional strain shows up as rock folding and thickening. Shear stress involves transverse forces; the strain shows up as opposing blocks or regions of material moving past each other.

Table showing types of stress and resulting strain:

Type of Stress	Associated Plate Boundary type (see Ch. 2)	Resulting Strain	Associated fault and offset types
Tensional	divergent	Stretching and thinning	Normal
Compressional	convergent	Shortening and thickening	Reverse
Shear	transform	Tearing	Strike-slip

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9.2 Deformation

When rocks are stressed, the resulting strain can be elastic, ductile, or brittle. This change is generally called deformation. Elastic deformationis strain that is reversible after a stress is released. For example, when you stretch a rubber band, it elastically returns to its original shape after you release it. Ductile deformationoccurs when enough stress is applied to a material that the changes in its shape are permanent, and the material is no longer able to revert to its original shape. For example, if you bend a metal bar too far, it can be permanently bent out of shape. The point at which elastic deformation is surpassed and strain becomes permanent is called the yield point. In the figure, yield point is where the line transitions from elastic deformation to ductile deformation (the end of the dashed line). Brittle deformation is another critical point of no return, when rock integrity fails and the rock fractures under increasing stress.

The type of deformation a rock undergoes depends on pore pressure, strain rate, rock strength, temperature, stress intensity, time, and confining pressure. Pore pressure is exerted on the rock by fluids in the open spaces or pores embedded within rock or sediment. Strain rate measures how quickly a material is deformed. For example, applying stress slowly makes it is easier to bend a piece of wood without breaking it. Rock strength measures how easily a rock deforms under stress. Shale has low strength and granite has high strength. Removing heat, or decreasing the temperature, makes materials more rigid and susceptible to brittle deformation. On the other hand, heating materials make them more ductile and less brittle. Heated glass can be bent and stretched.

Table showing relationship between factors operating on rock and the resulting strains:

Factor	Strain Response
Increase Temperature	More Ductile
Increase Strain Rate	More Brittle
Increase Rock Strength	More Brittle

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9.3 Geological Maps

Geologic maps are two dimensional (2D) representations of geologic formations and structures at the Earth’s surface, including formations, faults, folds, inclined strata, and rock types. Formations are recognizable rock units. Geologists use geologic maps to represent where geologic formations, faults, folds, and inclined rock units are. Geologic formations are recognizable, mappable rock units. Each formation on the map is indicated by a color and a label. For examples of geologic maps, see the Utah Geological Survey (UGS) geologic map viewer.

Formation labels include symbols that follow a specific protocol. The first one or more letters are uppercase and represent the geologic time period of the formation. More than one uppercase letter indicates the formation is associated with multiple time periods. The following lowercase letters represent the formation name, abbreviated rock description, or both.

9.3.1 Cross sections

Cross sections are subsurface interpretations made from surface and subsurface measurements. Maps display geology in the horizontal plane, while cross sections show subsurface geology in the vertical plane. For more information on cross sections, check out the AAPG wiki.

9.3.2 Strike and Dip

Geologists use a special symbol called strike and dip to represent inclined beds. Strike and dip map symbols look like the capital letter T, with a short trunk and extra-wide top line. The short trunk represents the dip and the top line represents the strike. Dip is the angle that a bed plunges into the Earth from the horizontal. A number next to the symbol represents dip angle. One way to visualize the strike is to think about a line made by standing water on the inclined layer. That line is horizontal and lies on a compass direction that has some angle with respect to true north or south (see figure). The strike angle is that angle measured by a special compass. E.g., N 30° E (read north 30 degrees east) means the horizontal line points northeast at an angle of 30° from true north. The strike and dip symbol is drawn on the map at the strike angle with respect to true north on the map. The dip of the inclined layer represents the angle down to the layer from horizontal, in the figure 45o SE (read dipping 45 degrees to the SE). The direction of dip would be the direction a ball would roll if set on the layer and released. A horizontal rock bed has a dip of 0° and a vertical bed has a dip of 90°. Strike and dip considered together are called rock attitude.

This video illustrates geologic structures and associated map symbols.

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9.4 Folds

Geologic folds are layers of rock that are curved or bent by ductile deformation. Folds are most commonly formed by compressional forces at depth, where hotter temperatures and higher confining pressures allow ductile deformation to occur.

Folds are described by the orientation of their axes, axial planes, and limbs. The plane that splits the fold into two halves is known as the axial plane. The fold axis is the line along which the bending occurs and is where the axial plane intersects the folded strata. The hinge line follows the line of greatest bend in a fold. The two sides of the fold are the fold limbs.

Symmetrical folds have a vertical axial plane and limbs have equal but opposite dips. Asymmetrical folds have dipping, non-vertical axial planes, where the limbs dip at different angles. Overturned folds have steeply dipping axial planes and the limbs dip in the same direction but usually at different dip angles. Recumbent folds have horizontal or nearly horizontal axial planes. When the axis of the fold plunges into the ground, the fold is called a plunging fold. Folds are classified into five categories: anticline, syncline, monocline, dome, and basin.

9.4.1 Anticline

Anticlines are arch-like, or A-shaped, folds that are convex-upward in shape. They have downward curving limbs and beds that dip down and away from the central fold axis. In anticlines, the oldest rock strata are in the center of the fold, along the axis, and the younger beds are on the outside. Since geologic maps show the intersection of surface topography with underlying geologic structures, an anticline on a geologic map can be identified by both the attitude of the strata forming the fold and the older age of the rocks inside the fold. An antiform has the same shape as an anticline, but the relative ages of the beds in the fold cannot be determined. Oil geologists are interested in anticlines because they can form oil traps, where oil migrates up along the limbs of the fold and accumulates in the high point along the fold axis.

9.4.2 Syncline

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Synclinal fold – Macigno Formation
by alanpitts on Sketchfab

Synclines are trough-like, or U shaped, folds that are concave-upward in shape. They have beds that dip down and in toward the central fold axis. In synclines, older rock is on the outside of the fold and the youngest rock is inside of the fold axis. A synform has the shape of a syncline but like an antiform, does not have distinguishable age zones.

9.4.3 Monocline

Monoclines are step-like folds, in which flat rocks are upwarped or downwarped, then continue flat. Monoclines are relatively common on the Colorado Plateau where they form “reefs,” which are ridges that act as topographic barriers and should not be confused with ocean reefs (see Chapter 5). Capitol Reef is an example of a monocline in Utah. Monoclines can be caused by bending of shallower sedimentary strata as faults grow below them. These faults are commonly called “blind faults” because they end before reaching the surface and can be either normal or reverse faults.

9.4.4 Dome

A dome is a symmetrical to semi-symmetrical upwarping of rock beds. Domes have a shape like an inverted bowl, similar to an architectural dome on a building. Examples of domes in Utah include the San Rafael Swell, Harrisburg Junction Dome, and Henry Mountains. Domes are formed from compressional forces, underlying igneous intrusions(see Chapter 4), by salt diapirs, or even impacts, like upheaval dome in Canyonlands National Park.

9.4.5 Basin

A basin is the inverse of a dome, a bowl-shaped depression in a rock bed. The Uinta Basin in Utah is an example of a basin. Some structural basins are also sedimentary basins that collect large quantities of sediment over time. Sedimentary basins can form as a result of folding but are much more commonly produced in mountain building, forming between mountain blocks or via faulting. Regardless of the cause, as the basin sinks or subsides, it can accumulate more sediment because the weight of the sediment causes more subsidence in a positive-feedback loop. There are active sedimentary basins all over the world. An example of a rapidly subsiding basin in Utah is the Oquirrh Basin, dated to the Pennsylvanian-Permian age, which has accumulated over 9,144 m (30,000 ft) of fossiliferous sandstones, shales, and limestones. These strata can be seen in the Wasatch Mountains along the east side of Utah Valley, especially on Mt. Timpanogos and in Provo Canyon.

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9.5 Faults

Faults are the places in the crust where brittle deformation occurs as two blocks of rocks move relative to one another. Normal and reverse faults display vertical, also known as dip-slip, motion. Dip-slip motion consists of relative up-and-down movement along a dipping fault between two blocks, the hanging wall and footwall. In a dip-slip system, the footwall is below the fault plane and the hanging wall is above the fault plane. A good way to remember this is to imagine a mine tunnel running along a fault; the hanging wall would be where a miner would hang a lantern and the footwall would be at the miner’s feet.

Faulting as a term refers to rupture of rocks. Such ruptures occur at plate boundaries but can also occur in plate interiors as well. Faults slip along the fault plane. The fault scarp is the offset of the surface produced where the fault breaks through the surface. Slickensides are polished, often grooved surfaces along the fault plane created by friction during the movement.

A joint or fracture is a plane of brittle deformation in rock created by movement that is not offset or sheared. Joints can result from many processes, such as cooling, depressurizing, or folding. Joint systems may be regional affecting many square miles.

9.5.1 Normal Faults

Normal faults move by a vertical motion where the hanging-wall moves downward relative to the footwall along the dip of the fault. Normal faults are created by tensional forces in the crust. Normal faults and tensional forces commonly occur at divergent plate boundaries, where the crust is being stretched by tensional stresses (see Chapter 2). Examples of normal faults in Utah are the Wasatch Fault, the Hurricane Fault, and other faults bounding the valleys in the Basin and Range province.

Grabens, horsts, and half-grabens are blocks of crust or rock bounded by normal faults (see Chapter 2). Grabens drop down relative to adjacent blocks and create valleys. Horsts rise up relative to adjacent down-dropped blocks and become areas of higher topography. Where occurring together, horsts and grabens create a symmetrical pattern of valleys surrounded by normal faults on both sides and mountains. Half-grabens are a one-sided version of a horst and graben, where blocks are tilted by a normal fault on one side, creating an asymmetrical valley-mountain arrangement. The mountain-valleys of the Basin and Range Province of Western Utah and Nevada consist of a series of full and half-grabens from the Salt Lake Valley to the Sierra Nevada Mountains.

Normal faults do not continue clear into the mantle. In the Basin and Range Province, the dip of a normal fault tends to decrease with depth, i.e., the fault angle becomes shallower and more horizontal as it goes deeper. Such decreasing dips happen when large amounts of extension occur along very low-angle normal faults, known as detachment faults. The normal faults of the Basin and Range, produced by tension in the crust, appear to become detachment faults at greater depths.

9.5.2 Reverse Faults

In reverse faults, compressional forces cause the hanging wall to move up relative to the footwall. A thrust fault is a reverse fault where the fault plane has a low dip angle of less than 45°. Thrust faults carry older rocks on top of younger rocks and can even cause repetition of rock units in the stratigraphic record.

Convergentplate boundaries with subduction zones create a special type of “reverse” fault called a megathrust fault where denser oceanic crust drives down beneath less dense overlying crust. Megathrust faults cause the largest magnitude earthquakes yet measured and commonly cause massive destruction and tsunamis.

9.5.3 Strike-slip Faults

Strike-slipfaults have side-to-side motion. Strike-slip faults are most commonly associated with transform plate boundaries and are prevalent in transform fracture zones along mid-ocean ridges. In pure strike-slip motion, fault blocks on either side of the fault do not move up or down relative to each other, rather move laterally, side to side. The direction of strike-slip movement is determined by an observer standing on a block on one side of the fault. If the block on the opposing side of the fault moves left relative to the observer’s block, this is called sinistral motion. If the opposing block moves right, it is dextral motion.

Video showing motion in a strike-slip fault.

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Bends along strike-slip faults create areas of compression or tension between the sliding blocks (see Chapter 2). Tensional stresses create transtensional features with normal faults and basins, such as the Salton Sea in California. Compressional stresses create transpressional features with reverse faults and cause small-scale mountain building, such as the San Gabriel Mountains in California. The faults that splay off transpression or transtension features are known as flower structures.

An example of a dextral, right-lateral strike-slip fault is the San Andreas Fault, which denotes a transform boundary between the North American and Pacific plates. An example of a sinistral, left-lateral strike-slip fault is the Dead Sea fault in Jordan and Israel.

Video showing how faults are classified.

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9.6 Earthquake Essentials

Earthquakes are felt at the surface of the Earth when energy is released by blocks of rock sliding past each other, i.e. faulting has occurred. Seismic energy thus released travels through the Earth in the form of seismic waves. Most earthquakes occur along active plate boundaries. Intraplate earthquakes (not along plate boundaries) occur and are still poorly understood. The USGS Earthquakes Hazards Program has a real time map showing the most recent earthquakes..

9.6.1 How Earthquakes Happen

The release of seismic energy is explained by the elastic rebound theory. When rock is strained to the point that it undergoes brittle deformation, The place where the initial offsetting rupture takes place between the fault blocks is called the focus. This offset propagates along the fault, which is known as the fault plane.

The fault blocks of persistent faults like the Wasatch Fault (Utah), that show recurring movements, are locked together by friction. Over hundreds to thousands of years, stress builds up along the fault until it overcomes frictional resistance, rupturing the rock and initiating fault movement. The deformed unbroken rocks snap back toward their original shape in a process called elastic rebound. Think of bending a stick until it breaks; stored energy is released, and the broken pieces return to near their original orientation.

Bending, the ductile deformation of the rocks near a fault, reflects a build-up of stress. In earthquake-prone areas like California, strain gauges are used to measure this bending and help seismologists, scientists who study earthquakes, understand more about predicting them. In locations where the fault is not locked, seismic stress causes continuous, gradual displacement between the fault blocks called fault creep. Fault creep occurs along some parts of the San Andreas Fault (California).

After an initial earthquake, continuous application of stress in the crust causes elastic energy to begin to build again during a period of inactivity along the fault. The accumulating elastic strain may be periodically released to produce small earthquakes on or near the main fault called foreshocks. Foreshocks can occur hours or days before a large earthquake, or may not occur at all. The main release of energy during the major earthquake is known as the mainshock. Aftershocks may follow the mainshock to adjust new strain produced during the fault movement and generally decrease over time.

9.6.2 Focus and Epicenter

The earthquake focus, also called hypocenter, is the initial point of rupture and displacement of the rock moves from the hypocenter along the fault surface. The earthquake focus or hypocenter is the point along the fault plane from which initial seismic waves spread outward and is always at some depth below the ground surface. From the focus, rock displacement propagates up, down, and laterally along the fault plane. This displacement produces shock waves called seismic waves. The larger the displacement between the opposing fault blocks and the further the displacement propagates along the fault surface, the more seismic energy is released and the greater the amount and time of shaking is produced. The epicenter is the location on the Earth’s surface vertically above the focus. This is the location that most news reports give because it is the center of the area where people are affected.

9.6.3 Seismic Waves

To understand earthquakes and how earthquake energy moves through the Earth, consider the basic properties of waves. Waves describe how energy moves through a medium, such as rock or unconsolidated sediments in the case of earthquakes. Wave amplitude indicates the magnitude or height of earthquake motion. Wavelength is the distance between two successive peaks of a wave. Wave frequency is the number of repetitions of the motion over a period of time, cycles per time unit. Period, which is the amount of time for a wave to travel one wavelength, is the inverse of frequency. When multiple waves combine, they can interfere with each other (see figure). When waves combine in sync, they produce constructive interference, where the influence of one wave adds to and magnifies the other. If waves are out of sync, they produce destructive interference, which diminishes the amplitudes of both waves. If two combined waves have the same amplitude and frequency but are one-half wavelength out of sync, the resulting destructive interference can eliminate each wave. These processes of wave amplitude, frequency, period, and constructive and destructive interference determine the magnitude and intensity of earthquakes.

Seismic waves are the physical expression of energy released by the elastic rebound of rock within displaced fault blocks and are felt as an earthquake. Seismic waves occur as body waves and surface waves. Body waves pass underground through the Earth’s interior body and are the first seismic waves to propagate out from the focus. Body waves include primary (P) waves and secondary (S) waves. P waves are the fastest body waves and move through rock via compression, very much like sound waves move through air. Rock particles move forward and back during passage of the P waves, enabling them to travel through solids, liquids, plasma, and gases. S waves travel slower, following P waves, and propagate as shear waves that move rock particles from side to side. Because they are restricted to lateral movement, S waves can only travel through solids but not liquids, plasma, or gases.

P-waves are compressional.

During an earthquake, body waves pass through the Earth and into the mantle as a sub-spherical wave front. Considering a point on a wave front, the path followed by a specific point on the spreading wave front is called a seismic ray and a seismic ray reaches a specific seismograph located at one of thousands of seismic monitoring stations scattered over the Earth. Density increases with depth in the Earth, and since seismic velocity increases with density, a process called refraction causes earthquake rays to curve away from the vertical and bend back toward the surface, passing through different bodies of rock along the way.

Surface waves are produced when body waves from the focus strike the Earth’s surface. Surface waves travel along the Earth’s surface, radiating outward from the epicenter. Surface waves take the form of rolling waves called Raleigh Waves and side to side waves called Love Waves (watch videos for wave propagation animations). Surface waves are produced primarily as the more energetic S waves strike the surface from below with some surface wave energy contributed by P waves (videos courtesy blog.Wolfram.com). Surface waves travel more slowly than body waves and because of their complex horizontal and vertical movement, surface waves are responsible for most of the damage caused by an earthquake. Love waves produce predominantly horizontal ground shaking and, ironically from their name, are the most destructive. Rayleigh waves produce an elliptical motion with longitudinal dilation and compression, like ocean waves. However, Raleigh waves cause rock particles to move in a direction opposite to that of water particles in ocean waves.

The Earth has been described as ringing like a bell after an earthquake with earthquake energy reverberating inside it. Like other waves, seismic waves refract (bend) and bounce (reflect) when passing through rocks of differing densities. S waves, which cannot move through liquid, are blocked by the Earth’s liquid outer core, creating an S wave shadow zone on the side of the planet opposite to the earthquake focus. P waves, on the other hand, pass through the core, but are refracted into the core by the difference of density at the core–mantle boundary. This has the effect of creating a cone shaped P wave shadow zone on parts of the other side of the Earth from the focus.

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2011 Tohoku Earthquake, Mag. 9.0. Body and Surface Waves from seismicsoundlab on Vimeo.

9.6.4 Induced Seismicity

Earthquakes known as induced seismicity occur near natural gas extraction sites because of human activity. Injection of waste fluids in the ground, commonly a byproduct of an extraction process for natural gas known as fracking, can increase the outward pressure that liquid in the pores of a rock exerts, known as pore pressure. The increase in pore pressure decreases the frictional forces that keep rocks from sliding past each other, essentially lubricating fault planes. This effect is causing earthquakes to occur near injection sites, in a human induced activity known as induced seismicity. The significant increase in drilling activity in the central United States has spurred the requirement for the disposal of significant amounts of waste drilling fluid, resulting in a measurable change in the cumulative number of earthquakes experienced in the region.

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9.7 Measuring Earthquakes

9.7.1 Seismographs

People feel approximately 1 million earthquakes a year, usually when they are close to the source and the earthquake registers at least moment magnitude 2.5. Major earthquakes of moment magnitude 7.0 and higher are extremely rare. The U. S. Geological Survey (USGS) Earthquakes Hazards Program real-time map shows the location and magnitude of recent earthquakes around the world.

To accurately study seismic waves, geologists use seismographs that can measure even the slightest ground vibrations. Early 20th-century seismograms use a weighted pen (pendulum) suspended by a long spring above a recording device fixed solidly to the ground. The recording device is a rotating drum mounted with a continuous strip of paper. During an earthquake, the suspended pen stays motionless and records ground movement on the paper strip. The resulting graph a seismogram. Digital versions use magnets, wire coils, electrical sensors, and digital signals instead of mechanical pens, springs, drums, and paper. A seismograph array is multiple seismographs configured to measure vibrations in three directions: north-south (x axis), east-west (y axis), and up-down (z axis).

To pinpoint the location of an earthquake epicenter, seismologists use the differences in arrival times of the P, S, and surface waves. After an earthquake, P waves will appear first on a seismogram, followed by S waves, and finally surface waves, which have the largest amplitude. It is important to note that surface waves lose energy quickly, so they are not measurable at great distances from the epicenter. These time differences determine the distance but not the direction of the epicenter. By using wave arrival times recorded on seismographs at multiple stations, seismologists can apply triangulation to pin point the location of the epicenter of an earthquake. At least three seismograph stations are needed for triangulation. The distance from each station to the epicenter is plotted as the radius of a circle. The epicenter is demarked where the circles intersect. This method also works in 3D, using multi-axis seismographs and sphere radii to calculate the underground depth of the focus.

This video shows the method of triangulation to locate the epicenter of an earthquake.

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9.7.2 Seismograph Network

The International Registry of Seismograph Stationslists more than 20,000 seismographs on the planet. By comparing data from multiple seismographs, scientists can map the properties of the inside of the Earth, detect detonations of large explosive devices, and predict tsunamis. The Global Seismic Network, a worldwide set of linked seismographs that electronically distribute real-time data, includes more than 150 stations that meet specific design and precision standards. The USArray is a network of hundreds of permanent and transportable seismographs in the United States that are used to map the subsurface activity of earthquakes (see video).

Along with monitoring for earthquakes and related hazards, the Global Seismograph Network helps detect nuclear weapons testing, which is monitored by the Comprehensive Nuclear Test Ban Treaty Organization. Most recently, seismographs have been used to determine nuclear weapons testing by North Korea.

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Nepal Earthquake M7.9 Ground Motion Visualization

9.7.3 Seismic tomography

Very much like a CT (Computed Tomography) scan uses X-rays at different angles to image the inside of a body, seismictomographyuses seismic rays from thousands of earthquakes that occur each year, passing at all angles through masses of rock, to generate images of internal Earth structures.

Using the assumption that the earth consists of homogenous layers, geologists developed a model of expected properties of earth materials at every depth within the earth called the PREM (Preliminary Reference Earth Model). These properties include seismic wave transmission velocity, which is dependent on rock density and elasticity. In the mantle, temperature differences affect rock density. Cooler rocks have a higher density and therefore transmit seismic waves faster. Warmer rocks have a lower density and transmit earthquake waves slower. When the arrival times of seismic rays at individual seismic stations are compared to arrival times predicted by PREM, differences are called seismic anomaliesand can be measured for bodies of rock within the earth from seismic rays passing through them at stations of the seismic network. Because seismic rays travel at all angles from lots of earthquakes and arrive at lots of stations of the seismic network, like CT scans of the body, variations in the properties of the rock bodies allow 3D images to be constructed of the rock bodies through which the rays passed. Seismologists are thus able to construct 3D images of the interior of the Earth..

For example, seismologists have mapped the Farallon Plate, a tectonic plate that subducted beneath North America during the last several million years, and the Yellowstone magma chamber, which is a product of the Yellowstone hot spot under the North American continent. Peculiarities of the Farallon Plate subduction are thought to be responsible for many features of western North America including the Rocky Mountains (See chapter 8).

9.7.4 Earthquake Magnitude and Intensity

9.7.4.1 RICHTER SCALE

Magnitude is the measure of the energy released by an earthquake. The Richter scale (ML), the first and most well-known magnitude scale, was developed by Charles F. Richter (1900-1985) at the California Institute of Technology. This was the magnitude scale used historically by early seismologists. Used by early seismologists, Richter magnitude (ML) is determined from the maximum amplitude of the pen tracing on the seismogram recording. Adjustments for epicenter distance from the seismograph are made using the arrival-time differences of S and P waves.

The Richter Scale is logarithmic, based on powers of 10. This means an increase of one Richter unit represents a 10-fold increase in seismic-wave amplitude or in other words, a magnitude 6 earthquake shakes the ground 10 times more than a magnitude 5. However, the actual energy released for each magnitude unit is 32 times greater, which means a magnitude 6 earthquake releases 32 times more energy than a magnitude 5.

The Richter Scale was developed for earthquakes in Southern California, using local seismographs. It has limited applications for larger distances and very large earthquakes. Therefore, most agencies no longer use the Richter Scale. Moment magnitude (MW), which is measured using seismic arrays and generates values comparable to the Richter Scale, is more accurate for measuring earthquakes across the Earth, including large earthquakes, although they require more time to calculate. News media often report Richter magnitudes right after an earthquake occurs even though scientific calculations now use moment magnitudes.

9.7.4.2 MOMENT MAGNITUDE SCALE

The Moment Magnitude scale depicts the absolute size of earthquakes, comparing information from multiple locations and using a measurement of actual energy released calculated from cross-sectional area of rupture, amount of slippage, and the rigidity of the rocks. Because each earthquake occurs in a unique geologic setting and the rupture area is often hard to measure, estimates of moment magnitude can take days or even months to calculate.

Like the Richter Scale, the moment magnitude scale is logarithmic. Magnitude values of the two scales are approximately equal, except for very large earthquakes. Both scales are used for reporting earthquake magnitude. The Richter Scale provides a quick magnitude estimate immediately following the quake and thus, is usually reported in news accounts. Moment magnitude calculations take much longer but are more accurate and thus, more useful for scientific analysis.

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9.7.4.3 Modified Mercalli Intensity Scale

The Modified Mercalli Intensity Scale (MMI) is a qualitative rating of ground-shaking intensity based on observable structural damage and people’s perceptions. This scale uses a I (Roman numeral one) rating for the lowest intensity and X (ten) for the highest (see table) and can vary depending on epicenter location and population density, such as urban versus rural settings. Historically, scientists used the MMI Scale to categorize earthquakes before they developed quantitative measurements of magnitude. Intensity maps show locations of the most severe damage, based on residential questionnaires, local news articles, and on-site assessment reports.

Intensity	Shaking	Description/Damage
I	Not felt	Not felt except by a very few under especially favorable conditions.
II	Weak	Felt only by a few persons at rest,especially on upper floors of buildings.
III	Weak	Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.
IV	Light	Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
V	Moderate	Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
VI	Strong	Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
VII	Very strong	Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
VIII	Severe	Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
IX	Violent	Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X	Extreme	Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

Table. Abridged Mercalli Scale from USGS General Interest Publication 1989-288-913.

9.7.4.4 Shake Maps

Shake maps, written ShakeMaps by the USGS, use high-quality, computer-interpolated data from seismograph networks to show areas of intense shaking. Shake maps are useful in the crucial minutes after an earthquake, as they show emergency personnel where the greatest damage likely occurred and help them locate possibly damaged gas lines and other utility facilities.

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9.8 Earthquake Risk

9.8.1 Factors that Determine Shaking

Earthquake magnitude is an absolute value that measures pure energy release. Intensity however, i.e. how much the ground shakes, is a determined by several factors.

Earthquake Magnitude—In general, the larger the magnitude, the stronger the shaking and the longer the shaking will last.

This table is taken from from the USGS and shows scales of magnitude and Mercalli Intensity, and descriptions of shaking and resulting damage.

Magnitude	Modified Mercalli Intensity	Shaking/Damage Description
1.0 – 3.0	I	Only felt by a very few.
3.0 – 3.9	II – III	Noticeable indoors, especially on upper floors.
4.0 – 4.9	IV – V	Most to all feel it. Dishes, doors, cars shake and possibly break.
5.0 – 5.9	VI – VII	Everyone feels it. Some items knocked over or broken. Building damage possible.
6.0 – 6.9	VII – IX	Frightening amounts of shaking. Significant damage especially with poorly constructed buildings
≥ 7.0	≥ VIII	Significant destruction of buildings. Potential for objects to be thrown in air from shaking.

Location and Direction—Shaking is more severe closer to the epicenter. The severity of shaking is influenced by the location of the observer relative to epicenter, direction of rupture propagation, and path of greatest rupture.

Local Geologic Conditions—Seismic waves are affected by the nature of the ground materials through which they pass. Different materials respond differently to an earthquake. Think of shaking a block of Jello versus a meatloaf, one will jiggle much more when hit by waves of the same amplitude. The ground’s response to shaking depends on the degree of substrate consolidation. Solid sedimentary, igneous, or metamorphic bedrock shakes less than unconsolidated sediments.

This video shows how different substrates behave in response to different seismic waves and their potential for destruction.

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Seismic waves move fastest through consolidated bedrock, slower through unconsolidated sediments, and slowest through unconsolidated sediments with a high water content. Seismic energy is transmitted by wave velocity and amplitude. When seismic waves slow down, energy is transferred to the amplitude, increasing the motion of surface waves, which in turn amplifies ground shaking.

Focus depth—Deeper earthquakes cause less surface shaking because much of their energy, transmitted as body waves, is lost before reaching the surface. Recall that surface waves are generated by P and S waves impacting the Earth’s surface.

9.8.2 Factors that Determine Destruction

Just as certain conditions will impact intensity of ground-shaking, several factors affect how much destruction is caused.

Building Materials—The flexibility of a building material determines its resistance to earthquake damage. Unreinforced masonry (URM) is the material most devastated by ground shaking. Wood framing fastened with nails bends and flexes during seismic wave passage and is more likely to survive intact. Steel also has the ability to deform elastically before brittle failure. The Fix the Bricks campaign in Salt Lake City, Utah has good information on URMs and earthquake safety.

Intensity and Duration—Greater shaking and duration of shaking causes more destruction than lower and shorter shaking.

Resonance—Resonance occurs when seismic wave frequency matches a building’s natural shaking frequency and increases the shaking happened in the 1985 Mexico City Earthquake, where buildings of heights between 6 and 15 stories were especially vulnerable to earthquake damage. Skyscrapers designed with earthquake resilience have dampers and base isolation features to reduce resonance.

Resonance is influenced by the properties of the building materials. Changes in the structural integrity of a building can alter resonance. Conversely, changes in measured resonance can indicate a potentially altered structural integrity.

These two videos discuss why buildings fall during earthquakes and a modern procedure to reduce potential earthquake destruction for larger buildings.

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9.8.3 Earthquake Recurrence

A long hiatus in activity on along a fault segment with a history of recurring earthquakes is known as a seismic gap. The lack of activity may indicate the fault segment is locked, which may produce a buildup of strain and higher probability of an earthquake recurring. Geologists dig earthquake trenches across faults to estimate the frequency of past earthquake occurrences. Trenches are effective for faults with relatively long recurrence intervals, roughly 100s to 10,000s of years between significant earthquakes. Trenches are less useful in areas with more frequent earthquakes because they usually have more recorded data.

9.8.4 Earthquake Distribution

This video shows the distribution of significant earthquakes on the Earth during the years 2010 through 2012. Like volcanoes, earthquakes tend to aggregate around active boundaries of tectonic plates. The exception is intraplate earthquakes, which are comparatively rare..

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Subduction Zones—Subduction zones, found at convergent plate boundaries, are where the largest and deepest earthquakes, called megathrust earthquakes, occur. Examples of subduction-zone earthquake areas include the Sumatran Islands, Aleutian Islands, west coast of South America, and Cascadia Subduction Zone off the coast of Washington and Oregon. See Chapter 2 for more information about subduction zones.

Collision Zones—Collisions between converging continental plates create broad earthquake zones that may generate deep, large earthquakes from the remnants of past subduction events or other deep-crustal processes. The Himalayan Mountains (northern border of the Indian subcontinent) and Alps (southern Europe and Asia) are active regions of collision-zone earthquakes. See Chapter 2 for more information about collision zones.

TransformBoundaries—Strike-slipfaults created at transform boundaries produce moderate-to-large earthquakes, usually having a maximum moment magnitude of about 8. The San Andreas fault (California) is an example of a transform-boundary earthquake zone. Haiti’s Enriquillo-Plantain Garden fault system, which caused the 2010 earthquake near Port-au-Prince (see below), and Septentrional Fault, which destroyed Cap-Haïtien in 1842 and shook Cuba in 2020, are also transform faults. Other examples are the Alpine Fault (New Zealand) and Anatolian Faults (Turkey). See Chapter 2 for more information about transform boundaries.

Divergent Boundaries—Continentalrifts and mid-ocean ridges found at divergent boundaries generally produce moderate earthquakes. Examples of active earthquake zones include the East African Rift System (southwestern Asia through eastern Africa), Iceland, and Basin and Range province (Nevada, Utah, California, Arizona, and northwestern Mexico). See Chapter 2 for more information about divergent boundaries.

Intraplate Earthquakes—Intraplate earthquakes are not found near tectonic plate boundaries, but generally occur in areas of weakened crust or concentrated tectonic stress. The New Madrid seismic zone, which covers Missouri, Illinois, Tennessee, Arkansas, and Indiana, is thought to represent the failed Reelfoot rift. The failed rift zone weakened the crust, making it more responsive to tectonic plate movement and interaction. Geologists theorize the infrequently occurring earthquakes are produced by low strain rates

9.8.5 Secondary Hazards Caused by Earthquakes

Most earthquake damage is caused by ground shaking and fault block displacement In addition, there are secondary hazards that endanger structures and people, in some cases after the shaking stops.

Buildings toppled from liquefaction during a 7.5 magnitude earthquake in Japan.

Liquefaction—Liquefaction occurs when water-saturated, unconsolidated sediments, usually silt or sand, become fluid-like from shaking. The shaking breaks the cohesion between grains of sediment, creating a slurry of particles suspended in water. Buildings settle or tilt in the liquified sediment, which looks very much like quicksand in the movies. Liquefaction also creates sand volcanoes, cone-shaped features created when liquefied sand is squirted through an overlying and usually finer-grained layer.

This video demonstrates how liquefaction takes place.

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This video shows liquefaction occurring during the 2011 earthquake in Japan.

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Tsunamis—Among the most devastating natural disasters are tsunamis, earthquake-induced ocean waves. When the sea floor is offset by fault movement or an underwater landslide, the ground displacement lifts a volume of ocean water and generates the tsunami wave. Ocean wave behavior, which includes tsunamis, is covered in Chapter 12. Tsunami waves are fast-moving with low amplitude in deep ocean water but grow significantly in amplitude in the shallower waters approaching shore. When a tsunami is about to strike land, the drawback of the trough preceding the wave crest causes the water to recede dramatically from shore. Tragically, curious people wander out and follow the disappearing water, only to be overcome by an oncoming wall of water that can be upwards of a 30 m (100 ft) high. Early warning systems help mitigate the loss of life caused by tsunamis.

Landslides—Shaking can triggerlandslides (see Chapter 10). In 1992 a moment magnitude 5.9 earthquake in St. George, Utah, caused a landslide that destroyed several structures in the Balanced Rock Hills subdivision in Springville, Utah.

Seiches—Seiches are waves generated in lakes by earthquakes. The shaking may cause water to slosh back-and-forth or sometimes change the lake depth. Seiches in Hebgen Lake during a 1959 earthquake caused major destruction to nearby structures and roads.

This video shows a seich generated in a swimming pool by an earthquake in Nepal in 2015.

https://youtu.be/27GMnYEWL0M

Land Elevation Changes—Elastic rebound and displacement along the fault plane can cause significant land elevation changes, such as subsidence or upheaval. The 1964 Alaska earthquake produced significant land elevation changes, with the differences in height between the hanging wall and footwall ranging from one to several meters (3–30 ft). The Wasatch Mountains in Utah represent an accumulation of fault scarps created a few meters at a time, over a few million years.

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9.9 Case Studies

Video explaining the seismic activity and hazards of the Intermountain Seismic Belt and the Wasatch Fault, a large intraplate area of seismic activity.

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9.9.1 North American Earthquakes

Basin and Range Earthquakes—Earthquakes in the Basin and Range Province, from the Wasatch Fault (Utah) to the Sierra Nevada (California), occur primarily in normal faults created by tensional forces. The Wasatch Fault, which defines the eastern extent of the Basin and Range province, has been studied as an earthquake hazard for more than 100 years.

New Madrid Earthquakes (1811-1812)—Historical accounts of earthquakes in the New Madrid seismic zone date as far back as 1699 and earthquakes continue to be reported in modern times. A sequence of large (Mw >7) occurred from December 1811 to February 1812 in the New Madrid area of Missouri. The earthquakes damaged houses in St. Louis, affected the stream course of the Mississippi River, and leveled the town of New Madrid. These earthquakes were the result of intraplate seismic activity.

Charleston (1886)—The 1886 earthquake in Charleston South Carolina was a moment magnitude 7.0, with a Mercalli intensity of X, caused significant ground motion, and killed at least 60 people. This intraplate earthquake was likely associated with ancient faults created during the breakup of Pangea. The earthquake caused significant liquefaction. Scientists estimate the recurrence of destructive earthquakes in this area with an interval of approximately 1500 to 1800 years.

Great San Francisco Earthquake and Fire (1906)—On April 18, 1906, a large earthquake, with an estimated moment magnitude of 7.8 and MMI of X, occurred along the San Andreas fault near San Francisco California. There were multiple aftershocks followed by devastating fires, resulting in about 80% of the city being destroyed. Geologists G.K. Gilbert and Richard L. Humphrey, working independently, arrived the day following the earthquake and took measurements and photographs.

Alaska (1964)—The 1964 Alaska earthquake, moment magnitude 9.2, was one of the most powerful earthquakes ever recorded. The earthquake originated in a megathrust fault along the Aleutian subduction zone. The earthquake caused large areas of land subsidence and uplift, as well as significant mass wasting.

Video from the USGS about the 1964 Alaska earthquake.

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Loma Prieta (1989)—The Loma Prieta, California, earthquake was created by movement along the San Andreas Fault. The moment magnitude 6.9 earthquake was followed by a magnitude 5.2 aftershock. It caused 63 deaths, buckled portions of the several freeways, and collapsed part of the San Francisco-Oakland Bay Bridge.

This video shows how shaking propagated across the Bay Area during the 1989 Loma Prieta earthquake.

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This video shows destruction caused by the 1989 Loma Prieta earthquake.

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9.9.2 Global Earthquakes

Many of history’s largest earthquakes occurred in megathrust zones, such as the Cascadia Subduction Zone (Washington and Oregon coasts) and Mt. Rainier (Washington).

Shaanxi, China (1556)—On January 23, 1556 an earthquake of an approximate moment magnitude 8 hit central China, killing approximately 830,000 people in what is considered the most deadly earthquake in history. The high death toll was attributed to the collapse of cave dwellings (yaodong) built in loess deposits, which are large banks of windblown, compacted sediment (see Chapter 5). Earthquakes in this are region are believed to have a recurrence interval of 1000 years.

Lisbon, Portugal (1755)—On November 1, 1755 an earthquake with an estimated moment magnitude range of 8–9 struck Lisbon, Portugal, killing between 10,000 to 17,400 people. The earthquake was followed by a tsunami, which brought the total death toll to between 30,000-70,000 people.

Valdivia, Chile (1960)—The May 22, 1960 earthquake was the most powerful earthquake ever measured, with a moment magnitude 9.4–9.6 and lasting an estimated 10 minutes. It triggered tsunamis that destroyed houses across the Pacific Ocean in Japan and Hawaii and caused vents to erupt on the Puyehue-Cordón Caulle (Chile).

Video describing the tsunami produced by the 1960 Chili earthquake.

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Tangshan, China (1976)—Just before 4 a.m. (Beijing time) on July 28, 1976 a moment magnitude 7.8 earthquake struck Tangshan (Hebei Province), China, and killed more than 240,000 people. The high death-toll is attributed to people still being asleep or at home and most buildings being made of unreinforced masonry.

Sumatra, Indonesia (2004)—On December 26, 2004, slippage of the Sunda megathrust fault generated a moment magnitude 9.0–9.3 earthquake off the coast of Sumatra, Indonesia. This megathrust fault is created by the Australia plate subducting below the Sunda plate in the Indian Ocean. The resultant tsunamis created massive waves as tall as 24 m (79 ft) when they reached the shore and killed more than an estimated 200,000 people along the Indian Ocean coastline.

Haiti (2010)—The moment magnitude 7 earthquake that occurred on January 12, 2010, was followed by many aftershocks of magnitude 4.5 or higher. More than 200,000 people are estimated to have died as result of the earthquake. The widespread infrastructure damage and crowded conditions contributed to a cholera outbreak, which is estimated to have caused thousands more deaths.

Tōhoku, Japan (2011)—Because most Japanese buildings are designed to tolerate earthquakes, the moment magnitude 9.0 earthquake on March 11, 2011, was not as destructive as the tsunami it created. The tsunami caused more than 15,000 deaths and tens of billions of dollars in damage, including the destructive meltdown of the Fukushima nuclear power plant.

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Summary

Geologic stress, applied force, comes in three types: tension, shear, and compression. Strain is produced by stress and produces three types of deformation: elastic, ductile, and brittle. Geological maps are two-dimensional representations of surface formations which are the surface expression of three-dimensional geologic structures in the subsurface. The map symbol called strike and dip or rock attitude indicates the orientation of rock strata with reference to north-south and horizontal. Folded rock layers are categorized by the orientation of their limbs, fold axes and axial planes. Faults result when stress forces exceed rock integrity and friction, leading to brittle deformation and breakage. The three major fault types are described by the movement of their fault blocks: normal, strike-slip, and reverse.

Earthquakes, or seismic activity, are caused by sudden brittle deformation accompanied by elastic rebound. The release of energy from an earthquake focus is generated as seismic waves. P and S waves travel through the Earth’s interior. When they strike the outer crust, they create surface waves. Human activities, such as mining and nuclear detonations, can also cause seismic activity. Seismographs measure the energy released by an earthquake using a logarithmic scale of magnitude units; the Moment Magnitude Scale has replaced the original Richter Scale. Earthquake intensity is the perceived effects of ground shaking and physical damage. The location of earthquake foci is determined from triangulation readings from multiple seismographs.

Earthquake rays passing through rocks of the Earth’s interior and measured at the seismographs of the worldwide Seismic Network allow 3-D imaging of buried rock masses as seismic tomographs.

Earthquakes are associated with plate tectonics. They usually occur around the active plate boundaries, including zones of subduction, collision, and transform and divergent boundaries. Areas of intraplate earthquakes also occur. The damage caused by earthquakes depends on a number of factors, including magnitude, location and direction, local conditions, building materials, intensity and duration, and resonance. In addition to damage directly caused by ground shaking, secondary earthquake hazards include liquefaction, tsunamis, landslides, seiches, and elevation changes.

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References

Christenson, G.E., 1995, The September 2, 1992 ML 5.8 St. George earthquake, Washington County, Utah: Utah Geological Survey Circular 88, 48 p.

Coleman, J.L., and Cahan, S.M., 2012, Preliminary catalog of the sedimentary basins of the United States: U.S. Geological Survey Open-File Report 1111, 27 p.

Earle, S., 2015, Physical geology OER textbook: BC Campus OpenEd.

Feldman, J., 2012, When the Mississippi Ran Backwards: Empire, Intrigue, Murder, and the New Madrid Earthquakes of 1811 and 1812: Free Press, 320 p.

Fuller, M.L., 1912, The New Madrid earthquake: Central United States Earthquake Consortium Bulletin 494, 129 p.

Gilbert, G.K., and Dutton, C.E., 1877, Report on the geology of the Henry Mountains: Washington, U.S. Government Printing Office, 160 p.

Hildenbrand, T.G., and Hendricks, J.D., 1995, Geophysical setting of the Reelfoot rift and relations between rift structures and the New Madrid seismic zone: U.S. Geological Survey Professional Paper 1538-E, 36 p.

Means, W.D., 1976, Stress and Strain – Basic Concepts of Continuum Mechanics: Berlin, Springe, 273 p.

Ressetar, R. (Ed.), 2013, The San Rafael Swell and Henry Mountains Basin: geologic centerpiece of Utah: Utah Geological Association, Utah Geological Association, 250 p.

Satake, K., and Atwater, B.F., 2007, Long-Term Perspectives on Giant Earthquakes and Tsunamis at Subduction Zones: Annual Review of Earth and Planetary Sciences, v. 35, no. 1, p. 349–374., doi: 10.1146/annurev.earth.35.031306.140302.

Talwani, P., and Cox, J., 1985, Paleoseismic evidence for recurrence of Earthquakes near Charleston, South Carolina: Science, v. 229, no. 4711, p. 379–381.

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10 Mass Wasting

10 Mass Wasting

KEY CONCEPTS

At the end of this chapter, students should be able to:

Explain what mass wasting is and why it occurs on a slope

Explain the basic triggers of mass-wasting events and how they occur

Identify types of mass wasting

Identify risk factors for mass-wasting events

Evaluate landslides and their contributing factors

This chapter discusses the fundamental processes driving mass-wasting, types of mass wasting, examples and lessons learned from famous mass-wasting events, how mass wasting can be predicted, and how people can be protected from this potential hazard. Mass wasting is the downhill movement of rock and soil material due to gravity. The term landslide is often used as a synonym for mass wasting, but mass wasting is a much broader term referring to all movement downslope. Geologically, landslide is a general term for mass wasting that involves fast-moving geologic material. Loose material along with overlying soils are what typically move during a mass-wasting event. Moving blocks of bedrock are called rock topples, rock slides, or rock falls, depending on the dominant motion of the blocks. Movements of dominantly liquid material are called flows. Movement by mass wasting can be slow or rapid. Rapid movement can be dangerous, such as during debris flows. Areas with steep topography and rapid rainfall, such as the California coast, Rocky Mountain Region, and Pacific Northwest, are particularly susceptible to hazardous mass-wasting events.

10.1 Slope Strength

Mass wasting occurs when a slope fails. A slope fails when it is too steep and unstable for existing materials and conditions. Slope stability is ultimately determined by two principal factors: the slope angle and the strength of the underlying material. Force of gravity, which plays a part in mass wasting, is constant on the Earth’s surface for the most part, although small variations exist depending on the elevation and density of the underlying rock. In the figure, a block of rock situated on a slope is pulled down toward the Earth’s center by the force of gravity (fg). The gravitational force acting on a slope can be divided into two components: the shear or driving force (fs) pushing the block down the slope, and the normal or resisting force (fn) pushing into the slope, which produces friction. The relationship between shear force and normal force is called shear strength. When the normal force, i.e., friction, is greater than the shear force, then the block does not move downslope. However, if the slope angle becomes steeper or if the earth material is weakened, shear force exceeds normal force, compromising shear strength, and downslope movement occurs.

In the figure, the force vectors change as the slope angle increases. The gravitational force doesn’t change, but the shear force increases while the normal force decreases. The steepest angle at which rock and soil material is stable and will not move downslope is called the angle of repose. The angle of repose is measured relative from the horizontal. When a slope is at the angle of repose, the shear force is in equilibrium with the normal force. If the slope becomes just slightly steeper, the shear force exceeds the normal force, and the material starts to move downhill. The angle of repose varies for all material and slopes depending on many factors such as grain size, grain composition, and water content. The figure shows the angle of repose for sand that is poured into a pile on a flat surface. The sand grains cascade down the sides of the pile until coming to rest at the angle of repose. At that angle, the base and height of the pile continue to increase, but the angle of the sides remains the same.

Water is a common factor that can significantly change the shear strength of a particular slope. Water is located in pore spaces, which are empty air spaces in sediments or rocks between the grains. For example, assume a dry sand pile has an angle of repose of 30 degrees. If water is added to the sand, the angle of repose will increase, possibly to 60 degrees or even 90 degrees, such as a sandcastle being built at a beach. But if too much water is added to the pore spaces of the sandcastle, the water decreases the shear strength, lowers the angle of repose, and the sandcastle collapses.

Another factor influencing shear strength are planes of weakness in sedimentary rocks. Bedding planes (see Chapter 5) can act as significant planes of weakness when they are parallel to the slope but less so if they are perpendicular to the slope. At locations A and B, the bedding is nearly perpendicular to the slope and relatively stable. At location D, the bedding is nearly parallel to the slope and quite unstable. At location C, the bedding is nearly horizontal, and the stability is intermediate between the other two extremes [1]. Additionally, if clay minerals form along bedding planes, they can absorb water and become slick. When a bedding plane of shale (clay and silt) becomes saturated, it can lower the shear strength of the rock mass and cause a landslide, such as at the 1925 Gros Ventre, Wyoming rock slide. See the case studies section for details on this and other landslides.

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10.2 Mass-Wasting Triggers & Mitigation

Mass-wasting events often have a trigger: something changes that causes a landslide to occur at a specific time. It could be rapid snowmelt, intense rainfall, earthquake shaking, volcanic eruption, storm waves, rapid-stream erosion, or human activities, such as grading a new road. Increased water content within the slope is the most common mass-wasting trigger. Water content can increase due to rapidly melting snow or ice or an intense rain event. Intense rain events can occur more often during El Niño years. Then, the west coast of North America receives more precipitation than normal, and landslides become more common. Changes in surface-water conditions resulting from earthquakes, previous slope failures that dam up streams, or human structures that interfere with runoff, such as buildings, roads, or parking lots can provide additional water to a slope. In the case of the 1959 Hebgen Lake rock slide, Madison Canyon, Montana, the shear strength of the slope may have been weakened by earthquake shaking. Most landslide mitigation diverts and drains water away from slide areas. Tarps and plastic sheeting are often used to drain water off of slide bodies and prevent infiltration into the slide. Drains are used to dewater landslides and shallow wells are used to monitor the water content of some active landslides.

An oversteepened slope may also trigger landslides. Slopes can be made excessively steep by natural processes of erosion or when humans modify the landscape for building construction. An example of how a slope may be oversteepened during development occurs where the bottom of the slope is cut into, perhaps to build a road or level a building lot, and the top of the slope is modified by depositing excavated material from below. If done carefully, this practice can be very useful in land development, but in some cases, this can result in devastating consequences. For example, this might have been a contributing factor in the 2014 North Salt Lake City, Utah landslide. A former gravel pit was regraded to provide a road and several building lots. These activities may have oversteepened the slope, which resulted in a slow moving landslide that destroyed one home at the bottom of the slope. Natural processes such as excessive stream erosion from a flood or coastal erosion during a storm can also oversteepen slopes. For example, natural undercutting of the riverbank was proposed as part of the trigger for the famous 1925 Gros Ventre, Wyoming rock slide.

Slope reinforcement can help prevent and mitigate landslides . For rockfall-prone areas, sometimes it is economical to use long steel bolts. Bolts, drilled a few meters into a rock face, can secure loose pieces of material that could pose a hazard. Shockcrete, a reinforced spray-on form of concrete, can strengthen a slope face when applied properly. Buttressing a slide by adding weight at the toe of the slide and removing weight from the head of the slide, can stabilize a landslide. Terracing, which creates a stairstep topography, can be applied to help with slope stabilization, but it must be applied at the proper scale to be effective.

A different approach in reducing landslide hazard is to shield, catch, and divert the runout material. Sometimes the most economical way to deal with a landslide hazard is to divert and slow the falling material. Special stretchable fencing can be applied in areas where rockfall is common to protect pedestrians and vehicles. Runout channels, diversion structures, and check dams can be used to slow debris flows and divert them around structures. Some highways have special tunnels that divert landslides over the highway. In all of these cases the shielding has to be engineered to a scale that is greater than the slide, or catastrophic loss in property and life could result.

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10.3 Landslide Classification & Identification

Mass-wasting events are classified by type of movement and type of material, and there are several ways to classify these events. The figure and table show terms used. In addition, mass-wasting types often share common morphological features observed on the surface, such as the head scarp—commonly seen as crescent shapes on a cliff face; hummocky or uneven surfaces; accumulations of talus—loose rocky material falling from above; and toe of slope, which covers existing surface material.

10.3.1 Types of Mass Wasting

The most common mass-wasting types are falls, rotational and translational slides, flows, and creep. Falls are abrupt rock movements that detach from steep slopes or cliffs. Rocks separate along existing natural breaks such as fractures or bedding planes. Movement occurs as free-falling, bouncing, and rolling. Falls are strongly influenced by gravity, mechanical weathering, and water. Rotational slides commonly show slow movement along a curved rupture surface. Translational slides often are rapid movements along a plane of distinct weakness between the overlying slide material and more stable underlying material. Slides can be further subdivided into rock slides, debris slides, or earth slides depending on the type of the material involved (see table).

Table of Mass Wasting Types. Mass wasting movement type and primary earth material. Modified from [zotpressInText item=”{948446:KG8X6AAJ},{948446:HN9CI37K}” format=”(%num%)” brackets=”yes”].

Type of Movement	Primary Material Type and Common Name of Slide
Bedrock	Soil Types
Mostly Coarse-Grained	Mostly Fine-Grained
Falls	Rock Fall	—	—
Rock Avalanche	Rock Avalanche	—	—
Rotational Slide (Slump)	—	Rotational Debris Slide (Slump)	Rotational Earth Slide (Slump)
Translational Slide	Translational Rock Slide	Translational Debris Slide	Translational Earth Slide
Flows	—	Debris Flow	Earth flow
Soil Creep	—	Creep	Creep

Flows are rapidly moving mass-wasting events in which the loose material is typically mixed with abundant water, creating long runouts at the slope base. Flows are commonly separated into debris flow (coarse material) and earthflow (fine material) depending on the type of material involved and the amount of water. Some of the largest and fastest flows on land are called sturzstroms, or long runout landslides. They are still poorly understood, but are known to travel for long distances, even in places without significant atmospheres like the Moon.

Creep is the imperceptibly slow downward movement of material caused by a regular cycle of nighttime freezing followed by daytime thawing in unconsolidated material such as soil. During the freeze, expansion of ice pushes soil particles out away from the slope, while the next day following the thaw, gravity pulls them directly downward. The net effect is a gradual movement of surface soil particles downhill. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges. A special type of soil creep is solifluction, which is the slow movement of soil lobes on low-angle slopes due to soil seasonally freezing and thawing in high-latitude, typically sub-Arctic, Arctic, and Antarctic locations.

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Landslide Hazards, David Applegate

10.3.2 Parts of a Landslide

Landslides have several identifying features that can be common across the different types of mass wasting. Note that there are many exceptions, and a landslide does not have to have these features. Displacement of material by landslides causes the absence of material uphill and the deposition of new material downhill, and careful observation can identify the evidence of that displacement. Other signs of landslides include tilted or offset structures or natural features that would normally be vertical or in place.
Many landslides have escarpments or scarps. Landslide scarps, like fault scarps, are steep terrain created when movement of the adjacent land exposes a part of the subsurface. The most prominent scarp is the main scarp, which marks the uphill extent of the landslide. As the disturbed material moves out of place, a step slope forms and develops a new hillside escarpment for the undisturbed material. Main scarps are formed by movement of the displaced material away from the undisturbed ground and are the visible part of slide rupture surface.

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The slide rupture surface is the boundary of the body of movement of the landslide. The geologic material below the slide surface does not move, and is marked on the sides by the flanks of the landslide and at the end by the toe of the landslide.

The toe of the landslide marks the end of the moving material. The toe marks the runout, or maximum distance traveled, of the landslide. In rotational landslides, the toe is often a large, disturbed mound of geologic material, forming as the landslide moves past its original rupture surface.

Rotational and translational landslides often have extensional cracks, sag ponds, hummocky terrain and pressure ridges. Extensional cracks form when a landslide’s toe moves forward faster than the rest of landslide, resulting in tensional forces. Sag ponds are small bodies of water filling depressions formed where landslide movement has impounded drainage. Hummocky terrain is undulating and uneven topography that results from the ground being disturbed. Pressure ridges develop on the margins of the landslide where material is forced upward into a ridge structure.

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10.4 Examples of Landslides

Landslides in United States

1925, Gros Ventre, Wyoming:On June 23, 1925, a 38 million cubic meter (50 million cu yd) translational rock slide occurred next to the Gros Ventre River (pronounced “grow vont”) near Jackson Hole, Wyoming. Large boulders dammed the Gros Ventre River and ran up the opposite side of the valley several hundred vertical feet. The dammed river created Slide Lake, and two years later in 1927, lake levels rose high enough to destabilize the dam. The dam failed and caused a catastrophic flood that killed six people in the small downstream community of Kelly, Wyoming.

A combination of three factors caused the rock slide: 1) heavy rains and rapidly melting snow saturated the Tensleep Sandstone causing the underlying shale of the Amsden Formation to lose its shear strength, 2) the Gros Ventre River cut through the sandstone creating an oversteepened slope, and 3) soil on top of the mountain became saturated with water due to poor drainage. The cross-section diagram shows how the parallel bedding planes between the Tensleep Sandstone and Amsden Formation offered little friction against the slope surface as the river undercut the sandstone. Lastly, the rockslide may have been triggered by an earthquake.

1959, Madison Canyon, Montana:In 1959, the largest earthquake in Rocky Mountain recorded history, magnitude 7.5, struck the Hebgen Lake, Montana area, causing a destructive seiche on the lake (see Chapter 9). The earthquake caused a rock avalanche that dammed the Madison River, creating Quake Lake, and ran up the other side of the valley hundreds of vertical feet. Today, there are still house-sized boulders visible on the slope opposite their starting point. The slide moved at a velocity of up to 160.9 kph (100 mph), creating an incredible air blast that swept through the Rock Creek Campground. The slide killed 28 people, most of whom were in the campground and remain buried there. In a manner like the Gros Ventre slide, foliation planes of weakness in metamorphic rock outcrops were parallel with the surface, compromising shear strength.

1980, Mount Saint Helens, Washington: On May 18, 1980 a 5.1-magnitude earthquake triggered the largest landslide observed in the historical record. This landslide was followed by the lateral eruption of Mount Saint Helens volcano, and the eruption was followed by volcanic debris flows known as lahars. The volume of material moved by the landslide was 2.8 cubic kilometers (0.67 mi3).

1995 and 2005, La Conchita, California: On March 4, 1995, a fast-moving earthflow damaged nine houses in the southern California coastal community of La Conchita. A week later, a debris flow in the same location damaged five more houses. Surface-tension cracks at the top of the slide gave early warning signs in the summer of 1994. During the rainy winter season of 1994/1995, the cracks grew larger. The likely trigger of the 1995 event was unusually heavy rainfall during the winter of 1994/1995 and rising groundwater levels. Ten years later, in 2005, a rapid-debris flow occurred at the end of a 15-day period of near-record rainfall in southern California. Vegetation remained relatively intact as it was rafted on the surface of the rapid flow, indicating that much of the landslide mass simply was being carried on a presumably much more saturated and fluidized layer beneath. The 2005 slide damaged 36 houses and killed 10 people.

2014, Oso Landslide, Washington: On March 22, 2014, a landslide of approximately 18 million tons (10 million yd3) traveled at 64 kph (40 mph), extended for nearly a 1.6 km (1 m), and dammed the North Fork of the Stillaguamish River. The landslide covered 40 homes and killed 43 people in the Steelhead Haven community near Oso, Washington. It produced a volume of material equivalent to 600 football fields covered in material 3 m (10 ft) deep. The winter of 2013-2014 was unusually wet with almost double the average amount of precipitation. The landslide occurred in an area of the Stillaguamish River Valley historically active with many landslides, but previous events had been small.

Yosemite National Park Rock Falls:The steep cliffs of Yosemite National Park cause frequent rock falls. Fractures created to tectonic stresses and exfoliation and expanded by frost wedging can cause house-sized blocks of granite to detach from the cliff-faces of Yosemite National Park. The park models potential runout, the distance landslide material travels, to better assess the risk posed to the millions of park visitors.

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Utah Landslides

Markagunt Gravity Slide: About 21–22 million years ago, one of the biggest land-based landslides yet discovered in the geologic record displaced more than 1,700 cu km (408 cu mi) of material in one relatively fast event. Evidence for this slide includes breccia conglomerates (see Chapter 5), glassy pseudotachylytes, (see Chapter 6), slip surfaces (similar to faults) see Chapter 9), and dikes (see Chapter 7). The landslide is estimated to encompass an area the size of Rhode Island and to extend from near Cedar City, Utah to Panguitch, Utah. This landslide was likely the result of material released from the side of a growing laccolith (a type of igneous intrusion) see Chapter 4), after being triggered by an eruption-related earthquake.

1983, Thistle Slide: Starting in April of 1983 and continuing into May of that year, a slow-moving landslide traveled 305 m (1,000 ft) downhill and blocked Spanish Fork Canyon with an earthflow dam 61 m (200 ft) high. This caused disastrous flooding upstream in the Soldier Creek and Thistle Creek valleys, submerging the town of Thistle. As part of the emergency response, a spillway was constructed to prevent the newly formed lake from breaching the dam. Later, a tunnel was constructed to drain the lake, and currently the river continues to flow through this tunnel. The rail line and US-6 highway had to be relocated at a cost of more than $200 million.

2013, Rockville Rock Fall:Rockville, Utah is a small community near the entrance to Zion National Park. In December of 2013, a 2,700 ton (1,400 yd3) block of Shinarump Conglomerate fell from the Rockville Bench cliff, landed on the steep 35-degree slope below, and shattered into several large pieces that continued downslope at a high speed. These boulders completely destroyed a house located 375 feet below the cliff (see the before and after photographs) and killed two people inside the home. The topographic map shows other rock falls in the area prior to this catastrophic event.

2014, North Salt Lake Slide:In August 2014 after a particularly wet period, a slow moving rotational landslide destroyed one home and damaged nearby tennis courts.

Reports from residents suggested that ground cracks had been seen near the top of the slope at least a year prior to the catastrophic movement. The presence of easily-drained sands and gravels overlying more impermeable clays weathered from volcanic ash, along with recent regrading of the slope, may have been contributing causes of this slide. Local heavy rains seem to have provided the trigger. In the two years after the landslide, the slope has been partially regraded to increase its stability. Unfortunately, in January 2017, parts of the slope have shown reactivation movement. Similarly, in 1996 residents in a nearby subdivision started reporting distress to their homes. This distress continued until 2012 when 18 homes became uninhabitable due to extensive damage and were removed. A geologic park was constructed in the now vacant area.

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North Salt Lake Landslide

2013, Bingham Canyon Copper Mine Landslide, Utah: At 9:30 pm on April 10, 2013, more than 65 million cubic meters of steep terraced mine wall slid down into the engineered pit of Bingham Canyon mine, making it one of the largest historic landslides not associated with volcanoes. Radar systems maintained by the mine operator warned of movement of the wall, preventing the loss of life and limiting the loss of property.

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10.5 Chapter Summary

Mass wasting is a geologic term describing all downhill rock and soil movement due to gravity. Mass wasting occurs when a slope is too steep to remain stable with existing material and conditions. Loose rock and soil, called regolith, are what typically move during a mass-wasting event. Slope stability is determined by two factors: the angle of the slope and the shear strength of the accumulated materials. Mass-wasting events are triggered by changes that oversteepen slope angles and weaken slope stability, such as rapid snow melt, intense rainfall, earthquake shaking, volcanic eruption, storm waves, stream erosion, and human activities. Excessive precipitation is the most common trigger. Mass-wasting events are classified by their type of movement and material, and they share common morphological surface features. The most common types of mass-wasting events are rockfalls, slides, flows, and creep.

Mass-wasting movement ranges from slow to dangerously rapid. Areas with steep topography and rapid rainfall, such as the California coast, Rocky Mountain Region, and Pacific Northwest, are particularly susceptible to hazardous mass-wasting events. By examining examples and lessons learned from famous mass-wasting events, scientists have a better understanding of how mass-wasting occurs. This knowledge has brought them closer to predicting where and how these potentially hazardous events may occur and how people can be protected.

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References

Haugerud, R.A., 2014, Preliminary interpretation of pre-2014 landslide deposits in the vicinity of Oso, Washington: US Geological Survey.

Highland, L., 2004, Landslide types and processes: pubs.er.usgs.gov.

Highland, L.M., and Bobrowsky, P., 2008, The Landslide Handbook – A Guide to Understanding Landslides: U.S. Geological Survey USGS Numbered Series 1325, 147 p.

Highland, L.M., and Schuster, R.L., 2000, Significant landslide events in the United States: United States Geological Survey.

Hildenbrand, T.G., and Hendricks, J.D., 1995, Geophysical setting of the Reelfoot rift and relations between rift structures and the New Madrid seismic zone: U.S. Geological Survey Professional Paper 1538-E, 36 p.

Hungr, O., Leroueil, S., and Picarelli, L., 2013, The Varnes classification of landslide types, an update: Landslides, v. 11, no. 2, p. 167–194.

Jibson, R.W., 2005, Landslide hazards at La Conchita, California: United States Geological Survey Open-File Report 2005-1067.

Lipman, P.W., and Mullineaux, D.R., 1981, The 1980 eruptions of Mount St. Helens, Washington: US Geological Survey USGS Numbered Series 1250, 844 p., doi: 10.3133/pp1250.

Lund, W.R., Knudsen, T.R., and Bowman, S.D., 2014, Investigation of the December 12, 2013, Fatal Rock Fall at 368 West Main Street, Rockville, Utah: Utah Geological Survey 273, 24 p.

United States Forest Service, 2016, A Brief History of the Gros Ventre Slide Geological Site: United States Forest Service.

Media Attributions

Thistlelandslideusgs © R.L. Schuster, U.S. Geological Survey is licensed under a Public Domain license

Block_on_Incline © Paul Inkenbrandt

blocks-on-incline © Paul Inkenbrandt

Angleofrepose © Captain Sprite is licensed under a CC BY-SA (Attribution ShareAlike) license

Relative-stability-of-slopes-as-a-function-of-the-orientation-of-weaknesses © Steven Earle is licensed under a CC BY (Attribution) license

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Fig3grouping-2LG © Margo Johnson (U.S. Geological Survey) is licensed under a Public Domain license

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10.3 Rotational Landslide QR Code

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GrosVentre © Paul Inkenbrandt

Gros_Ventre-Cross-section © U.S. Geological Survey is licensed under a Public Domain license

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La_Conchita_oblique LIDAR © U.S. Geological Survey is licensed under a Public Domain license

La_Conchita_1995 © U.S. Geological Survey is licensed under a Public Domain license

USGS_MR_Oso_Aerial_clipped_adjusted © U.S. Geological Survey is licensed under a Public Domain license

JdLC-G_Oso Landslide Area 2014 Lidar Map © U.S. Geological Survey is licensed under a Public Domain license

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markagunt-megabreccia-map-1030×741 © Robert F. Biek, Utah Geological Survey, Survey Notes, v. 45 no. 2, May 2013 is licensed under a CC BY-SA (Attribution ShareAlike) license

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Ch.10 Review QR Code

11 Water

11 Water

KEY CONCEPTS

Describe the processes of the water cycle

Describe drainage basins, watershed protection, and water budget

Describe reasons for water laws, who controls them, and how water is shared in the western U.S.

Describe zone of transport, zone of sediment production, zone of deposition, and equilibrium

Describe stream landforms: channel types, alluvial fans, floodplains, natural levees, deltas, entrenched meanders, and terraces

Describe the properties required for a good aquifer; define confining layer water table

Describe three major groups of water contamination and three types of remediation

Describekarst topography, how it is created, and the landforms that characterize it

All life on Earth requires water. The hydrosphere (Earth’s water) is an important agent of geologic change. Water shapes our planet by depositing minerals, aiding lithification, and altering rocks after they are lithified. Water carried by subducted oceanic plates causes flux melting of upper mantle material. Water is among the volatiles in magma and emerges at the surface as steam in volcanoes.

Humans rely on suitable water sources for consumption, agriculture, power generation, and many other purposes. In pre-industrial civilizations, the powerful controlled water resources [1, 2]. As shown in the figures, two thousand year old Roman aqueducts still grace European, Middle Eastern, and North African skylines. Ancient Mayan architecture depicts water imagery such as frogs, water-lilies, water fowl to illustrate the importance of water in their societies [3]. In the drier lowlands of the Yucatan Peninsula, mask facades of the hooked-nosed rain god, Chac (or Chaac) are prominent on Mayan buildings such as the Kodz Poop (Temple of the Masks, sometimes spelled Coodz Poop) at the ceremonial site of Kabah. To this day government controlled water continues to be an integral part of most modern societies.

11.1 Water Cycle

The water cycle is the continuous circulation of water in the Earth’s atmosphere. During circulation, water changes between solid, liquid, and gas (water vapor) and changes location. The processes involved in the water cycle are evaporation, transpiration, condensation, precipitation, and runoff.

Evaporation is the process by which a liquid is converted to a gas. Water evaporates when solar energy warms the water sufficiently to excite the water molecules to the point of vaporization. Evaporation occurs from oceans, lakes, and streams and the land surface. Plants contribute significant amounts of water vapor as a byproduct of photosynthesis called transpiration that occurs through the minute pores of plant leaves. The term evapotranspiration refers to these two sources of water entering the atmosphere and is commonly used by geologists.

Water vapor is invisible. Condensation is the process of water vapor transitioning to a liquid. Winds carrywater vapor in the atmosphere long distances. When water vapor cools or when air masses of different temperatures mix,water vapor may condense back into droplets of liquid water. These water droplets usually form around a microscopic piece of dust or salt called condensation nuclei. Thesesmall droplets of liquid water suspended in the atmosphere becomevisible as in a cloud. Water droplets inside clouds collide and stick together, growing into largerdroplets. Once the water droplets become big enough, they fall to Earth as rain, snow, hail, or sleet.

Once precipitation has reached the Earth’s surface, it can evaporate or flow as runoff into streams, lakes, and eventually back to the oceans. Water in streams and lakes is called surface water. Or water can also infiltrate into the soil and fill the pore spaces in the rock or sediment underground to become groundwater. Groundwater slowly moves through rock and unconsolidated materials. Some groundwater may reach the surface again, where it discharges as springs, streams, lakes, and the ocean. Also, surface water in streams and lakes can infiltrate again to recharge groundwater. Therefore, the surface water and groundwater systems are connected.

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11.2 Water Basins and Budgets

The basic unit of division of the landscape is the drainage basin, also known as a catchment or watershed. It is the area of land that captures precipitation and contributes runoff to a stream or stream segment [4]. Drainage divides are local topographic high points that separate one drainage basin from another [5]. Water that falls on one side of the divide goes to one stream, and water that falls on the other side of the divide goes to a different stream. Each stream, tributary and streamlet has its own drainage basin. In areas with flatter topography, drainage divides are not as easily identified but they still exist [6].

The headwater is where the stream begins. Smaller tributarystreams combine downhill to make the larger trunk of the stream. The mouth is where the stream finally reaches its end. The mouth of most streams is at the ocean. However, a rare number of streams do not flow to the ocean, but rather end in a closed basin (or endorheic basin) where the only outlet is evaporation. Most streams in the Great Basin of Western North America end in endorheic basins. For example, in Salt Lake County, Utah, Little Cottonwood Creek and the Jordan River flow into the endorheic Great Salt Lake where the water evaporates.

Perennial streams flow all year round. Perennial streams occur in humid or temperate climates where there is sufficient rainfall and low evaporation rates. Water levels rise and fall with the seasons, depending on the discharge. Ephemeral streams flow only during rain events or the wet season. In arid climates, like Utah, many streams are ephemeral. These streams occur in dry climates with low amounts of rainfall and high evaporation rates. Their channels are often dry washes or arroyos for much of the year and their sudden flow causes flash floods [7].

Along Utah’s Wasatch Front, the urban area extending north to south from Brigham City to Provo, there are several watersheds that are designated as “watershed protection areas” that limit the type of use allowed in those drainages in order to protect culinary water. Dogs and swimming are limited in those watersheds because of the possibility of contamination by harmful bacteria and substances to the drinking supply of Salt Lake City and surrounding municipalities.

Water in the water cycle is very much like money in a personal budget. Income includes precipitation and stream and groundwater inflow. Expenses include groundwater withdrawal, evaporation, and stream and groundwater outflow. If the expenses outweigh the income, the water budget is not balanced. In this case, water is removed from savings, i.e. water storage, if available. Reservoirs, snow, ice, soil moisture, and aquifers all serve as storage in a water budget. In dry regions, the water is critical for sustaining human activities. Understanding and managing the water budgetis an ongoing political and social challenge.

Hydrologists create groundwater budgets within any designated area, but they are generally made for watershed (basin) boundaries, because groundwater and surface water are easier to account for within these boundaries. Water budgets can be created for state, county, or aquifer extent boundaries as well. The groundwater budget is an essential component of the hydrologic model; hydrologists use measured data with a conceptual workflow of the model to better understand the water system.

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11.3 Water Use and Distribution

In the United States, 1,344 billion L (355 billion gallons) of ground and surface water are used each day, of which 288 billion L (76 billion gallons) are fresh groundwater. The state of California uses 16% of national groundwater [8].

Utah is the second driest state in the United States. Nevada, having a mean statewide precipitation of 31 cm (12.2 inches) per year, is the driest. Utah also has the second highest per capita rate of total domestic water use of 632.16 L (167 gallonsL per day per person [8]. With the combination of relatively high demand and limited quantity, Utah is at risk for water budget deficits.

11.3.1 Surface Water Distribution

Fresh water is a precious resource and should not be taken for granted, especially in dry climates. Surface water makes up only 1.2% of the fresh water available on the planet, and 69% of that surface water is trapped in ground ice and permafrost. Stream water accounts for only 0.006% of all freshwater and lakes contain only 0.26% of the world’s fresh water [9].

Global circulation patterns are the most important factor in distributing surface water through precipitation. Due to the Coriolis effect and the uneven heating of the Earth, air rises near the equator and near latitudes 60° north and south. Air sinks at the poles and latitudes 30° north and south (see Chapter 13). Land masses near rising air are more prone to humid and wet climates. Land masses near sinking air, which inhibits precipitation, are prone to dry conditions [10, 11]. Prevailing winds, ocean circulation patterns such as the Gulf Stream’s effects on eastern North America, rain shadows (the dry leeward sides of mountains), and even the proximity of bodies of water can affect local climate patterns. When this moist air collides with the nearby mountains causing it to rise and cool, the moisture may fall out as snow or rain on nearby areas in a phenomenon known as “lake-effect precipitation.” [12]

In the United States, the 100th meridian roughly marks the boundary between the humid and arid parts of the country. Growing crops west of the 100th meridian requires irrigation [13]. In the west, surface water is stored in reservoirs and mountain snowpacks [14], then strategically released through a system of canals during times of high water use.

Some of the driest parts of the western United States are in the Basin and Range Province. The Basin and Range has multiple mountain ranges that are oriented north to south. Most of the basin valleys in the Basin and Range are dry, receiving less than 30 cm (12 inches) of precipitation per year. However, some of the mountain ranges can receive more than 1.52 m (60 inches) of water as snow or snow-water-equivalent. The snow-water equivalent is the amount of water that would result if the snow were melted, as the snowpack is generally much thicker than the equivalent amount of water that it would produce [12].

11.3.2 Groundwater Distribution

Water source	Water volume (cubic miles)	Fresh water (%)	Total water (%)
Oceans, Seas, & Bays	321,000,000	—	96.5
Ice caps, Glaciers , & Permanent Snow	5,773,000	68.7	1.74
Groundwater	5,614,000	—	1.69
— Fresh	2,526,000	30.1	0.76
— Saline	3,088,000	—	0.93
Soil Moisture	3,959	0.05	0.001
Ground Ice & Permafrost	71,970	0.86	0.022
Lakes	42,320	—	0.013
— Fresh	21,830	0.26	0.007
— Saline	20,490	—	0.006
Atmosphere	3,095	0.04	0.001
Swamp Water	2,752	0.03	0.0008
Rivers	509	0.006	0.0002
Biological Water	269	0.003	0.0001
Source: Igor Shiklomanov’s chapter “World fresh water resources” in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World’s Fresh Water Resources (Oxford University Press, New York)[zotpressInText item=”{P7VGIQT4}” format=”%num%” brackets=”yes” separator=”comma”]

Groundwater makes up 30.1% of the fresh water on the planet, making it the most abundant reservoir of fresh water accessible to most humans. The majority of freshwater, 68.7%, is stored in glaciers and ice caps as ice [9]. As the glaciers and ice caps melt due to global warming, this fresh water is lost as it flows into the oceans.

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11.4 Water Law

Federal and state governments have put laws in place to ensure the fair and equitable use of water. In the United States, the states are tasked with creating a fair and legal system for sharing water.

11.4.1 Water Rights

Because of the limited supply of water, especially in the western United States, states disperse a system of legal water rights defined as a claim to a portion or all of a water source, such as a spring, stream, well, or lake. Federal law mandates that states control water rights, with the special exception of federally reserved water rights, such as those associated with national parks and Native American tribes, and navigation servitude that maintains navigable water bodies. Each state in the United States has a different way to disperse and manage water rights.

A person, entity, company, or organization, must have a water right to legally extract or use surface or groundwater in their state. Water rights in some western states are dictated by the concept of prior appropriation, or “first in time, first in right,” where the person with the oldest water right gets priority water use during times when there is not enough water to fulfill every water right.

The Colorado River and its tributaries pass through a desert region, including seven states (Wyoming, Colorado, Utah, New Mexico, Arizona, Nevada, California), Native American reservations, and Mexico. As the western United States became more populated and while California was becoming a key agricultural producer, the states along the Colorado River realized that the river was important to sustaining life in the West.

To guarantee certain perceived water rights, these western states recognized that a water budget was necesary for the Colorado River Basin. Thus was enacted the Colorado River Compact in 1922 to ensure that each state got a fair share of the river water. The Compact granted each state a specific volume of water based on the total measured flow at the time. However, in 1922, the flow of the river was higher than its long-term average flow, consequently, more water was allocated to each state than is typically available in the river [16].

Over the next several decades, lawmakers have made many other agreements and modifications regarding the Colorado River Compact, including those agreements that brought about the Hoover Dam (formerly Boulder Dam), and Glen Canyon Dam,and a treaty between the American and Mexican governments. Collectively, the agreements are referred to as “The Law of the River“ by the United States Bureau of Reclamation. Despite adjustments to the Colorado River Compact, many believe that the Colorado River is still over-allocated, as the Colorado Riverflow no longer reaches the Pacific Ocean, its original terminus (base level). Dams along the Colorado River have caused water to divert and evaporate, creating serious water budget concerns in the Colorado River Basin. Predicted drought associated with global warming is causing additional concerns about over-allocating the Colorado River flow in the future.

The Law of the River highlights the complex and prolonged nature of interstate water rights agreements, as well as the importance of water.

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The Snake Valley straddles the border of Utah and Nevada withmore of the irrigable land area lying on the Utah side of the border.In 1989, the Southern Nevada Water Authority (SNWA) submitted applications for water rights to pipe up to 191,189,707 cu m (155,000 ac-ft) of water per year (an acre-foot of water is one acre covered with water one foot deep) from Spring, Snake, Delamar, Dry Lake, and Cave valleys to southern Nevada, mostly for Las Vegas [17]. Nevada and Utah have attempted a comprehensive agreement, but negotiations have not yet been settled.

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NPR story on Snake Valley

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Dean Baker Story

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11.4.2 Water Quality and Protection

Two major federal laws that protect water quality in the United States are the Clean Water Act and the Safe Drinking Water Act. The Clean Water Act, an amendment of the Federal Water Pollution Control Act, protects navigable waters from dumping and point-source pollution. The Safe Drinking Water Act ensures that water that is provided by public water suppliers, like cities and towns, is safe to drink [18].

The U.S. Environmental Protection Agency Superfund program ensures the cleanup of hazardous contamination, and can be applied to situations of surface water and groundwater contamination. It is part of the Comprehensive Environmental Response, Compensation, and Liability Act of 1980. Under this act, state governments and the U.S. Environmental Protection Agency can use the superfund to pay for remediation of a contaminated site and then file a lawsuit against the polluter to recoup the costs. Or to avoid being sued, the polluter that caused the contamination may take direct action or provide funds to remediatethe contamination.

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11.5 Surface Water

Geologically, a streamis a body of flowing surface water confined to a channel. Terms such as river, creek and brook are social terms not used in geology. Streams erode and transport sediments, making them the most important agents of the earth’s surface, along with wave action (see Chapter 12) in eroding and transporting sediments. They create much of the surface topography and are an important water resource.

Several factors cause streams to erode and transport sediment, but the two main factors are stream–channel gradient and velocity. Stream–channel gradient is the slope of thestream usually expressed in meters per kilometer or feet per mile. A steeper channel gradient promotes erosion. When tectonic forces elevate a mountain, the stream gradient increases, causing themountain stream to erode downward and deepen its channel eventually forming a valley. Stream–channel velocity is the speed at which channel water flows. Factors affecting channel velocityinclude channel gradient which decreases downstream, discharge and channel size which increaseas tributaries coalesce, and channel roughness which decreases as sediment lining the channel walls decreases in size thus reducing friction.The combined effect of these factors is that channel velocity actually increases from mountain brooks to the mouth of the stream.

11.5.1 Discharge

Stream size is measured in terms of discharge, the volume of water flowing past a point in the stream over a defined time interval. Volume is commonly measured in cubic units (length x width x depth), shown as feet3 (ft3) or meter3 (m3). Therefore, the units of discharge are cubic feet per second (ft3/sec or cfs). Therefore, the units of discharge are cubic meters per second, (m³/s or cms, or cubic feet per second (ft³/sec or cfs). Stream discharge increases downstream. Smaller streams have less discharge than larger streams. For example, the Mississippi River is the largest river in North America, with an average flow of about 16,990.11 cms (600,000 cfs) [19]. For comparison, the average discharge of the Jordan River at Utah Lake is about 16.25 cms (574 cfs) [20] and for the annual discharge of the Amazon River, (the world’s largest river), annual discharge is about 175,565 cms (6,200,000 cfs) [21].

Discharge can be expressed by the following equation:

Q = V A

Q = discharge cms (or ft3/sec),

A = cross-sectional area of the stream channel [width times average depth]as m2 (or in2 or ft2),

V = average channel velocity m/s (or ft/sec) [7]

At a given location along the stream, velocity varies with stream width, shape, and depth within the stream channel as well. Whenthe stream channel narrows but discharge remains constant, the same volume of water must flows through a narrower space causing the velocity to increase, similar to putting a thumb over the end of a backyard water hose. In addition, during rain storms or heavy snow melt, runoffincreases, which increases stream discharge and velocity.

When the streamchannel curves, the highest velocity will be on the outside of the bend. When the streamchannel is straight and uniformly deep, the highest velocity is in the channel center at the top of the water where it is the farthest from frictional contact with the stream channel bottom and sides. In hydrology, the thalweg of a river is the line drawn that shows its natural progression and deepest channel, as is shown in the diagram.

11.5.2 Runoff vs. Infiltration

Factors that dictate whether water will infiltrate into the ground or run off over the land include the amount, type, and intensity of precipitation; the type and amount of vegetation cover; the slope of the land; the temperature and aspect of the land; preexisting conditions; and the type of soil in the infiltrated area. High– intensityrain will cause more runoff than the same amount of rain spread out over a longer duration. If the rain falls faster than the soil’s properties allow it to infiltrate, then the water that cannot infiltrate becomes runoff.Dense vegetation can increase infiltration, as the vegetative cover slows the water particle’s overland flow giving them more time to infiltrate. If a parcel of land has more direct solar radiation or higher seasonal temperatures, there will be less infiltration and runoff, as evapotranspiration rates will be higher. As the land’s slope increases, so does runoff, because the water is more inclined to move downslope than infiltrate into the ground. Extreme examples are a basin and a cliff, where water infiltrates much quicker into a basin than a cliff that has the same soil properties. Because saturated soil does not have the capacity to take more water, runoff is generally greater over saturated soil. Clay-rich soil cannot accept infiltration as quickly as gravel-rich soil.

11.5.3 Drainage Patterns

The pattern of tributaries within a region is called drainage pattern. They depend largely on the type of rock beneath, and on structures within that rock (such as folds and faults). The main types of drainage patterns are dendritic, trellis, rectangular, radial, and deranged. Dendritic patternsare the most common and develop in areas where the underlying rock or sediments are uniform in character, mostly flat lying, and can be eroded equally easily in all directions. Examples are alluvial sediments or flat lying sedimentary rocks. Trellis patternstypically develop where sedimentary rocks have been folded or tilted and then eroded to varying degrees depending on their strength. The Appalachian Mountains in eastern United States have many good examples of trellis drainage. Rectangular patternsdevelop in areas that have very little topography and a system of bedding planes, joints, or faults that form a rectangular network. A radial patternforms when streams flow away from a central high point such as a mountain top or volcano, with the individual streams typically having dendritic drainage patterns. In places with extensive limestone deposits, streams can disappear into the groundwater via caves and subterranean drainage and this creates a deranged pattern[4].

11.5.4 Fluvial Processes

Fluvial processes dictate how a stream behaves and include factors controlling fluvial sediment production, transport, and deposition. Fluvial processes include velocity, slope and gradient, erosion, transportation, deposition, stream equilibrium, and base level.

Streams can be divided into three main zones: the many smaller tributaries in the source area, the main trunk stream in the floodplain and the distributaries at the mouth of the stream. Major stream systems like the Mississippi are composed of many source areas, many tributaries and trunk streams, all coalescing into the one main stream draining the region. The zones of a stream are defined as 1) the zone of sediment production (erosion), 2) the zone of transport, and 3) the zone of deposition. The zone of sediment production is located in the headwaters of the stream. In the zone of sediment transport, there is a general balance between erosion of the finer sediment in its channel and transport of sediment across the floodplain. Streams eventually flow into the ocean or end in quiet water with a delta which is a zone of sedimentdepositionlocated at the mouthof a stream [6]. The longitudinal profileof a stream is a plot of the elevation of the stream channel at all points along its course and illustrates the location of the three zones [22]

Zone of Sediment Production

The zone of sediment production is located in the headwaters of a stream where rills and gullies erode sediment and contribute to larger tributary streams. These tributaries carry sediment and water further downstream to the main trunk of the stream. Tributaries at the headwaters have the steepest gradient; erosion there produces considerable sediment carried b the stream. Headwater streams tend to be narrow and straight with small or non-existent floodplains adjacent to the channel. Since the zone of sediment production is generally the steepest part of the stream, headwaters are generally located in relatively high elevations. The Rocky Mountains of Wyoming and Colorado west of the Continental Divide contain much of the headwaters for the Colorado River which then flows from Colorado through Utah and Arizona to Mexico. Headwaters of the Mississippi river system lie east of the Continental Divide in the Rocky Mountains and west of the Appalachian Divide.

Zone of Sediment TransPORT

Streams transport sediment great distances from the headwaters to the ocean, the ultimate depositional basins. Sediment transportation is directly related to stream gradient and velocity. Faster and steeper streams can transport larger sediment grains. When velocity slows down, larger sediments settle to the channel bottom. When the velocity increases, those larger sediments are entrained and move again.

Transported sediments are grouped into bedload, suspended load, and dissolved load as illustrated in the above image. Sediments moved along the channel bottom are the bedloadthat typically consists of the largest and densest particles. Bedload is moved by saltation (bouncing) and traction (being pushed or rolled along by the force of the flow). Smaller particles are picked up by flowing water and carried in suspension as suspended load. The particle size that is carried in suspended and bedload depends on the flow velocity of the stream. Dissolved load in a stream is the total of the ions in solution from chemical weathering, including such common ions such as bicarbonate (-HCO3–), calcium (Ca+2), chloride (Cl-1), potassium (K+1), and sodium (Na+1). The amounts of these ions are not affected by flow velocity.

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A floodplainis the flat area of land adjacent to a stream channel inundated with flood water on a regular basis. Stream flooding is a natural process that adds sediment to floodplains. A stream typically reaches its greatest velocity when it is close to flooding, known as the bankfull stage. As soon as the flooding stream overtops its banks and flows onto its floodplain, the velocity decreases. Sediment that was being carried by the swiftly moving water is deposited at the edge of the channel, forming a low ridge or natural levée. In addition, sediments are added to the floodplain during this flooding process contributing to fertile soils [4].

Zone of SEDIMENT Deposition

Deposition occurs when bedload and suspended load come to rest on the bottom of the stream channel, lake, or ocean due to decrease in stream gradient and reduction in velocity. While both deposition and erosion occur in the zone of transport such as on point bars and cut banks, ultimate deposition where the stream reaches a lake or ocean. Landforms called deltas form where the stream enters quiet water composed of the finest sediment such as fine sand, silt, and clay.

Equilibrium and Base Level

All three stream zones are present in the typical longitudinal profileof a stream which plots the elevation of the channel at all points along its course (see figure). All streams have a long profile. The long profile shows the stream gradient from headwater to mouth. All streams attempt to achieve an energetic balance among erosion, transport, gradient, velocity, discharge, and channel characteristics along the stream’s profile. This balance is called equilibrium, a state called grade.

Another factor influencing equilibrium is base level, the elevation of the stream‘s mouth representingthe lowest level to which a stream can erode. The ultimate base level is, of course, sea-level. A lake or reservoir may also represent base level for a stream entering it. The Great Basin of western Utah, Nevada, and parts of some surrounding states contains no outlets to the sea and provides internal base levels for streams within it. Base level for a stream entering the ocean changes if sea-level rises or falls. Base level also changes if a natural or human-made dam is added along a stream‘s profile. When base level is lowered, a stream will cut down and deepen its channel. When base level rises, deposition increases as the stream adjusts attempting to establish a new state of equilibrium. A stream that has approximately achieved equilibrium is called a gradedstream.

11.5.5 Fluvial Landforms

Stream landforms are the land features formed on the surface by either erosion or deposition. The stream-related landforms described here are primarily related to channel types.

Channel Types

Stream channels can be straight, braided, meandering, or entrenched. The gradient, sediment load, discharge, and location of base level all influence channel type. Straight channelsare relatively straight, located near the headwaters, have steep gradients, low discharge, and narrow V-shaped valleys. Examples of these are located in mountainous areas.

Braided streamshave multiple channels splitting and recombining around numerous mid-channel bars. These are found in floodplains with low gradients in areas with near sources of coarse sediment such as trunk streams draining mountains or in front of glaciers.

Meandering streamshave a single channel that curves back and forth like a snake within its floodplainwhere it emerges from its headwaters into the zone of transport. Meandering streams are dynamic creating a wide floodplain by eroding and extending meander loops side-to-side. The highest velocity water is located on the outside of a meander bend. Erosion of the outside of the curve creates a feature called a cut bank and the meander extends its loop wider by this erosion.

The thalwegof the stream is the deepest part of the stream channel. In the straight parts of the channel, the thalweg and highest velocity are in the center of the channel. But at the bend of a meandering stream, the thalweg shifts toward the cut bank. Opposite the cutbank on the inside bend of the channel is the lowest stream velocity and is an area of deposition called a point bar.

In areas of tectonic uplift such as on the Colorado Plateau, meandering streams that once flowed on the plateau surface have become entrenched or incised as uplift occurred and the stream cut its meandering channel down into bedrock. Over the past several million years, the Colorado River and its tributaries have incised into the flat lying rocks of the plateau by hundreds, even thousands of feet creating deep canyons including the Grand Canyon in Arizona.

Many fluvial landforms occur on a floodplain associated with a meandering stream. Meander activity and regular flooding contribute to widening the floodplain by eroding adjacent uplands. The stream channels are confined by natural leveesthat have been built up over many years of regular flooding.Natural levees can isolate and direct flow from tributary channels on the floodplain from immediately reaching the main channel. These isolated streams are called yazoo streams and flow parallel to the main trunk stream until there is an opening in the levee to allow for a belated confluence.

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To limit flooding, humans build artificial levees on flood plains. Sediment that breaches the levees during flood stage is called crevasse splaysand delivers silt and clay onto the floodplain. These deposits are rich in nutrients and often make good farm land. When floodwaters crest over human-made levees, the levees quickly erode with potentially catastrophic impacts. Because of the good soils, farmers regularly return after floods and rebuild year after year.

Through erosion on the outsides of the meanders and deposition on the insides, the channels of meandering streams move back and forth across their floodplain over time. On very broad floodplains with very low gradients, the meander bends can become so extreme that they cut across themselves at a narrow neck (see figure) called a cutoff. The former channel becomes isolated and forms an oxbow lake seen on the right of the figure. Eventually the oxbow lake fills in with sediment and becomes a wetland and eventually a meander scar. Stream meanders can migrate and form oxbow lakes in a relatively short amount of time. Where stream channels form geographic and political boundaries, this shifting of channels can cause conflicts.

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Alluvial fans are a depositional landform created where streams emerge from mountain canyons into a valley. The channel that had been confined by the canyon walls is no longer confined, slows down and spreads out, dropping its bedload of all sizes, forming a delta in the air of the valley. As distributary channels fill with sediment, the stream is diverted laterally, and the alluvial fan develops into a cone shaped landform with distributaries radiating from the canyon mouth. Alluvial fans are common in the dry climates of the West where ephemeral streams emerge from canyons in the ranges of the Basin and Range.

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A delta is formedwhen a stream reaches a quieter body of water such as a lake or the ocean and the bedload and suspended load is deposited. If wave erosion from the water body is greater than deposition from the river, a delta will not form. The largest and most famous delta in the United States is the Mississippi River delta formed where the Mississippi River flows into the Gulf of Mexico. The Mississippi River drainage basin is the largest in North America, draining 41%of the contiguous United States [24]. Because of the large drainage area, the river carries a large amount of sediment. The Mississippi River is a major shipping route and human engineering has ensured that the channel has been artificially straightened and remains fixed within the floodplain. The river is now 229 km shorter than it was before humans began engineering it [24]. Because of these restraints, the delta is now focused on one trunk channel and has created a “bird’s foot” pattern. The two NASA imagesbelow of the delta show how the shoreline has retreated and land was inundated with water while deposition of sediment was focused at end of the distributaries. These images have changed over a 25 year period from 1976 to 2001. These are stark changes illustrating sea-level rise and land subsidence from the compaction of peat due to the lack of sediment resupply [25].

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The formation of the Mississippi River delta started about 7500 years ago when postglacial sea level stopped rising. In the past 7000 years, prior to anthropogenic modifications, the Mississippi River delta formed several sequential lobes. The river abandoned each lobe for a more preferred route to the Gulf of Mexico. These delta lobes were reworked by the ocean waves of the Gulf of Mexico [26]. After each lobe was abandoned by the river, isostatic depression and compaction of the sediments caused basin subsidence and the land to sink.

A clear example of how deltas form came from an earthquake. During the 1959 Madison Canyon 7.5 magnitude earthquake in Montana, a large landslide dammed the Madison River forming Quake Lake still there today [27]. A small tributary stream that once flowed into the Madison River, now flows into Quake Lake forming a delta composed of coarse sediment actively eroded from the mountainous upthrown block to the north.

Deltas can be further categorized as wave-dominated or tide-dominated. Wave-dominated deltas occur where the tides are small and wave energy dominates. An example is the Nile River delta in the Mediterranean Sea that has the classic shape of the Greek character (Δ) from which the landform is named. A tide-dominated delta forms when ocean tides are powerful and influence the shape of the delta. For example, Ganges-Brahmaputra Delta in the Bay of Bengal (near India and Bangladesh) is the world’s largest delta and mangrove swamp called the Sundarban [29].

At the Sundarban Delta in Bangladesh, tidal forces create linear intrusions of seawater into the delta. This delta also holds the world’s largest mangrove swamp.

Lake Bonneville was a large, pluvial lake that occupied the western half of Utah and parts of eastern Nevada from about 30,000 to 12,000 years ago. The lake filled to a maximum elevation as great as approximately 5100 feet above mean sea level, filling the basins, leaving the mountains exposed, many as islands. The presence of the lake allowed for deposition of both fine grained lake mud and silt and coarse gravels from the mountains. Variations in lake level were controlled by regional climate and a catastrophic failure of Lake Bonneville’s main outlet, Red Rock Pass [31]. during extended periods of time in which the lake level remained stable, wave-cut terraces were produced that can be seen today on the flanks of many mountains in the region. Significant deltas formed at the mouths of major canyons in Salt Lake, Cache, and other Utah valleys. The Great Salt Lake is the remnant of Lake Bonneville and cities have built up on these delta deposits.

Stream terracesare remnants of older floodplains located above the existing floodplain and river. Like entrenched meanders, stream terraces form when uplift occurs or base level drops and streams erode downward, their meanders widening a new flood plain. Stream terraces can also form from extreme flood events associated with retreating glaciers. A classic example of multiple stream terraces are along the Snake River in Grand Teton National Park in Wyoming [32; 33].

Terraces along the Snake River, Wyoming.	Terraces in Glen Roy, Scotland

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11.6 Groundwater

Groundwater is an important source of freshwater. It can be found at varying depths in all places under the ground, but is limited by extractable quantity and quality.

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11.6.1 Porosity and Permeability

An aquifer is a rock unit that contains extractable ground water. A good aquifer must be both porous and permeable. Porosity is the space between grains that can hold water, expressed as the percentage of open space in the total volume of the rock.Permeability comes from connectivity of the spaces that allows water to move in the aquifer. Porosity can occur as primary porosity, as space between sand grains or vesicles in volcanic rocks, or secondary porosity as fractures or dissolved spaces in rock). Compaction and cementation during lithification of sediments reduces porosity (see chapter 5.3).

A combination of a place to contain water (porosity) and the ability to move water (permeability) makes a good aquifer—a rock unit or sediment that allows extraction of groundwater. Well-sorted sediments have higher porosity because there are not smaller sediment particles filling in the spaces between the larger particles. Shales made of clays generally have high porosity, but the pores are poorly connected, thereby causing low permeability.

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While permeability is an important measure of a porous material’s ability to transmit water, hydraulic conductivity is more commonly used by geologists to measure how easily a fluid is transmitted. Hydraulic conductivitymeasures both the permeability of the porous material and the properties of the water, or whatever fluid is being transmitted like oil or gas.Because hydraulic conductivity also measures the properties of the fluid, such as viscosity, it is used by both petroleum geologists and hydrogeologists to describe both the production capability of oil reservoirs and of aquifers. High hydraulic conductivity indicates that fluid transmits rapidly through an aquifer.

11.6.2 Aquifers

Aquifers are rock layers with sufficient porosity and permeability to allow water to be both contained and move within them. For rock or sediment to be considered an aquifer, its pores must be at least partially filled with water and it must be permeable enough to transmit water. Drinking water aquifers must also contain potable water. Aquifers can vary dramatically in scale, from spanning several formations covering large regions to being a local formation in a limited area. Aquifers adequate for water supply are both permeable, porous, and potable.

11.6.3 Groundwater Flow

When surface water infiltrates or seeps into the ground, it usually enters the unsaturated zone also called the vadose zone, or zone of aeration. The vadose zoneis the volume of geologic material between the land surface and the zone of saturation where the pore spaces are not completely filled with water [34]. Plant roots inhabit the upper vadose zone and fluid pressure in the pores is less than atmospheric pressure. Below the vadose zone is the capillary fringe. Capillary fringe is the usually thin zone below the vadose zone where the pores are completely filled with water (saturation), but the fluid pressure is less than atmospheric pressure. The pores in the capillary fringe are filled because of capillary action, which occurs because of a combination of adhesion and cohesion. Below the capillary fringe is the saturated zone or phreatic zone, where the pores are completely saturated and the fluid in the pores is at or above atmospheric pressure. The interface between the capillary fringe and the saturated zone marks the location of the water table.

Wells are conduits that extend into the ground with openings to the aquifers, to extract from, measure, and sometimes add water to the aquifer. Wells are generally the way that geologists and hydrologist measure the depth to groundwater from the land surface as well as withdraw water from aquifers.

Water is found throughout the pore spaces in sediments and bedrock. The water table is the area below which the pores are fully saturated with water. The simplest case of a water table is when the aquifer is unconfined, meaning it does not have a confining layer above it. Confining layers can pressurize aquifers by trapping water that is recharged at a higher elevation underneath the confining layer, allowing for a potentiometric surface higher than the top of the aquifer, and sometimes higher than the land surface.

A confining layeris a low permeability layer above and/or below an aquifer that restricts the water from moving in and out of the aquifer. Confining layers include aquicludes, which are so impermeable that no water travels through them, and aquitards, which significantly decrease the speed at which water travels through them. The potentiometric surface represents the height that water would rise in a well penetrating the pressurized aquifer system. Breaches in the pressurized aquifer system, like faults or wells, can cause springs or flowing wells, also known as artesian wells.

The water table will generally mirror surface topography, though more subdued, because hydrostatic pressure is equal to atmospheric pressure along the surface of the water table. If the water table intersects the ground surface the result will be water at the surface in the form of a gaining stream, spring, lake, or wetland. The water table intersects the channel for gaining streams which then gains water from the water table. The channels for losing streams lie below the water table, thus losing streams lose water to the water table. Losing streams may be seasonal during a dry season or ephemeral in dry climates where they may normally be dry and carry water only after rain storms. Ephemeral streams pose a serious danger of flash flooding in dry climates.

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Mentioned in the video is the USGS Groundwater Watch site.

Using wells, geologists measure the water table’s height and the potentiometric surface. Graphs of the depth to groundwater over time, are known as hydrographs and show changes in the water table over time. Well–water level is controlled by many factors and can change very frequently, even every minute, seasonally, and over longer periods of time.

In 1856, French engineer Henry Darcy developed a hypothesis to show how discharge through a porous medium is controlled by permeability, pressure, and cross– sectional area. To prove this relationship, Darcy experimented withtubes of packed sediment with water running through them. The results of his experiments empirically established a quantitative measure of hydraulic conductivity and discharge that is known as Darcy’s law. The relationships described by Darcy’s Law have close similarities to Fourier’s law in the field of heat conduction, Ohm’s law in the field of electrical networks, or Fick’s law in diffusion theory.

Q=KA(Δh/L)

Q = flow (volume/time)

K = hydraulic conductivity (length/time)

A = cross-sectional area of flow (area)

Δh = change in pressure head (pressure difference)

L = distance between pressure (h) measurements (length)

Δh/L is commonly referred to as the hydraulic gradient

Pumping water from an unconfined aquifer lowers the water table. Pumping water from a confined aquifer lowers the pressure and/or potentiometric surface around the well. In an unconfined aquifer, the water table is lowered as water is removed from the aquifer near the well producing drawdown and a cone of depression (see figure). In a confined aquifer, pumping on an artesian well reduces the pressure or potentiometric surface around the well.

When one cone of depression intersects another cone of depression or a barrier feature like an impermeable mountain block, drawdown is intensified. When a cone of depression intersects a recharge zone, the cone of depression is lessened.

11.6.4 Recharge

The rechargearea is where surface water enters an aquifer through the process of infiltration. Recharge areas are generally topographically high locations of an aquifer. They are characterized by losing streams and permeable rock that allows infiltration into the aquifer. Recharge areas mark the beginning of groundwater flow paths.

In the Basin and Range Province, recharge areas for the unconsolidated aquifers of the valleys are along mountain foothills. In the foothills of Salt Lake Valley, losing streams contribute water to the gravel-rich deltaic deposits of ancient Lake Bonneville, in some cases feeding artesian wells in the Salt Lake Valley.

An aquifer management practice is to induce rechargethrough storage and recovery. Geologists and hydrologists can increase the recharge rate into an aquifer system using injection wells and infiltration galleries or basins [35]. Injection wells pump water into an aquifer where it can be stored. Injection wells are regulated by state and federal governments to ensure that the injected water is not negatively impacting the quality or supply of the existing groundwater in the aquifer. Some aquifers can store significant quantities of water, allowing water managers to use the aquifer system like a surface reservoir. Water is stored in the aquifer during periods of low water demand and high water supply and later extracted during times of high water demand and low water supply.

11.6.5 Discharge

Discharge areas are where the water table or potentiometric surface intersects the land surface. Discharge areas mark the end of groundwater flow paths. These areas are characterized by springs, flowing (artesian) wells, gaining streams, and playas in thedry valley basins of the Basin and Range Province of the western United States.

11.6.6 Groundwater mining and subsidence

Like other natural resources on our planet, the quantity of fresh and potable water is finite. The only natural source of water on land is from the sky in the form of precipitation. In many places, groundwater is being extracted faster than it is being replenished. When groundwater is extracted faster than it is recharged, groundwater levels and potentiometric surfaces decline, and discharge areas diminish or dry up completely. Regional pumping-induced groundwater decline is known as groundwater miningor groundwater overdraft. Groundwater mining is a serious situation and can lead to dry wells, reduced spring and stream flow, and subsidence. Groundwater mining is happeningis places where more water is extracted by pumping than is being replenished by precipitation, and the water tableis continually lowered. In these situations, groundwater must be viewed as a ore body and in its depletion, the possibility of producing ghost towns.

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In many places, water actually helps hold up an aquifer’s skeleton by the water pressure exerted on the grains in an aquifer.This pressure is called porepressure and comes from the weight of overlying water. If pore pressure decreases because of groundwater mining, the aquifer can compact, causing the surface of the ground to sink. Areas especially susceptible to this effect are aquifers made of unconsolidated sediments. Unconsolidated sediments with multiple layers of clay and other fine-grained material are at higher risk because when water is drained, clay compactsconsiderably [36; 37].

Subsidencefrom groundwater mining has been documented in southwestern Utah, notably Cedar Valley, Iron County, Utah. Groundwater levels have declined more than 100 feet in certain parts of Cedar Valley, causing earth fissures and measurable amounts of land subsidence.

This photo shows documentation of subsidence from pumping of groundwater for irrigation in the Central Valley in California. The pole shows subsidence from groundwater pumping over a period of time.

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11.7 Water Contamination and Remediation

Water can be contaminated by natural features like mineral-rich geologic formations and by human activities such as agriculture, industrial operations, landfills, animal operations, and sewage treatment processes, among many other things. As water runs over the land or infiltrates into the ground, it dissolves material left behind by these potential contaminant sources. There are three major groups of contamination: organic and inorganic chemicals and biological agents. Small sediments that cloudwater, causing turbidity, is also an issue with some wells, but it is not considered contamination. The risks and type of remediation for a contaminant depends on the type of chemicals present.

Contamination occurs as point–source and nonpoint–source pollution. Point sourcepollution can be attributed to a single, definable source, whilenonpoint sourcepollution is from multiple dispersed sources. Point sources include waste disposal sites, storage tanks, sewage treatment plants, and chemical spills. Nonpoint sources are dispersed and indiscreet, where the whole of the contribution of pollutants is harmful, but the individual components do not have harmful concentrations of pollutants. A good example of nonpoint pollution is residential areas, where lawn fertilizer on one person’s yard may not contribute much pollution to the system, but the combined effect of many residents using fertilizer can lead to significant nonpoint pollution. Other nonpoint sources include nutrients (nitrate and phosphate), herbicides, pesticides contributed by farming, nitrate contributed by animal operations, and nitrate contributed by septic systems.

Organic chemicals are common pollutants. They consist of strands and rings of carbon atoms, usually connected by covalent bonds. Other types of atoms, like chlorine, and molecules, like hydroxide (OH–), are attached to the strands and rings. The number and arrangement of atoms will decide how the chemical behaves in the environment, its danger to humans or ecosystems, and where the chemical ends up in the environment. The different arrangements of carbon allow for tens of thousands of organic chemicals, many of which have never been studied for negative effects on human health or the environment. Common organic pollutants are herbicides and pesticides, pharmaceuticals, fuel, and industrial solvents and cleansers.

Organic chemicals include surfactants such as cleaning agents and synthetic hormones associated with pharmaceuticals, which can act as endocrine disruptors. Endocrine disruptors mimic hormones, and can cause long-term effects in developing sexual reproduction systems in developing animals. Only very small quantities of endocrine disruptors are needed to cause significant changes in animal populations.

An example of organic chemical contamination is the Love Canal, Niagara Falls, New York. From 1942 to 1952, the Hooker Chemical Company disposed of over 21,337 mt (21,000 t) of chemical waste, including chlorinated hydrocarbons, into a canal and covered it with a thin layer of clay. Chlorinated hydrocarbons are a large group of organic chemicals that have chlorine functional groups, most of which are toxic and carcinogenic to humans. The company sold the land to the New York School Board, who developed it into a neighborhood. After residents began to suffer from serious health ailments and pools of oily fluid started rising into residents’ basements, the neighborhood had to be evacuated. This site became a U.S. Environmental Protection Agency Superfund site, a site with federal funding and oversight to ensure its cleanup.

Inorganic chemicals are another set of chemical pollutants. They can contain carbon atoms, but not in long strands or links. Inorganic contaminants include chloride, arsenic, and nitrate (NO3). Nutrients can be from geologic material, like phosphorous-rich rock, but are most often sourced from fertilizer and animal and human waste. Untreated sewage and agricultural runoff contain concentrates of nitrogen and phosphorus which are essential for the growth of microorganisms. Nutrients like nitrate and phosphate in surface water can promote growth of microbes, like blue-green algae (cyanobacteria), which in turn use oxygen and create toxins (microcystins and anatoxins) in lakes [38]. This process is known as eutrophication.

Metals are common inorganic contaminants. Lead, mercury, and arsenic are some of the more problematic inorganic groundwater contaminants. Bangladesh has a well documented case of arsenic contamination from natural geologic material dissolving into the groundwater. Acid–minedrainage can also cause significant inorganic contamination (see Chapter 16).

Salt, typically sodium chloride, is a common inorganic contaminant. It can be introduced into groundwater from natural sources, such as evaporite deposits like the Arapien Shale of Utah, or from anthropogenic sources like the salts applied to roads in the winter to keep ice from forming. Salt contamination can also occur near ocean coasts from saltwater intruding into the cones of depression around fresh groundwater pumping, inducing the encroachment of saltwater into the freshwater body.

Biological agents are another common groundwater contaminant which includes harmful bacteria and viruses. A common bacteria contaminant is Escherichia coli (E. coli). Generally, harmful bacteria are not present in groundwater unless the groundwater source is closely connected with a contaminated surface source, such as a septic system. Karst, landforms created from dissolved limestone, is especially susceptible to this form of contamination, because water moves relatively quickly through theconduits of dissolved limestone. Bacteria can also be used for remediation.

Table. Groundwater contaminants.

Remediation is the act of cleaning contamination. Hydrologists use three types of remediation: biological, chemical, and physical. Biological remediation uses specific strains of bacteria to break down a contaminant into safer chemicals. This type of remediation is usually used on organic chemicals, but also works on reducing or oxidizing inorganic chemicals like nitrate. Phytoremediation is a type of bioremediation that uses plants to absorb the chemicals over time.

Chemical remediation uses chemicals to remove the contaminant or make it less harmful. One example is to use a reactive barrier, a permeable wall in the ground or at a discharge point that chemically reacts with contaminants in the water. Reactive barriers made of limestone can increase the pH of acid mine drainage, making the water less acidic and more basic, which removes dissolved contaminants by precipitation into solid form.

Physical remediation consists of removing the contaminated water and either treating it with filtration, called pump–and–treat, or disposing of it. All of these options are technically complex, expensive, and difficult, with physical remediation typically being the most costly.

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11.8 Karst

Karst refers to landscapes and hydrologic features created by the dissolving of limestone. Karst can be found anywherethere is limestone and other soluble subterranean substances like salt deposits. Dissolving of limestone creates features like sinkholes, caverns, disappearing streams, and towers.

Dissolving of underlying salt deposits has caused sinkholes to form in the Kaibab Limestone on the Colorado Plateau in Arizona.

Collapse of the surface into an underground cavern caused this sinkhole in the front yard of a home in Florida.

CO2 in the atmosphere dissolves readily in the water droplets that form clouds from which precipitation comes in the form of rain and snow. This precipitation is slightly acidic with carbonic acid. Karst forms when carbonic acid dissolves calcite (calcium carbonate) in limestone.

H2O + CO2 = H2CO3

Water + Carbon Dioxide Gas equals Carbonic Acid in Water

CaCO3 + H2CO3 = Ca2++ 2HCO3 -1

Solid Calcite + Carbonic Acid in Water Dissolved equals Calcium Ion + Dissolved Bicarbonate Ion

After the slightly acidic water dissolves the calcite, changes in temperature or gas content in the water can cause the water to redeposit the calcite in a different place as tufa (travertine), often deposited by a springor in a cave. Speleothems are secondary deposits, typically made of travertine, deposited in a cave. Travertine speleothems form by water dripping through cracks and dissolved openings in caves and evaporating, leaving behind the travertine deposits. Speleothems commonly occur in the form of stalactites, when extending from the ceiling, and stalagmites, when standing up from the floor.

Surface water enters the karst system through sinkholes, losing streams, and disappearing streams. Changes in base level can cause rivers running over limestone to dissolve the limestone and sink into the ground. As the water continues to dissolve its way through the limestone, it can leave behind intricate networks of caves and narrow passages. Often dissolution will follow and expand fractures in the limestone. Water exits the karst system as springs and rises. In mountainous terrane, dissolution can extend all the way through the vertical profile of the mountain, with caverns dropping thousands of feet.

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Summary

Water is essential for all living things. It continuously cycles through the atmosphere, over land, and through the ground. In much of the United States and other countries, water is managed through a system of regional laws and regulations and distributed on paper in a system collectively known as “water rights”. Surface water follows a watershed, which is separate from other areas by its divides (highest ridges). Groundwater exists in the pores within rocks and sediment. It moves predominantly due to pressure and gravitational gradients through the rock. Human and natural causes can make water unsuitable for consumption. There are different ways to deal with this contamination. Karst is when limestone is dissolved by water, forming caves and sinkholes.

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References

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Maupin, M.A., Kenny, J.F., Hutson, S.S., Lovelace, J.K., Barber, N.L., and Linsey, K.S., 2014, Estimated use of water in the United States in 2010: US Geological Survey.

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12 Coastlines

12 Coastlines

KEY CONCEPTS

By the end of this chapter, students should be able to:

Describe how waves occur, move, and carry energy

Explain wave behavior approaching the shoreline

Describe shoreline features and zones

Describe wave refraction and its contribution to longshore currents and longshore drift

Explain how longshore currents cause the formation of spits and baymouth bars

Distinguish between submergent and emergent coasts and describe coastal features associated with each

Describe the relationship between the natural river of sand in the littoral zone and human attempts to alter it for human convenience

Describe the pattern of the main ocean currents and explain the different factors involved in surface currents and deep ocean currents

Explain how ocean tides occur and distinguish among diurnal, semidiurnal, and mixed tide patterns

The Earth’s surface is 29% land and 71% water. Coastlines are the interfaces between, and as such, the longest visible boundaries on Earth. To understand the processes that occur at these boundaries, it is important to first understand wave energy.

12.1 Waves and Wave Processes

Wind blowing over the surface of water transfers energy to the water through friction. The energy transferred from wind to water causes waves to form. Waves move as individual oscillating particles of water. As the wave crest passes, the water is moving forward. As the wave trough passes, the water is moving backward. To see wave movement in action, watch a cork or some floating object as a wave passes.

Important terms to understand in the operation of waves include: the wave crest is the highest point of the wave; the trough is the lowest point of the wave. Wave heightis the vertical distance from the trough to the crest and is determined by wave energy. Wave amplitude is half the wave height, or the distance from either the crest or trough to the still water line. Wavelength is the horizontal distance between consecutive wave crests. Wave velocity is the speed at which a wave crest moves forward and is related to the wave’s energy. Wave period is the time interval it takes for adjacent wave crests to pass a given point.

The circular motion of water particles diminishes with depth and is negligible at about one-half wavelength, an important dimension to remember in connection with waves. Wave base is the vertical depth at which water ceases to be disturbed by waves. In water shallower than wave base, waves will disturb the bottom and ripple shore sand. Wave base is measured at a depth of about one-half wavelength, where the water particles’ circular motion diminishes to zero. If waves approaching a beach have crests at about 6 m (~20 ft) intervals, this wave motion disturbs water to about 3 m (~10 ft) deep. This motion is known as fair-weather wave base. In strong storms such as hurricanes, both wavelength and wave base increase dramatically to a depth known as storm wave base, which is approximately 91 m (~300 ft) [1].

Waves are generated by wind blowing across the ocean surface. The amount of energy imparted to the water depends on wind velocity and the distance across which the wind is blowing. This distance is called fetch. Waves striking a shore are typically generated by storms hundreds of miles from the coast and have been traveling across the ocean for days.

Winds blowing in a relatively constant direction generate waves moving in that direction. Such a group of approximately parallel waves traveling together is called a wave train. A wave train coming from one fetch can produce various wavelengths. Longer wavelengths travel at a faster velocity than shorter wavelengths, so they arrive first at a distant shore. Thus, there is a wavelength–sorting process that takes place during the wave train’s travel. This sorting process is called wave dispersion.

12.1.1 Behavior of Waves Approaching Shore

On the open sea, waves generally appear choppy because wave trains from many directions are interacting with each other, a process called wave interference. Constructive interference occurs where crests align with other crests. The aligned wave height is the sum of the individual wave heights, a process referred to as wave amplification. Constructive interference also produces hollows where troughs align with other troughs. Destructive interference occurs where crests align with troughs and cancel each other out. As waves approach shore and begin to make frictional contact with the sea floor at a depth of about one-half wavelength or less, they begin to slow down. However, the energy carried by the wave remains the same, so the waves build up higher. Remember that water moves in a circular motion as a wave passes, and each circle is fed from the trough in front of the advancing wave. As the wave encounters shallower water at the shore, there is eventually insufficient water in the trough in front of the wave to supply a complete circle, so the crest pours over creating a breaker.

A special type of wave is called a tsunami, sometimes incorrectly called a “tidal wave.” Tsunamis are generated by energetic events affecting the sea floor, such as earthquakes, submarine landslides, and volcanic eruptions (see Chapter 9 and Chapter 4). During earthquakes for example, tsunamis can be produced when the moving crustal rocks below the sea abruptly elevate a portion of the seafloor. Water is suddenly lifted creating a bulge at the surface and a wave train spreads out in all directions traveling at tremendous speeds [over 322 kph (200 mph)] and carrying enormous energy. Tsunamis may pass unnoticed in the open ocean because they move so fast, the wavelength is very long, and the wave height is very low. But, as the wave train approaches shore and each wave begins to interact with the shallow seafloor, friction increases and the wave slows down. Still carrying its enormous energy, wave height builds up and the wave strikes the shore as a wall of water that can be over 30 m (~100 ft) high. The massive wave, called a tsunami runup, may sweep inland well beyond the beach destroying structures far inland. Tsunamis can deliver a catastrophic blow to people at the beach. As the trough water in front of the tsunami wave is drawn back, the seafloor is exposed. Curious and unsuspecting people on the beach may run out to see exposed offshore sea life only to be overwhelmed when the breaking crest hits.

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12.2 Shoreline Features

Coastlines are dynamic, high energy, and geologically complicated places where many different erosional and depositional features exist (see Chapter 5). They include all parts of the land-sea boundary directly affected by the sea, including land far above high tide and seafloor well below normal wave base. But, the shoreline itself is the direct interface between water and land that shifts with the tides. This shifting interface at the shoreline is called the littoral zone. The combination of waves, currents, climate, coastal morphology, and gravity, all act on this land-sea boundary to create shoreline features.

12.2.1 Shoreline Zones

Shorelines are divided into five primary zones—offshore, nearshore, surf, foreshore, and backshore. The offshore zone is below water, but it is still geologically active due to flows of turbidity currents that cascade over the continental slope and accumulate in the continental rise. The nearshore zone is the area of the shore affected by the waves where water depth is one-half wavelength or less. The width of this zone depends on the maximum wavelength of the approaching wave train and the slope of the seafloor. The nearshore zone includes the shoreface, which is where sand is disturbed and deposited. The shoreface is broken into two segments: upper and lower shoreface. Upper shoreface is affected by everyday wave action and consists of finely-laminated and cross-bedded sand. The lower shoreface is the only area moved by storm waves and consists of hummocky cross-stratified sand. The surf zone is where the waves break.

The foreshore zone overlaps the surf zone and is periodically wet and dry due to waves and tides. The foreshore zone is where planer-laminated, well-sorted sand accumulates. The beach face is the part of the foreshore zone where the breaking waves swash up and the backwash flows back down. Low ridges above the beach face in the foreshore zone are called berms. During the summer in North America, when most people visit the beach, the zone where people spread their towels and beach umbrellas is the summer berm. Wave energy is typically lower in the summer, which allows sand to pile onto the beach. Behind the summer berm is a low ridge of sand called the winter berm. In winter, higher storm energy moves the summer berm sand off the beach and piles it in the nearshore zone. The next year, that sand is replaced on the beach and moved back onto the summer berm. The backshore zone is the area always above sea level in normal conditions. In the backshore zone, onshore winds may blow sand behind the beach and the berms, creating dunes.

12.2.2 Refraction, Longshore Currents, and Longshore Drift

As waves enter shallower water less than one-half wavelength depth, they slow down. Waves usually approach the shoreline at an angle, with the end of the waves nearest the beach slowing down first. This causes the wave crests to bend, called wave refraction. From the beach face, this causes it to look like waves are approaching the beach straight on, parallel to the beach. However, as refracted waves actually approach the shoreline at a slight angle, they create a slight difference between the swash as it moves up the beach face at a slight angle and the backwash as it flows straight back down under gravity. This slight angle between swash and backwash along the beach creates a current called the longshore current. Waves stir up sand in the surf zone and move it along the shore. This movement of sand is called longshore drift. Longshore drift along both the west and east coasts of North America moves sand north to south on average.

Longshore currents can carry longshore drift down a coast until it reaches a bay or inlet where it will deposit sand in the quieter water (see Chapter 11). Here, a spit can form. As the spit grows, it may extend across the mouth of the bay forming a barrier called a baymouth bar. Where the bay or inlet serves as boat anchorage, spits and baymouth bars are a severe inconvenience. Often, inconvenienced communities create methods to keep their bays and harbors open.

One way to keep a harbor open is to build a jetty, a long concrete or stone barrier constructed to deflect the sand away from a harbor mouth or other ocean waterway. If the jetty does not deflect the sand far enough out, sand may continue to flow along the shore, forming a spit around the end of the jetty. A more expensive but effective method to keep a bay mouth open is to dredge the sand from the growing spit, put it on barges, and deliver it back to the drift downstream of the harbor mouth. An even more expensive but more effective option is to install large pumps and pipes to draw in the sand upstream of the harbor, pump it through pipes, and discharge it back into the drift downstream of the harbor mouth. Because natural processes work continuously, human efforts to mitigate inconvenient spits and baymouth bars require ongoing modifications. For example, the community of Santa Barbara, California, tried several methods to keep their harbor open before settling on pumps and piping [2].

Rip currents are another coastal phenomenon related to longshore currents. Rip currents occur in the nearshore seafloor when wave trains come straight onto the shoreline. In areas where wave trains push water directly toward the beach face or where the shape of the nearshore seafloor refracts waves toward a specific point on the beach, the water piles up on shore. But this water must find an outlet back to the sea. The outlet is relatively narrow, and rip currents carry the water directly away from the beach. Swimmers caught in rip currents are carried out to sea. Swimming back to shore directly against the strong current is fruitless. A solution for good swimmers is to ride out the current to where it dissipates, swim around it, and return to the beach. Another solution for average swimmers is to swim parallel to the beach until out of the current, then return to the beach. Where rip currents are known to exist, warning signs are often posted. The best solution is to understand the nature of rip currents, have a plan before entering the water, or watch the signs and avoid them all together.

Like rip currents, undertow is a current that moves away from the shore. However, unlike rip currents, undertow occurs underneath the approaching waves and is strongest in the surf zone where waves are high and water is shallow. Undertow is another return flow for water transported onshore by waves.

12.2.3 Emergent and Submergent Coasts

Emergent coasts occur where sea levels fall relative to land level. Submergent coasts occur where sea levels rise relative to land level. Tectonic shifts and sea level changes cause the long-term rise and fall of sea level relative to land. Some features associated with emergent coasts include high cliffs, headlands, exposed bedrock, steep slopes, rocky shores, arches, stacks, tombolos, wave-cut platforms, and wave notches.

In emergent coasts, wave energy, wind, and gravity erode the coastline. The erosional features are elevated relative to the wave zone. Sea cliffs are persistent features as waves cut away at their base and higher rocks calve off by mass wasting. Refracted waves that attack bedrock at the base of headlands may erode or carve out a sea arch, which can extend below sea level in a sea cave. When a sea arch collapses, it leaves one or more rock columns called stacks.

A stack or near shore island creates a quiet water zone behind it. Sand moving in the longshore drift accumulates in this quiet zone forming a tombolo: a sand strip that connects the island or stack to the shoreline. Where sand supply is low, wave energy may erode a wave-cut platform across the surf zone, exposed as bare rock with tidal pools at low tide. This bench-like terrace extends to the cliff’s base. When wave energy cuts into the base of a sea cliff, it creates a wave notch.

Submergentcoasts occur where sea levels rise relative to land. This may be due to tectonicsubsidence—when the Earth’s crust sinks—or when sea levels rise due to glacier melt. Features associated with submergent coasts include flooded river mouths, fjords, barrier islands, lagoons, estuaries, bays, tidal flats, and tidal currents. In submergent coastlines, river mouths are flooded by the rising water, for example Chesapeake Bay. Fjords are glacial valleys flooded by post-ice age sea level rise (see Chapter 14). Barrier islands are elongated bodies of sand that formed from old beach sands that used to parallel the shoreline. Often, lagoons lie behind barrier islands [3]. Barrier island formation is controversial: some scientists believe that they formed when ice sheets melted after the last ice age, raising sea levels. Another hypothesis is that barrier islands formed from spits and bars accumulating far offshore.

Tidal flats—or mudflats, form where tides alternately flood and expose low areas along the coast. Tidal currents create combinations of symmetrical and asymmetrical ripple marks on mudflats, and drying mud creates mud cracks. In the central Wasatch Mountains of Utah, ancient tidal flat deposits are exposed in the Precambrian strata of the Big Cottonwood Formation. These ancient deposits provide an example of applying Hutton’s principle of uniformitarianism (see Chapter 1). Sedimentary structures common on modern tidal flats indicate that these ancient deposits were formed in a similar environment: there were shorelines, tides, and shoreline processes acting at that time, yet the ancient age indicates that there were no land plants to hold products of mechanical weathering in place (see Chapter 5), so erosion rates would have been different.

Geologically, tidal flats are broken into three different sections: barren zones, marshes, and salt pans. These zones may be present or absent in each individual tidal flat. Barren zones are areas with strong flowing water, coarser sediment, with ripple marks and cross bedding common. Marshes are vegetated with sand and mud. Salt pans or flats, less often submerged than the other zones, are the finest-grained parts of tidal flats, with silty sediment and mud cracks (see Chapter 5) [4].

Lagoons are locations where spits, barrier islands, or other features partially cut off a body of water from the ocean. Estuaries are a vegetated type of lagoon where fresh water flows into the area making the water brackish—a salinity between salt and fresh water. However, terms like lagoon, estuary, and even bay are often loosely used in place of one another [5]. Lagoons and estuaries are certainly transitional between land and water environments where littoral, shallow shorelines; lacustrine, lakes or lagoons; and fluvial, rivers or currents can overlap. For more information on lagoons and estuaries, see Chapter 5.

12.2.4 Human impact on coastal beaches

Humans impact coastal beaches when they build homes, condominiums, hotels, businesses, and harbors—and then again when they try to manage the natural processes of erosion. Waves, currents, longshore drift, and dams at river mouths deplete sand from expensive beachfront property and expose once calm harbors to high-wave energy. To protect their investment, keep sand on their beach, and maintain calm harbors, cities and landowners find ways to mitigate the damage by building jetties, groins, dams, and breakwaters.

Jetties are large manmade piles of boulders or concrete barriers built at river mouths and harbors. A jetty is designed to divert the current or tide, to keep a channel to the ocean open, and to protect a harbor or beach from wave action. Groins are similar but smaller than jetties. Groins are fences of wire, wood or concrete built across the beach perpendicular to the shoreline and downstream of a property. Unlike jetties, groins are used to preserve sand on a beach rather than to divert it. Sand erodes on the downstream side of the groin and collects against the upstream side. Every groin on one property thus creates a need for another one on the property downstream. A series of groins along a beach develops a scalloped appearance along the shoreline.

Inland streams and rivers flow to the ocean carrying sand to the longshore current which distributes it to beaches. When dams are built, they trap sand and keep sediment from reaching beaches. To replenish beaches, sand may be hauled in from other areas by trucks or barges and dumped on the depleted beach. Unfortunately, this can disrupt the ecosystem that exists along the shoreline by exposing native creatures to foreign ecosystems and microorganisms and by introducing foreign objects to humans. For example, visitors to one replenished east coast beach found munitions and metal shards in the sand, which had been dredged from abandoned military test ranges [6].

An approach to protect harbors and moorings from high-energy wave action is to build a breakwater—an offshore structure against which the waves break, leaving calmer waters behind it. Unfortunately, breakwaters keep waves from reaching the beach and stop sand moving with longshore drift. When longshore drift is interrupted, sand is deposited in quieter water, and the shoreline builds out forming a tombolo behind the breakwater. The tombolo eventually fill in behind the breakwater with sand [7]. When the city of Venice, California built a breakwater to create a quiet water harbor, longshore drift created a tombolo behind the breakwater, as seen in the image. The tombolo now acts as a large groin in the beach drift.

12.2.5 Submarine Canyons

Submarine canyons are narrow, deep underwater canyons located on continental shelves. Submarine canyons typically form at the mouths of large landward river systems. They form when rivers cut down into the continental shelf during low sea level and when material continually slumps or flows down from the mouth of a river or a delta. Underwater currents rich in sediment and more dense than sea water, can flow down the canyons, even erode and deepen them, then drain onto the ocean floor. Underwater landslides, called turbidity flows, occur when steep delta faces and underwater sediment flows are released down the continental slope [8]. Turbidity flows in submarine canyons can continue to erode the canyon, and eventually, fan-shaped deposits develop at the mouth of the canyon on the continental rise. See Chapter 5 for more information on turbidity flows.

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12.3 Currents and Tides

Ocean water moves as waves, currents, and tides. Ocean currents are driven by persistent global winds blowing over the water’s surface and by water density. Ocean currents are part of Earth’s heat engine in which solar energy is absorbed by ocean water and distributed by ocean currents. Water has another unique property, high specific heat, that relates to ocean currents. Specific heat is the amount of heat necessary to raise a unit volume of a substance one degree. For water it takes one calorie per cubic centimeter to raise its temperature one degree Celsius. This means the oceans, covering 71% of the Earth’s surface, soak up solar heat with little temperature change and distribute that heat around the Earth by ocean currents.

12.3.1 Surface Currents

The Earth’s rotation and the Coriolis effect exert significant influence on ocean currents (see Chapter 13). In the figure, the black arrows show global surface currents. Notice the large circular currents in the northern and southern hemispheres in the Atlantic, Pacific, and Indian Oceans. These currents are called gyres and are driven by atmospheric circulation—air movement [9]. Gyres rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere because of the Coriolis Effect. Western boundary currents flow from the equator toward the poles carrying warm water. They are key contributors to local climate. Western boundary currents are narrow and move poleward along the east coasts of adjacent continents. The Gulf Stream and the Kuroshio currents in the northern hemisphere and the Brazil, Mozambique, and Australian currents in the southern hemisphere are western boundary currents. Currents returning cold water toward the equator are broad and diffuse along the western coasts of adjacent land masses. These warm western boundary and cold eastern boundary currents affect climate of nearby lands making them warmer or colder than other areas at equivalent latitudes. For example, the warm Gulf Stream makes Northern Europe much milder than similar latitudes in northeastern Canada and Greenland. Another example is the cool Humboldt Current, also called the Peru Current, flowing north along the west coast of South America. Cold currents limit evaporation in the ocean, which is one reason the Atacama Desert in Chile is cool and arid [10].

12.3.2 Deep Currents

Whether an ocean current moves horizontally or vertically depends on its density. The density of seawater is determined by temperature and salinity.

Evaporation and freshwater influx from rivers affect salinity and, therefore, the density of seawater. As the western boundary currents cool at high latitudes and salinity increases due to evaporation and ice formation (recall that ice floats; water is densest just above its freezing point). So the cold, denser water sinks to become the ocean’s deep waters. Deep-water movement is called thermohaline circulation—thermo refers to temperature, and haline refers to salinity. This circulation connects the world’s deep ocean waters. Movement of the Gulf Stream illustrates the beginning of thermohaline circulation. Heat in the warm poleward moving Gulf Stream promotes evaporation which takes heat from the water and as heat thus dissipates, the water cools. The resulting water is much colder, saltier, and denser. As the denser water reaches the North Atlantic and Greenland, it begins to sink and becomes a deep-water current. As shown in the illustration above, this worldwide connection between shallow and deep-ocean circulation overturns and mixes the entire world ocean, bringing nutrients to marine life, and is sometimes referred to as the global conveyor belt [11].

12.3.3 Tides

Tides are the rising and lowering of sea level during the day and are caused by the gravitational effects of the Sun and Moon on the oceans [12]. The Earth rotates daily within the Moon and Sun’s gravity fields. Although the Sun is much larger and its gravitational pull is more powerful, the Moon is closer to Earth; hence, the Moon’s gravitational influence on tides is dominant. The magnitude of the tide at a given location and the difference between high and low tide—the tidal range, depends primarily on the configuration of the Moon and Sun with respect to the Earth orbit and rotation. Spring tide occurs when the Sun, Moon, and Earth line up with each other at the full or new Moon, and the tidal range is at a maximum. Neap tide occurs approximately two weeks later when the Moon and Sun are at right angles with the Earth, and the tidal range is lowest.

The Earth rotates within a tidal envelope, so tides rise and ebb daily. Tides are measured at coastal locations. These measurements and the tidal predictions based on them are published on the NOAA website [13]. Tides rising and falling create tidal patterns at any given shore location. The three types of tidal patterns are diurnal, semidiurnal, and mixed.

Diurnal tides go through one complete cycle each tidal day. A tidal day is the amount of time for the Moon to align with a point on the Earth as the Earth rotates, which is slightly longer than 24 hours. Semidiurnal tides go through two complete cycles in each tidal day—approximately 12 hours and 50 minutes, with the tidal range typically varying in each cycle. Mixed tides are a combination of diurnal and semidiurnal patterns and show two tidal cycles per tidal day, but the relative amplitudes of each cycle and their highs and lows vary during the tidal month. For example, there is a high, high tide and a high, low tide. The next day, there is a low, high tide and a low, low tide. Forecasting the tidal pattern and the times tidal phases arrive at a given shore location

is complicated and can be done for only a few days at a time. Tidal phases are determined by bathymetry: the depth of ocean basins and the continental obstacles that are in the way of the tidal envelope within which the Earth rotates. Local tidal experts make 48-hour tidal forecasts using tidal charts based on daily observations, as can be seen in the chart of different tide types. A typical tidal range is approximately 1 m (3 ft). Extreme tidal ranges occur where the tidal wave enters a narrow restrictive zone that funnels the tidal energy. An example is the English Channel between Great Britain and the European continent where the tidal range is 7 to 9.75 m (23 to 32 ft). The Earth’s highest tidal ranges occur at the Bay of Fundy, the funnel-like bay between Nova Scotia and New Brunswick, Canada, where the average range is nearly 12 m (40 ft) and the extreme range is around 18 m (60 ft). At extreme tidal range locations, a person who ventures out onto the seafloor exposed during ebb tide may not be able to outrun the advancing water during flood tide. NOAA has additional information on tides.

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Summary

Shoreline processes are complex, but important for understanding coastal processes. Waves, currents, and tides are the main agents that shape shorelines. Most coastal landforms can be attributed to moving sand via longshore drift, and long-term rising or falling sea levels.

The shoreline is the interface between water and land and is divided into five zones. Processes at the shoreline are called littoral processes. Waves approach the beach at an angle, which cause the waves to bend towards the beach. This bending action is called wave refraction and is responsible for creating the longshore current and longshore drift—the process that moves sand along the coasts. When the longshore current deposits sand along the coast into quieter waters, the sand can accumulate, creating a spit or barrier called a baymouth bar, which often blocks bays and harbors. Inconvenienced humans create methods to keep their harbors open and preserve sand on their beaches by creating jetties and groins, which negatively affect natural beach processes.

Emergent coasts are created by sea levels falling, while submergent coasts are caused by sea levels rising. Oceans absorb solar energy, which is distributed by currents throughout the world. Circular surface currents, called gyres, rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. Thermohaline deep circulation connects the world’s deep ocean waters: when shallow poleward moving warm water evaporates, the colder, saltier, and denser water sinks and becomes deep-water currents. The connection between shallow and deep-ocean circulation is called the global conveyor belt.

Tides are the rising and lowering of sea level during the day and are caused by the gravitational effects of the Sun and Moon on the oceans. There are three types of tidal patterns: diurnal, semidiurnal, and mixed. Typical tidal ranges are approximately 1 m (3 ft). Extreme tidal ranges are around 18 m (60 ft).

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References

Colling, Angela. 2001. Ocean Circulation. Edited by Open University Course Team. Butterworth-Heinemann.

Davis, Richard A., Jr., and Duncan M. Fitzgerald. 2009. Beaches and Coasts. John Wiley & Sons.

Davis, Richard Albert. 1997. The Evolving Coast. Scientific American Library New York.

Greene, Paul, George Follett, and Clint Henker. 2009. “Munitions and Dredging Experience on the United States Coast.” Marine Technology Society Journal 43 (4): 127–31.

Jackson, Nancy L., Mitchell D. Harley, Clara Armaroli, and Karl F. Nordstrom. 2015. “Beach Morphologies Induced by Breakwaters with Different Orientations.” Geomorphology 239 (June). Elsevier: 48–57.

“Littoral Bypassing and Beach Restoration in the Vicinity of Port Hueneme, California.” n.d. In Coastal Engineering 1966.

Munk, Walter H. 1950. “ON THE WIND-DRIVEN OCEAN CIRCULATION.” Journal of Meteorology 7 (2): 80–93.

Normark, William R., and Paul R. Carlson. 2003. “Giant Submarine Canyons: Is Size Any Clue to Their Importance in the Rock Record?” Geological Society of America Special Papers 370 (January): 175–90.

Reineck, H-E, and Indra Bir Singh. 2012. Depositional Sedimentary Environments: With Reference to Terrigenous Clastics. Springer Science & Business Media.

Rich, John Lyon. 1951. “THREE CRITICAL ENVIRONMENTS OF DEPOSITION, AND CRITERIA FOR RECOGNITION OF ROCKS DEPOSITED IN EACH OF THEM.” Geological Society of America Bulletin 62 (1). gsabulletin.gsapubs.org: 1–20.

Runyan, Kiki, and Gary Griggs. 2005. “Implications of Harbor Dredging for the Santa Barbara Littoral Cell.” In California and the World Ocean ’02, 121–35. Reston, VA: American Society of Civil Engineers.

Schwiderski, Ernst W. 1980. “On Charting Global Ocean Tides.” Reviews of Geophysics 18 (1): 243–68.

Stewart, Robert H. 2008. Introduction to Physical Oceanography. Texas A & M University Texas.

Stommel, Henry, and A. B. Arons. 2017. “On the Abyssal Circulation of the World ocean—I. Stationary Planetary Flow Patterns on a Sphere – ScienceDirect.” Accessed February 26. http://www.sciencedirect.com/science/article/pii/0146631359900656.

Stommel, Henry, and A. B. Arons. 2017. “On the Abyssal Circulation of the World ocean—I. Stationary Planetary Flow Patterns on a Sphere – ScienceDirect.” Accessed February 26. http://www.sciencedirect.com/science/article/pii/0146631359900656.

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13 Deserts

13 Deserts

KEY CONCEPTS

By the end of this chapter, students should be able to:

Explain the defining characteristic of a desert and distinguish between the three broad categories of deserts

Explain how geographic features, latitude, atmospheric circulation, and Coriolis Effect influence where deserts are located

List the primary desert weathering and erosion processes

Identify desert landforms

Explain how desert landforms are formed by erosion and deposition

Describe the main types of sand dunes and the conditions that form them

Identify the main features of the Basin and Range desert (United States)

Approximately 30% of the Earth’s terrestrial surface consists of deserts, which are defined as locations of low precipitation. While temperature extremes are often associated with deserts, they do not define them. Deserts exhibit extreme temperatures because of the lack of moisture in the atmosphere, including low humidity and scarce cloud cover. Without cloud cover, the Earth’s surface absorbs more of the Sun’s energy during the day and emits more heat at night.

Deserts are not randomly located on the Earth’s surface. Many deserts are located at latitudes between 15° and 30° in both hemispheres and at both the North and South Poles, created by prevailing wind circulation in the atmosphere. Sinking, dry air currents occurring at 30° north and south of the equator produce trade winds that create deserts like the African Sahara and Australian Outback.

Another type of desert is found in the rain shadow created from prevailing winds blowing over mountain ranges. As the wind drives air up and over mountains, atmospheric moisture is released as snow or rain. Atmospheric pressure is lower at higher elevations, causing the moisture-laden air to cool. Cool air holds less moisture than hot air, and precipitation occurs as the wind rises up the mountain. After releasing its moisture on the windward side of the mountains, the dry air descends on the leeward or downwind side of the mountains to create an arid region with little precipitation called a rain shadow. Examples of rain-shadow deserts include the Western Interior Desert of North America and Atacama Desert of Chile, which is the Earth’s driest, warm desert.

Finally, polar deserts, such as vast areas of the Antarctic and Arctic, are created from sinking cold air that is too cold to hold much moisture. Although they are covered with ice and snow, these deserts have very low average annual precipitation. As a result, Antarctica is Earth’s driest continent.

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13.1 The Origin of Deserts

13.1.1 Atmospheric circulation

Geographic location, atmospheric circulation, and the Earth’s rotation are the primary causal factors of deserts. Solar energy converted to heat is the engine that drives the circulation of air in the atmosphere and water in the oceans. The strength of the circulation is determined by how much energy is absorbed by the Earth’s surface, which in turn is dependent on the average position of the Sun relative to the Earth. In other words, the Earth is heated unevenly depending on latitude and angle of incidence. Latitude is a line circling the Earth parallel to the equator and is measured in degrees. The equator is 0° and the North and South Poles are 90° N and 90° S respectively (see the diagram of generalized atmospheric circulation on Earth). Angle of incidence is the angle made by a ray of sunlight shining on the Earth’s surface. Tropical zones are located near the equator, where the latitude and angle of incidence are close to 0°, and receive high amounts of solar energy. The poles, which have latitudes and angles of incidence approaching 90°, receive little or almost no energy.

The figure shows the generalized air circulation within the atmosphere. Three cells of circulating air span the space between the equator and poles in both hemispheres, the Hadley Cell, the Ferrel or Midlatitude Cell, and the Polar Cell. In the Hadley Cell located over the tropics and closest to the equatorial belt, the sun heats the air and causes it to rise. The rising air cools and releases its contained moisture as tropical rain. The rising dried air spreads away from the equator and toward the north and south poles, where it collides with dry air in the Ferrel Cell. The combined dry air sinks back to the Earth at 30° latitude. This sinking drier air creates belts of predominantly high pressure at approximately 30° north and south of the equator, called the horse latitudes. Arid zones between 15o and 30o north and south of the equator thus exist within which desert conditions predominate. The descending air flowing north and south in the Hadley and Ferrel cells also creates prevailing winds called trade winds near the equator, and westerlies in the temperate zone. Note the arrows indicating general directions of winds in these zones.

Other deserts, like the Great Basin Desert that covers parts of Utah and Nevada, owe at least part of their origin to other atmospheric phenomena. The Great Basin Desert, while somewhat affected by sinking air effects from global circulation, is a rain-shadow desert. As westerly moist air from the Pacific rises over the Sierra Nevada and other mountains, it cools and loses moisture as condensation and precipitation on the upwind or rainy side of the mountains.

One of the driest places on Earth is the Atacama Desert of northern Chile. The Atacama Desert occupies a strip of land along Chile’s coast just north of latitude 30°S, at the southern edge of the trade-wind belt. The desert lies west of the Andes Mountains, in the rain shadow created by prevailing trade winds blowing west. As this warm moist air crossing the Amazon basin meets the eastern edge of the mountains, it rises, cools, and precipitates much of its water out as rain. Once over the mountains, the cool, dry air descends onto the Atacama desert. Onshore winds from the Pacific are cooled by the Peru (Humboldt) ocean current. This super-cooled air holds almost no moisture and, with these three factors, some locations in the Atacama Desert have received no measured precipitation for several years. This desert is the driest, non-polar location on Earth.

Notice in the figure that the polar regions are also areas of predominantly high pressure created by descending cold dry air, the Polar Cells. As with the other cells, cold air, which holds much less moisture than warm air, descends to create polar deserts. This is why historically, land near the north and south poles has always been so dry.

13.1.2 Coriolis Effect

The Earth rotates toward the east where the sun rises. Think of spinning a weight on a string around your head. The speed of the weight depends on the length of the string. The speed of an object on the rotating Earth depends on its horizontal distance from the Earth’s axis of rotation. Higher latitudes are a smaller distance from the Earth’s rotational axis, and therefore do not travel as fast eastward as lower latitudes that are closer to the equator. When a fluid like air or water moves from a lower latitude to a higher latitude, the fluid maintains its momentum from moving at a higher speed, so it will travel relatively faster eastward than the Earth beneath at the higher latitudes. This factor causes deflection of movements that occur in north-south directions.

Another factor in the Coriolis effect also causes deflection of east-west movement due to the angle between the centripetal effect of Earth’s spin and gravity pulling toward the earth’s center (see figure). This produces a net deflection toward the equator. The total Coriolis deflection on a mass moving in any direction on the rotating Earth results from a combination of these two factors.

Since each hemisphere has three atmospheric cells moving respectively north and south relative to the Earth beneath them, the Coriolis effect deflects these moving air masses to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis effect also deflects moving masses of water in the ocean currents.

For example, in the northern hemisphere Hadley Cell, the lower altitude air currents are flowing south towards the equator. These are deflected to the right (or west) by the Coriolis effect. This deflected air generates the prevailing trade winds that European sailors used to cross the Atlantic Ocean and reach South America and the Caribbean Islands in their tall ships. This air movement is mirrored in the Hadley Cell in the southern hemisphere; the lower altitude air current flowing equatorward is deflected to the left, creating trade winds that blow to the northwest.

In the northern Mid-Latitude or Ferrel Cell, surface air currents flow from the horse latitudes (latitude 30°) toward the North Pole, and the Coriolis effect deflects them toward the east, or to the right, producing the zone of westerly winds. In the southern hemisphere Mid-Latitude or Ferrel Cell, the poleward flowing surface air is deflected to the left and flows southeast creating the Southern Hemisphere westerlies.

Another Coriolis-generated deflection produces the Polar Cells. At 60o north and south latitude, relatively warmer rising air flows poleward cooling and converging at the poles where it sinks in the polar high. This sinking dry air creates the polar deserts, the driest deserts on Earth. Persistence of ice and snow is a result of cold temperatures at these dry locations.

The Coriolis effect operates on all motions on the Earth. Artillerymen must take the Coriolis effect into account on ballistic trajectories when making long-distance targeting calculations. Geologists note how its effect on air and oceanic currents creates deserts in designated zones around the Earth as well as the surface currents in the ocean. The Coriolis effect causes the ocean gyres to turn clockwise in the northern hemisphere and counterclockwise in the southern. It also affects weather by creating high-altitude, polar jet streams that sometimes push lobes of cold arctic air into the temperate zone, down to as far as latitude 30° from the usual 60°. It also causes low pressure systems and intense tropical storms to rotate counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

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Explanation of Coriolis Effect.

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13.2 Desert weathering and erosion

Weathering takes place in desert climates by the same means as other climates, only at a slower rate. While higher temperatures typically spur faster chemical weathering, water is the main agent of weathering, and lack of water slows both mechanical and chemical weathering. Low precipitation levels also mean less runoff as well as ice wedging. When precipitation does occur in the desert, it is often heavy and may result in flash floods in which a lot of material may be dislodged and moved quickly.

One unique weathering product in deserts is desert varnish. Also known as desert patina or rock rust, this is thin dark brown layers of clay minerals and iron and manganese oxides that form on very stable surfaces within arid environments. The exact way this material forms is still unknown, though cosmogenic and biologic mechanisms have been proposed.

While water is still the dominant agent of erosion in most desert environments, wind is a notable agent of weathering and erosion in many deserts. This includes suspended sediment traveling in haboobs, or large dust storms, that frequent deserts. Deposits of windblown dust are called loess. Loess deposits cover wide areas of the midwestern United States, much of it from rock flour that melted out of the ice sheets during the last ice age. Loess was also blown from desert regions in the West. Possessing lower energy than water, wind transport nevertheless moves sand, silt, and dust. As noted in chapter 11, the load carried by a fluid (air is a fluid like water) is distributed among bedload and suspended load. As with water, in wind these components depend on wind velocity.

Sand size material moves by a process called saltationin which sand grains are lifted into the moving air and carried a short distance where they drop and splash into the surface dislodging other sand grains which are then carried a short distance and splash dislodging still others.

Since saltating sand grains are constantly impacting other sand grains, wind blown sand grains are commonly quite well rounded with frosted surfaces. Saltation is a cascading effect of sand movement creating a zone of wind blown sand up to a meter or so above the ground. This zone of saltating sand is a powerful erosive agent in which bedrock features are effectively sandblasted. The fine-grained suspended load is effectively sorted from the sand near the surface carrying the silt and dust into haboobs. Wind is thus an effective sorting agent separating sand and dust sized (≤70 µm) particles (See Chapter 5). When wind velocity is high enough to slide or roll materials along the surface, the process is called creep.

One extreme version of sediment movement was shrouded in mystery for years: Sliding stones. Also called sailing stones and sliding rocks, these are large moving boulders along flat surfaces in deserts, leaving trails. This includes the famous example of the Racetrack Playa in Death Valley National Park, California. For years, scientists and enthusiasts attempted to explain their movement, with little definitive results. In recent years, several experimental and observational studies have confirmed that the stones, imbedded in thin layers of ice, are propelled by friction from high winds. These studies include measurements of actual movement, as well as re-creations of the conditions, with resulting movement in the lab.

The zone of saltating sand is an effective agent of erosion through sand abrasion. A bedrock outcrop which has such a sandblasted shape is called a yardang. Rocks and boulders lying on the surface may be blasted and polished by saltating sand. When predominant wind directions shift, multiple sandblasted and polished faces may appear. Such wind abraded desert rocks are called ventifacts.

In places with sand and silt accumulations, clumps of vegetation often anchor sediment on the desert surface. Yet, winds may be sufficient to remove materials not anchored by vegetation. The bowl-shaped depression remaining on the surface is called a blowout.

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13.3 Desert landforms

In the American Southwest, as streams emerge into the valleys from the adjacent mountains, they create desert landforms called alluvial fans. When the stream emerges from the narrow canyon, the flow is no longer constrained by the canyon walls and spreads out. At the lower slope angle, the water slows down and drops its coarser load. As the channel fills with this conglomeratic material, the stream is deflected around it. This deposited material deflects the stream into a system of radial distributary channels in a process similar to how a delta is made by a river entering a body of water. This process develops a system of radial distributaries and constructs a fan shaped feature called an alluvial fan.

Alluvial fans continue to grow and may eventually coalesce with neighboring fans to form an apron of alluvium along the mountain front called a bajada.

As the mountains erode away and their sediment accumulates first in alluvial fans, then bajadas, the mountains eventually are buried in their own erosional debris. Such buried mountain remnants are called inselbergs, “island mountains,” as first described by the German geologist Wilhelm Bornhardt (1864–1946).

Where the desert valley is an enclosed basin, i.e. streams entering it do not drain out, the water is removed by evaporation and a dry lake bed is formed called a playa.

Playas are among the flattest of all landforms. Such a dry lake bed may cover a large area and be filled after a heavy thunderstorm to only a few inches deep. Playa lakes and desert streams that contain water only after rainstorms are called intermittent or ephemeral. Because of intense thunderstorms, the volume of water transported by ephemeral drainage in arid environments can be substantial during a short period of time. Desert soil structures lack organic matter that promotes infiltration by absorbing water. Instead of percolating into the soil, the runoff compacts the ground surface, making the soil hydrophobic or water-repellant. Because of this hardpan surface, ephemeral streams may gather water across large areas, suddenly fillingwith water from storms many miles away.

High-volume ephemeral flows, called flash floods, may move as sheet flows or sheetwash, as well as be channeled through normally dry arroyos or canyons. Flash floods are a major factor in desert deposition. Dry channels can fill quickly with ephemeraldrainage, creating a mass of water and debris that charges down the arroyo, and even overflowing the banks. Flash floods pose a serious hazard for desert travelers because the storm activity feeding the runoff may be miles away. People hiking or camping in arroyos that have been bone dry for months, or years, have been swept away by sudden flash floods.

13.3.1 Sand

The popular concept of a typical desert is a broad expanse of sand. Geologically, deserts are defined by a lack of water and arid regions resembling a sea of sand belong to the category of desert called an erg. An erg consists of fine-grained, loose sand grains, often blown by wind, or aeolian forces, into dunes. Probably the best known erg is the Rub’ al Khali, which means Empty Quarter, of the Arabian Peninsula. Ergs are also found in the Great Sand Dunes National Park (Colorado), Little Sahara Recreation Area (Utah), White Sands National Monument (New Mexico), and parts of Death Valley National Park (California). Ergs are not restricted to deserts, but may form anywhere there is a substantial supply of sand, including as far north as 60° N in Saskatchewan, Canada, in the Athabasca Sand Dunes Provincial Park . Coastal ergs exist along lakes and oceans as well, and examples are found in Oregon, Michigan, and Indiana.

An internal cross section of a sand dune shows a feature called cross bedding. As wind blows up the windward side of the dune, it carries sand to the dune crest depositing layers of sand parallel to the windward (or “stoss”) side. The sand builds up the crest of the dune and pours over the top until the leeward (downwind or slip) face of the dune reaches the angle of repose, the maximum angle which will support the slip face. Dunes are unstable features and move as the sand erodes from the stoss side and continues to drop down the leeward side covering previous stoss and slip-face layers and creating the cross beds. Mostly, these are reworked over and over again, but occasionally, the features are preserved in a depression, then lithified. Shifting wind directions and abundant sand sources create chaotic patterns of cross beds like those seen in Zion National Park of Utah.

In the Mesozoic Era, Utah was covered by a series of ergs, with the thickest being in Southern Utah, which lithified into sandstone (see Chapter 5). Perhaps the best known of these sandstone formations is the Navajo Sandstone of Jurassic age. This sandstone formation consists of dramatic cliffs and spires in Zion National Park and covers a large part of the Colorado Plateau. In Arches National Park, a later series of sand dunes covered the Navajo Sandstone and lithified to become the Entrada Formation also during the Jurassic. Erosion of overlying layers exposed fins of the underlying Entrada Sandstone and carved out weaker parts of the fins forming the arches.

As the cements that hold the grains together in these modern sand cliffs disintegrate and the freed grains gather at the base of the cliffs and move down the washes, sand grains may be recycled and redeposited. These Mesozoic sand ergs may represent ancient quartz sands recycled many times from igneous origins in the early Precambrian, just passing now through another cycle of erosion and deposition. An example of this is Coral Pink Sand Dunes State Park in Southwestern Utah, which contains sand that is being eroded from the Navajo Sandstone to form new dunes.

Dune Types

Dunes are complex features formed by a combination of wind direction and sand supply, in some cases interacting with vegetation. There are several types of dunes representing variables of wind direction, sand supply and vegetative anchoring. Barchan dunes or crescent dunesform where sand supply is limited and there is a fairly constant wind direction. Barchans move downwind and develop a crescent shape with wings on either side of a dune crest. Barchans are known to actually move over homes, even towns.

Longitudinal dunes or linear dunesform where sand supply is greater and the wind blows around a dominant direction, in a back-and-forth manner. They may form ridges tens of meters high lined up with the predominant wind directions.

Parabolic dunesform where vegetation anchors parts of the sand and unanchored parts blowout. Parabolic dune shape may be similar to barchan dunes but usually reversed, and it is determined more by the anchoring vegetation than a strict parabolic form.

Star dunesform where the wind direction is variable in all directions. Sand supply can range from limited to quite abundant. It is the variation in wind direction that forms the star.

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13.4 The Great Basin and the Basin and Range

The Great Basin is the largest area of interior drainage in North America, meaning there is no outlet to the ocean and all precipitation remains in the basin or is evaporated. It covers western Utah, most of Nevada, and extends into southeastern California, southern Oregon, and southern Idaho. Because there is not outlet to the ocean, streams in the Great Basin deliver runoff to lakes and playas within the basin. A subregion within the Great Basin is the Basin and Range which extends from the Wasatch Front in Utah west across Nevada to the Sierra Nevada Mountains of California. The basins and ranges referred to in the name are horstsand grabens, formed by normal fault blocks from crustal extension, as discussed in chapter 2 and chapter 9. The lithosphere of the entire area has stretched by a factor of about 2, meaning from end to end, the distance has doubled over the past 30 million years or so. Valleys without outlets form individual basins, each of which is filled with alluvial sediments leading into playa depositional environments. During the recent Ice Age, The climate was more humid and while glaciers were forming in some of the mountains, pluvial lakes formed covering large areas (see chapter 14.4.3). During the Ice Age, valleys in much of western Utah and eastern Nevada were covered by Lake Bonneville. As the climate became arid after the Ice Age, Lake Bonneville dried leaving as a remnant the Great Salt Lake in Utah.

The desert of the Basin and Range extends from about 35° to near 40° and results from a rain shadow effect created by westerly winds from the Pacific rising and cooling over the Sierras becoming depleted of moisture by precipitation on the western side. The result is relatively dry air descending across Nevada and western Utah.

A journey from the Wasatch Front southwest to the Pacific Ocean will show stages of desert landscape evolution from the fault block mountains of Utah with sharp peaks and alluvial fans at the mouths of canyons, through landscapes in Southern Nevada with bajadas along the mountain fronts, to the landscapes in the Mojave Desert of California with subdued inselbergs sticking up through a sea of coalesced bajadas. These landscapes illustrate the evolutionary stages of desert landscape development.

13.4.1 Desertification

When previously arable landsuitable for agriculture transforms into desert, this process is called desertification. Plants and humus-rich soil (see Chapter 5) promote groundwater infiltration and water retention. When an area becomes more arid due to changing environmental conditions, the plants and soil become less effective in retaining water, creating a positive feedback loop of desertification. Thisself-reinforcing loop spirals into increasingly arid conditions and further enlarges the desert regions.

Desertification may be caused by human activities, such as unsustainable crop cultivation practices, overgrazing by livestock, overuse of groundwater, and global climate change. Human-caused desertification is a serious worldwide problem. The world map figure above shows what areas are most vulnerable to desertification. Note the red and orange areas in the western and midwestern regions of the United States, which also cover large areas of arable land used for raising food crops and animals. The creation of the Dust Bowl in the 1930s (see Chapter 5) is a classic example of a high-vulnerability region impacted by human–caused desertification. As demonstrated in the Dust Bowl, conflicts may arise between agricultural practices and conservation measures. Mitigating desertification while allowing farmers to make a survivable living requires public and individual education to create community support and understanding of sustainable agriculture alternatives.

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Summary

Approximately 30% of Earth’s surface is arid lands, the location of which is determined by latitude, atmospheric circulation, and terrain. The arid belts between 15o and 30o north and south latitudes are produced by descending air masses associated with major cells in the atmosphere and include the major deserts like the Sahara in Africa and the Middle East. Rain shadow deserts lie behind mountain ranges or long land expanses in zones of prevailing winds like the deserts of western North America, the Atacama of South America, and the Gobi of Asia. Dry descending air also creates the polar deserts at the poles.

Major atmospheric circulation involves the Hadley cells, midlatitude or Ferrel cells, and the polar cells in each hemisphere. Warmed and rising air in the Hadley cells rains back on the tropics and moves toward the poles as dryer air. That air meets the dryer equatorward moving air of the Ferrel cells. This dry air descends in the arid zones, called the horse latitudes, to produce the arid belts in each hemisphere. Rotation of the Earth creates the Coriolis Effect that deflects these moving air masses to produce zones of prevailing winds, the Trade Winds in the subtropics and the Westerlies in the midlatitudes. A combination of latitude, rain shadow, and cold adjacent ocean currents causes the Atacama Desert of northern Chile, the driest desert on Earth.

Weatheringin deserts takes place just as in other climes only slower because of less water. Desert varnish is a weathering product unique to desert environments. As in more humid climes, water is the main agent of erosion although wind is a notable agent. Large dust storms called haboobs transport large amounts of sediment that may accumulate in sand seas called ergs or finer grained loess deposits. Sand transport occurs mainly by saltation in which grain to grain impact causes frosting of grain surfaces. Sandblasting by persistent winds produces stones with polished surfaces called ventifacts and sculpted bedrock features called yardangs.

Landforms produced in desert environments include alluvial fans, bajadas, inselbergs, and playas. Windblown sand can accumulate as dunes. The forms of dunes, like barchans, parabolic, longitudinal, and star dunes, relate to the abundance of sand supply and wind direction as well as presence of vegetation. The internal structure of dunes shows cross bedding. Fossil dunes in an ancient desert leave cross bedding in places like Zion and Arches national parks in Utah showing shifting wind directions in these ancient environments. Ephemeral streams in desert regions may carry water only after storms and pose risk of flash floods.

The Great Basin of North America is an enclosed basin with no drainage to the ocean. The only exit for precipitation there is by evaporation. Travels in the Great Basin show stages of development of desert landscapes from playas and alluvial fans, to bajadas, to inselbergs which are eroded mountains buried in their own erosional debris.

Poor land management can result in dying vegetation and loss of soil moisture producing an accelerating process of desertification in which once productive land is degraded into unproductive desert. This is a serious worldwide problem.

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References

Bagnold, R. A. 1941. “The Physics of Blown Sand and Desert Dunes.” Methum, London, UK, 265.

Boggs, Sam. 2006. “Principles of Sedimentology and Stratigraphy.” Pearson Prentice Hall. http://agris.fao.org/agris-search/search.do?recordID=US201300110702.

Clements, Thomas. 1952. “Wind-Blown Rocks and Trails on Little Bonnie Claire Playa, Nye County, Nevada.” Journal of Sedimentary Research 22 (3). Society for Sedimentary Geology. http://archives.datapages.com/data/sepm/journals/v01-32/data/022/022003/0182.htm.

Collado, Gonzalo A., Moisés A. Valladares, and Marco A. Méndez. “Hidden Diversity in Spring Snails from the Andean Altiplano, the Second Highest Plateau on Earth, and the Atacama Desert, the Driest Place in the World.” Zoological Studies 52 (1): 50.

Easterbrook, Don J. 1999. Surface Processes and Landforms. Pearson College Division.

Geist, Helmut. 2005. The Causes and Progression of Desertification. Ashgate Aldershot.

Grayson, Donald K. 1993. The Desert’s Past: A Natural Prehistory of the Great Basin. Smithsonian Inst Pr.

Hadley, Geo. 1735. “Concerning the Cause of the General Trade-Winds: By Geo. Hadley, Esq; FRS.” Philosophical Transactions 39 (436-444). The Royal Society: 58–62.

Hartley, Adrian J., and Guillermo Chong. 2002. “Late Pliocene Age for the Atacama Desert: Implications for the Desertification of Western South America.” Geology 30 (1). geology.gsapubs.org: 43–46.

Hedin, Sven Anders. 1903. Central Asia and Tibet. Vol. 1. Hurst and Blackett, limited.

Hooke, Roger Leb. 1967. “Processes on Arid-Region Alluvial Fans.” The Journal of Geology 75 (4). journals.uchicago.edu: 438–60.

King, Lester C. 1953. “CANONS OF LANDSCAPE EVOLUTION.” Geological Society of America Bulletin 64 (7). gsabulletin.gsapubs.org: 721–52.

Kletetschka, Gunther, Roger Leb Hooke, Andrew Ryan, George Fercana, Emerald McKinney, and Kristopher P. Schwebler. 2013. “Sliding Stones of Racetrack Playa, Death Valley, USA: The Roles of Rock Thermal Conductivity and Fluctuating Water Levels.” Geomorphology 195 (August). Elsevier: 110–17.

Laity, Julie E. 2009. “Landforms, Landscapes, and Processes of Aeolian Erosion.” In Geomorphology of Desert Environments, edited by Anthony J. Parsons and Athol D. Abrahams, 597–627. Springer Netherlands.

Littell, Eliakim, and Robert S. Littell. 1846. Littell’s Living Age. T.H. Carter & Company.

Livingstone, Ian, and Andrew Warren. 1996. Aeolian Geomorphology: An Introduction. Longman.

Muhs, Daniel R., and E. A. Bettis. 2003. “Quaternary Loess-Paleosol Sequences as Examples of Climate-Driven Sedimentary Extremes.” Special Papers-Geological Society of America. Boulder, Colo.; Geological Society of America; 1999, 53–74.

Norris, Richard D., James M. Norris, Ralph D. Lorenz, Jib Ray, and Brian Jackson. 2014. “Sliding Rocks on Racetrack Playa, Death Valley National Park: First Observation of Rocks in Motion.” PloS One 9 (8). journals.plos.org: e105948.

Shao, Yaping. 2008. Physics and Modelling of Wind Erosion. Springer Science & Business Media.

Stanley, George M. 1955. “ORIGIN OF PLAYA STONE TRACKS, RACETRACK PLAYA, INYO COUNTY, CALIFORNIA.” Geological Society of America Bulletin 66 (11). gsabulletin.gsapubs.org: 1329–50.

Walker, Alta S. 1996. “Deserts: Geology and Resources.” Government Printing Office. https://pubs.er.usgs.gov/publication/7000004.

Wilson, Ian Gordon. 1971. “Desert Sandflow Basins and a Model for the Development of Ergs.” The Geographical Journal 137 (2). [Wiley, Royal Geographical Society (with the Institute of British Geographers)]: 180–99.

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14 Glaciers

14 Glaciers

KEY CONCEPTS

At the end of this chapter, students should be able to:

Differentiate the different types of glaciers and contrast them with sea icebergs Describe how glaciers form, move, and create landforms Describe glacial budget; describe the zones of accumulation, equilibrium, and melting Identify glacial erosional and depositional landforms and interpret their origin; describe glacial lakes

Differentiate the different types of glaciers and contrast them with sea icebergs

Describe how glaciers form, move, and create landforms

Describe glacial budget; describe the zones of accumulation, equilibrium, and melting

Identify glacial erosional and depositional landforms and interpret their origin; describe glacial lakes

Describe the history and causes of past glaciations and their relationship to climate, sea-level changes, and isostatic rebound

Describe the history and causes of past glaciations and their relationship to climate, sea-level changes, and isostatic rebound

The Earth’s cryosphere, or ice, has a unique set of erosional and depositional features compared to its hydrosphere, or liquid water. This ice exists primarily in two forms, glaciers and icebergs. Glaciers are large accumulations of ice that exist year-round on the landsurface. In contrast, masses ice floating on the ocean are icebergs, although they may have had their origin in glaciers.

Glaciers cover about 10% of the Earth’s surface and are powerful erosional agents that sculpt the planet’s surface.These enormous masses of ice usually form in mountainous areas that experience cold temperatures and high precipitation. Glaciers also occur in low lying areassuch as Greenland and Antarctica that remain extremely cold year-round.

14.1 GlacierFormation

Glaciers form when repeated annual snowfall accumulates deep layers of snow that are not completely melted in the summer. Thus there is an accumulation of snow thatbuilds up into deep layers. Perennial snow is a snow accumulation that lasts all year. A thin accumulation of perennial snow is a snow field. Over repeated seasons of perennial snow, the snow settles, compacts, and bonds with underlying layers. The amount of void space between the snow grains diminishes. As the old snow gets buried by more new snow, the older snow layers compactinto firn, or névé, a granular mass of ice crystals. As the firn continues to be buried, compressed, and recrystallizes, the void spaces become smallerand the ice becomes less porous, eventually turninginto glacier ice. Solid glacial ice still retains a fair amount of void spaceand that traps air. These small air pockets provide records of the past atmospherecomposition.

There are three general types of glaciers: alpine or valley glaciers, ice sheets, and ice caps. Most alpine glaciers are located in the world’s major mountain ranges such as the Andes, Rockies, Alps, and Himalayas, usually occupying long, narrow valleys. Alpine glaciers may also form at lower elevations in areas that receive high annual precipitation such as the Olympic Peninsula in Washington state.

Ice sheets, also called continental glaciers,form across millions of square kilometers of land and are thousands of meters thick. Earth’s largest ice sheets are located on Greenland and Antarctica. The Greenland Ice Sheet is the largest ice mass in the Northern Hemisphere with an extensive surface area of over 2 million sq km (1,242,700 sq mi) and an average thickness of up to 1500 meters (5,000 ft, almost a mile) [1].

The Antarctic Ice Sheet is even larger and covers almost the entire continent. The thickest parts of the Antarctic ice sheet are over 4,000 meters thick (>13,000 ft or 2.5 mi). Its weight depresses the Antarctic bedrock to below sea level in many places. The cross-sectional diagram comparing the Greenland and Antarctica ice sheets illustrates the size difference between the two.

Ice cap glaciers are smaller versions of ice sheets that cover less than 50,000 km2 and usually occupy higher elevations and may cover tops of mountains. There are several ice caps on Iceland. A small ice cap called Snow Dome is near Mt. Olympus on the Olympic Peninsula in the state of Washington.

Snow Dome ice cap near Mt. Olympus, Washington	Vatnajökull ice cap in Iceland

The figure shows the size of the ancient Laurentide Ice Sheet in the Northern Hemisphere. This ice sheet was present during the last glacial maximum event, also known as the last Ice Age.

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14. 2 Glacier Movement

Glacial crevasses.	Cravasse on the Easton Glacier in the North Cascades

As the ice accumulates, it begins to flow downward under its own weight. In 1948,glaciologistsinstalled hollow vertical rods in into the Jungfraufirn Glacier in the Swiss Alps to measurechanges in its movement over two years. This study showed that the ice at the surface was fairly rigid and ice within the glacier was actually flowing downhill. The cross-sectional diagram of an alpine (valley) glaciershows that the rate of ice movement is slow near the bottom, and fastest in the middle with the top ice being carried along on the ice below.

One of the unique properties of ice is that it melts under pressure. About half of the overall glacial movement was from sliding on a film of meltwater along the bedrock surface and half from internal flow. Ice near the surface of the glacier is rigid and brittle to a depth of about 50 m (165 ft). In this brittle zone, large ice cracks called crevasses form on the glacier’s moving surface. These crevasses can be covered and hidden by a snow bridge and are a hazard for glacier travelers.

Below the brittle zone, the pressure typically exceeds 100 kilopascals (kPa), which is –approximately 100,000 times atmospheric pressure. Under this applied force, the iceno longer breaks, but rather it bends or flows in a zone called the plastic zone. This plastic zone represents the great majority of glacierice. The plastic zone contains a fair amount of sedimentof various grades from boulders to silt and clay. As the bottom of the glacier slides and grinds across the bedrock surface, these sediments act as grinding agentsand create a zone of significant erosion.

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14.3 Glacial Budget

A glacial budget is like a bank account, with the ice being the existing balance. If there is more income (snow accumulating in winter)than expense (snow and ice melting in summer), then the glacialbudget shows growth. A positive or negative balance of ice in the overall glacial budget determines whether a glacier advances or retreats, respectively. The area where the ice balance is growing is called the zone of accumulation. The area where ice balance is shrinking is called the zone of ablation.

The diagram shows these two zones and the equilibrium line. In the zone of accumulation, the snowaccumulation rate exceeds the snow meltingrate and the ice surface is always covered with snow.The equilibrium line, also called the snowlineor firnline,marks the boundary between the zones of accumulation and ablation. Below the equilibrium line in the zone of ablation, the melting rate exceeds snow accumulation leaving the bare ice surface exposed. The position of the firnline changes during the season and from year to year as a reflectionof a positive or negative ice balance in the glacial budget. Of the two variables affecting a glacier‘s budget, winter accumulation and summer melt, summer melt matters most to a glacier’s budget. Cool summers promote glacial advance and warm summers promote glacial retreat .

If a handful of warmer summers promote glacial retreat, then global climate warming over decades and centuries will accelerate glacial melting and retreateven faster. Global warmingdue to human burning of fossil fuelsis causing the ice sheets to lose in years, an amount of mass that would normally take centuries. Current glacial melting is contributing to rising sea–levels faster than expected based on previous history.

Asthe Antarctica and Greenland ice sheets melt during global warming, they become thinner or deflate. The edges of the ice sheets break off and fall into the ocean, a process called calving, becoming floating icebergs. A fjordis a steep-walled valley flooded with sea water. The narrow shape of a fjord has been carved out by a glacierduring a cooler climate period. During a warming trend, glacial meltwater may raisethe sea level in fjords and flood formerly dry valleys [8, 9]. Glacial retreat and deflation are well illustrated in the 2009 TED Talk by James Balog.

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14.4 Glacial Landforms

Both alpine and continental glaciers create two categories of landforms:erosional and depositional. Erosional landforms are formed by the removal of material. Depositional landforms are formed by the addition of material. Because glaciers were first studied by 18th and 19th century geologists in Europe, the terminology applied to glaciers and glacial features contains many terms derived from European languages.

14.4.1 Erosional Glacial Landforms

Erosional landforms are created when moving masses of glacial ice slide and grindover bedrock. Glacial ice contains large amounts of poorly sorted sand, gravel, and boulders that have been plucked and pried from the bedrock. As the glaciers slide across the bedrock, they grind these sediments into a fine powder called rock flour. Rock flour acts as fine grit that polishes the surface of the bedrock to a smooth finish called glacial polish. Larger rock fragments scrapeover the surfacecreating elongated grooves called glacial striations.

A polished and striated rock in Yosemite.	Glacial striations in Mt. Rainier National Park.

Alpine glaciers produce a variety of unique erosional landforms, such as U-shaped valleys, arêtes, cirques, tarns, horns, cols, hanging valleys, and truncated spurs. In contrast,stream-carved canyons have a V-shaped profile when viewed in cross-section. Glacial erosion transforms a former V-shaped stream valley into a U-shaped one. Glaciers are typically wider than streams of similar length, and since glaciers tend to erode both at their bases and their sides, they erode V-shaped valleys into relatively flat-bottomed broad valleys with steep sides and a distinctive “U” shape. As seen in the images, Little Cottonwood Canyon near Salt Lake City, Utah was occupied by an Ice Ageglacier that extended down to the mouth of the canyon and into Lake Bonneville. Today, that U-shaped valley hosts many erosional landforms,including polished and striated rock surfaces. In contrast, Big Cottonwood Canyon to the north of Little Cottonwood Canyon has retained the V-shape in its lower portion, indicating that its glacier did not extend clear to its mouth, but was confined to its upper portion.

The U-shape of the Little Cottonwood Canyon, Utah, as it enters into the Salt Lake Valley.	V-shaped Big Cottonwood Canyon, carved by river.

When glaciers carve two U-shaped valleys adjacent to each other, the ridge between them tends to be sharpened into a sawtooth feature called an arête. At the head of a glacially carved valley is a bowl-shaped feature called a cirque. The cirque represents where the head of the glacier eroded the mountain by plucking rock away from it and the weight of the thick ice eroded out a bowl. After the glacier is gone, the bowl at the bottom of the cirque often fills with precipitation and is occupied by a lake, called a tarn. When three or more mountain glaciers erode headwardat their cirques, theyproduce horns,steep-sided, spire-shaped mountains. Low points along arêtes or between horns are mountain passes termed cols. Where a smaller tributary glacier flows into a larger trunk glacier, the smaller glacier cuts down less. Once the ice has gone, the tributary valley is left as a hanging valley, sometimes with a waterfallplunging into the main valley. As the trunk glacier straightens and widens a V-shaped valley and erodes the ends of side ridges, a steep triangle-shaped cliff is formed called a truncated spur.

Cirque with Upper Thornton Lake in the North Cascades National Park, Washington. Photo by Walter Siegmund.	An example of a horn, Kinnerly Peak, Glacier National Park, Montana. Photo by USGS.	Bridalveil Falls in Yosemite National Park, California, is a good example of a hanging valley.

14.4.2 Depositional Glacial Landforms

Depositional landforms and materialsare produced from deposits left behind by a retreating glacier. All glacial deposits are called drift. Theseinclude till, tillites, diamictites,terminal moraines, recessional moraines, lateral moraines, medial moraines, ground moraines, silt, outwash plains, glacial erratics, kettles, kettle lakes, crevasses, eskers, kames, and drumlins.

Glacial ice carries a lot of sediment, which when deposited by a melting glacieris called till. Till is poorly sorted with grain sizes ranging from clay and silt to pebbles and boulders. These clasts may be striated. Many depositional landforms are composed of till. The term tilliterefers to lithified rockhaving glacial origins. Diamictite refers to a lithified rock that contains a wide range of clast sizes; this includes glacial till but is a more objective and descriptive term for any rock with a wide range of clast sizes.

Moraines are mounded deposits consisting of glacial till carried in the glacial ice and rock fragments dislodged by mass wasting from the U-shaped valleywalls. The glacier acts like a conveyor belt, carryingand depositing sedimentat the end of and along the sides of theice flow. Because the ice in the glacier is always flowing downslope, all glaciers have moraines build up at their terminus, even those not advancing.

Moraines are classified by their locationwith respect to the glacier. A terminal moraine is a ridge oftill located at the end or terminusof the glacier. Recessional moraines are left as glaciersretreat andthere are pauses in the retreat. Lateral morainesaccumulate along the sidesof the glacierfrom material mass wasted from the valley walls. When two tributary glaciers merge, the two lateral moraines combine to form a medial morainerunning down the center of the combined glacier.Ground moraine is a veneer of tilllefton the land as the glacier melts.

Lateral moraines of Little Cottonwood and Bell canyons, Utah.	Medial moraines where tributary glaciers meet. At least seven tributary glaciers from upstream have joined to form the trunk glacier flowing on out of the upper left of the picture.

In addition to moraines, glaciersleave behind other depositional landforms. Silt, sand, and gravel produced by the intense grinding process are carried by streams of water and deposited in front of the glacier in an area called the outwash plain. Retreating glaciersmay leave behind large boulders that don’t match the local bedrock. These are called glacial erratics. When continental glaciers retreat, they can leave behind large blocks of ice within the till. These ice blocks melt and create a depression in the till called a kettle. If the depression later fillswith water, it is called a kettle lake.

If meltwater flowing over the ice surface descends into crevasses in the ice, it may find a channel and continue to flow in sinuous channels within or at the base of the glacier. Within or under continental glaciers, these streams carry sediments. When the ice recedes, theaccumulated sediment is deposited as a long sinuous ridge known as an esker.Meltwater descending down through the ice or over the margins of the ice may deposit mounds oftill in hills called kames.

Drumlinsare common in continental glacial areas of Germany, New York, and Wisconsin, where they typically are found in fields with great numbers. A drumlin is an elongated asymmetrical teardrop-shaped hill reflecting ice movement with its steepest side pointing upstream to the flow of ice and its streamlinedor low–angled sidepointing downstream in the direction of ice movement.

Glacial scientists debate the origins of drumlins.A leading idea is that drumlins are created from accumulated till being compressed and sculpted under a glacier that retreated then advanced again over its own ground moraine. Another idea is that meltwater catastrophically flooded under the glacierand carved the till into these streamlined mounds. Still another proposes that the weight of the overlyingicestatically deformed the underlying till[12, 13].

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14.4.3 Glacial Lakes

Glacial lakes are commonlyfound in alpineenvironments. A lake confined within a glacial cirque is called a tarn. A tarn forms when the depression in the cirque fills with precipitation after the ice is gone. Examples of tarns include Silver Lake near Brighton Ski resortin Big Cottonwood Canyon, Utah and Avalanche Lake in Glacier National Park, Montana.

When recessional morainescreate a series of isolatedbasins in a glaciated valley, the resulting chain of lakesis called paternoster lakes.

Lakes filled by glacial meltwater often looks milky due to finely ground material called rock flour suspended in the water.

Long, glacially carved depressions filled with water are known as finger lakes. Proglacial lakes form along the edges of all the largest continental ice sheets, such as Antarctica and Greenland. The crust is depressed isostatically by the overlying ice sheet and these basins fill with glacial meltwater. Many such lakes, some of them huge, existed at various times along the southern edge of the Laurentide Ice Sheet.Lake Agassiz, Manitoba, Canada, is a classic example of a proglacial lake.Lake Winnipeg serves as the remnant of a much larger proglacial lake.

Other proglacial lakes were formed when glaciers dammed riversand flooded the river valley. A classic example isLake Missoula, which formed when a lobe of the Laurentide ice sheet blocked the Clark Fork Riverabout 18,000 years ago. Over about 2000 years the ice dam holding back Lake Missoula failed several times. During each breach, the lake emptied across parts of eastern Washington, Oregon, and Idaho into the Columbia River Valley and eventually the Pacific Ocean. After each breach, the dam reformed and the lake refilled. Each breach produced a catastrophic flood over a few days. Scientists estimate that this cycle of ice dam, proglacial lake, and torrential massive flooding happened at least 25 times over a spanof 20 centuries. The rate of each outflow is believed to have equaled the combined dischargeof all of Earth’s current rivers combined.

The landscape produced by these massive floods is preserved in the Channeled Scablands of Idaho, Washington, and Oregon.

Pluvial lakes form in humid environments that experience low temperatures and high precipitation. During the last glaciation, most of the western United States’ climate wascoolerand more humid than today. Under these low-evaporation conditions, many large lakes, called pluvial lakes, formed in the basins of the Basin and Range Province.Two of the largestwere Lake Bonneville and Lake Lahontan. Lake Lahontan was in northwestern Nevada. The figure illustrates the tremendous size of Lake Bonneville, which occupied much of western Utah and into eastern Nevada.The lake level fluctuated greatly over the centuries leaving several pronounced old shorelines marked by wave-cut terraces. These old shorelines can be seen on mountain slopes throughout the western portion of Utah, including the Salt Lake Valley, indicating that the now heavily urbanized valley was once filled with hundreds of feet of water.Lake Bonneville‘s level peaked around 18,000 years agowhen a breach occurred at Red Rock Pass in Idaho and water spilled into the Snake River. The flooding rapidly lowered the lake level and scoured the Idaho landscape across the Pocatello Valley, the Snake River Plain, and Twin Falls. The floodwaters ultimately flowed into the Columbia River across part of the scablands areaat an incredible discharge rate of about 4,750 cu km/sec (1,140 cumi/sec). For comparison, this discharge rate would drain the volume of Lake Michigan completely dry within a few days.

The five Great Lakes in North America’s upper Midwestare proglacial lakes that originated during the last ice age. The lake basins were originally carved by theencroaching continental ice sheet. The basins were later exposed as the ice retreated about 14,000 years ago and filled by precipitation.

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14.5 Ice Age Glaciations

A glaciationor ice age) occurs when the Earth’s climate becomes cold enough that continentalice sheetsexpand, covering large areas of land. Four major, well–documented glaciations have occurred in Earth’s history: one during the Archean-early Proterozoic Eon, ~2.5 billion years ago; another in late Proterozoic Eon, ~700 million years ago; another in the Pennsylvanian, 323 to 300 million years ago,and most recently during thePliocene-Pleistoceneepochs starting 2.5million years ago (Chapter 8). Some scientists also recognize a minor glaciationaround 440 million years ago in Africa.

The best–studied glaciation is, of course, the most recent. This infographic illustrates the glacial and climate changes over the last 20,000 years, ending with those caused by human actions since the Industrial Revolution. The Pliocene-Pleistocene glaciation was a series of several glacial cycles, possibly 18in total. Antarctic ice–core records exhibit especially strong evidence for eight glacial advances occurring within the last 420,000 years [16]. The last of theseis known in popular media as “The Ice Age,”but geologists refer to it as the Last Glacial Maximum. The glacial advancereached its maximum between 26,500 and 19,000 years ago [10, 17].

14.5.1 Causes of Glaciations

Glaciations occur due to both long-term and short–term factors. In the geologic sense, long-term means a scale of tensto hundreds of millions of yearsand short-term means a scale of hundredsto thousands of years.

Long-term causes include plate tectonics breaking up the supercontinents (see Wilson Cycle, Chapter 2), moving land massesto high latitudes near the north or south poles, and changingocean circulation.For example, the closing of the Panama Strait and isolation of the Pacific and Atlantic oceans may have triggered a change in precipitation cycles, which combined with a cooling climateto help expand the ice sheets.

Short-term causes of glacial fluctuations are attributed to the cycles in the Earth’s rotational–axis and to variations in the earth’s orbit around the Sun which affectthe distance between Earth and the Sun. Called Milankovitch Cycles, these cycles affect the amount of incoming solar radiation, causingshort-term cycles of warming and cooling.

During the Cenozoic Era, carbon dioxide levels steadily decreased from a maximum in the Paleocene, causing the climate to graduallycool. By the Pliocene, ice sheets began to form. The effects of the Milankovitch Cycles created short-term cycles of warming and coolingwithin the larger glaciation event.

Milankovitch Cycles are three orbital changes named after the Serbian astronomer Milutan Milankovitch. The three orbital changes are called precession, obliquity, and eccentricity. Precession is the wobbling of Earth’s axiswith a period of about 21,000 years; obliquity is changes in the angle of Earth’s axiswith a period of about 41,000 years; and eccentricity is variations in the Earth’s orbit around the sun leading to changes in distance from the sun with a period of 93,000 years [19]. These orbital changes created a 41,000–year-long glacial–interglacial Milankovitch Cycle from 2.5 to 1.0 million years ago, followed by another longer cycle of about 100,000 years from 1.0 million years ago to today (see Milankovitch Cycles).

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Watch the video to see summaries of the ice ages, including their characteristics and causes.

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14.5.2 Sea-Level Change and Isostatic Rebound

When glaciers melt and retreat, two things happen: water runs off into the ocean causing sea levels to rise worldwide, and the land, released from its heavy covering of ice, rises due to isostatic rebound. Since the Last Glacial Maximum about 19,000 years ago, sea–level has risen about 125 m (400 ft). A global change in sea level is called eustatic sea-level change. During a warming trend, sea–levelrises due to more water being added to the ocean and also thermal expansionof sea water. About half of the Earth’s eustatic sea-level rise during the last century has been the result of glaciers melting and about half due to thermal expansion [21, 22].Thermal expansion describes how a solid, liquid, or gas expands in volume with an increase in temperature. This 30 second videodemonstrates thermal expansion with the classic brass ball and ring experiment.

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Relative sea-level change includes vertical movement of both eustatic sea-level and continents on tectonic plates. In other words, sea-level change is measured relative tolandelevation. For example, if the land rises a lot and sea-level rises only a little, then the relative sea-level would appear to drop.

Continents sitting on the lithosphere can move vertically upward as a result of two main processes, tectonic uplift and isostatic rebound. Tectonic uplift occurs when tectonic plates collide (see Chapter 2). Isostatic rebound describes the upward movement of lithospheric crust sitting on top of the asthenospheric layer below it. Continental crust bearing the weight of continental ice sinks into the asthenosphere displacing it. After the ice sheet melts away, the asthenosphere flows back in and continental crust floatsback upward. Erosion can also create isostatic reboundby removing large masses like mountainsand transporting the sediment away (think of the Mesozoic removal of the Alleghanian Mountains and the uplift of the Appalachian plateau; Chapter 8), albeit this process occurs moreslowly than relatively rapid glaciermelting.

The isostatic–reboundmap below shows rates of vertical crustal movement worldwide.The highest rebound rate is indicated by the blue-to-purple zones (top end of the scale). The orange-to-red zones (bottom end of the scale) surrounding the high-rebound zones indicate isostatic lowering as adjustments in displaced subcrustal material have taken place.

Most glacial isostatic rebound is occurring where continental ice sheets rapidly melted about 19,000 years ago, such as in Canada and Scandinavia. Its effects can be seen wherever Ice Age ice or water bodies are or were present on continental surfaces and in terraces on river floodplains that cross these areas. Isostatic rebound occurred in Utah when the water from Lake Bonneville drained away [23]. North America’s Great Lakes also exhibit emergent coastline features caused by isostatic rebound since the continental ice sheet retreated.

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Summary

Glaciers form when average annual snowfall exceeds melting and snow compresses into glacial ice. There are three types of glaciers, alpine or valley glaciers that occupy valleys, ice sheets that cover continental areas, and ice caps that cover smaller areas usually at higher elevations. As the ice accumulates, it begins to flow downslope and outward under its own weight. Glacial ice is divided into two zones, the upper rigid or brittle zone where the ice cracks into crevasses and the lower plastic zone where under the pressure of overlying ice, the ice bends and flows exhibiting ductile behavior. Rock material that falls onto the ice by mass wasting or is plucked and carried by the ice is called moraine and acts as grinding agents against the bedrock creating significant erosion.

Glaciers have a budget of income and expense. The zone of income for the glacier is called the Zone of Accumulation, where snow is converted into firn then ice by compression and recrystallization, and the zone of expense called the Zone of Ablation, where ice melts or sublimes away. The line separating these two zones latest in the year is the Equilibrium or Firn Line and can be seen on the glacier separating bare ice from snow covered ice. If the glacial budget is balanced, even though the ice continues to flow downslope, the end or terminus of the glacier remains in a stable position. If income is greater than expense, the position of the terminus moves downslope. If expense is greater than income, a circumstance now affecting glaciers and ice sheets worldwide due to global warming, the terminus recedes. If this situation continues, the glaciers will disappear. An average of cooler summers affects the stability or growth of glaciers more than higher snowfall. As the Greenland and Antarctic ice sheets flow seaward, the edges calve off forming icebergs.

Glaciers create two kinds of landforms, erosional and depositional. Alpine glaciers carve U-shaped valleys and moraine carried in the ice polishes and grooves or striates the bedrock. Other landscape features produced by erosion include horns, arête ridges, cirques, hanging valleys, cols, and truncated spurs. Cirques may contain eroded basins that are occupied by post glacial lakes called tarns. Depositional features result from deposits left by retreating ice called drift. These include till, and moraine deposits (terminal, recessional, lateral, medial, and ground), eskers, kames, kettles and kettle lakes, erratics, and drumlins. A series of recessional moraines in glaciated valleys may create basins that are later filled with water to become paternoster lakes. Glacial meltwater carries fine grained sediment onto the outwash plain. Lakes containing glacial meltwater are milky in color from suspended finely ground rock flour. Ice Age climate was more humid and precipitation that did not become glacier ice filled regional depressions to become pluvial lakes. Examples of pluvial lakes include Lake Missoula dammed behind an ice sheet lobe and Lake Bonneville in Utah whose shoreline remnants can be seen on mountainsides. Repeated breaching of the ice lobe allowed Lake Missoula to rapidly drain causing massive floods that scoured the Channeled Scablands of Idaho, Washington, and Oregon.

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References

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15 Global Climate Change

15 Global Climate Change

KEY CONCEPTS

At the end of this chapter, students should be able to:

Describe the role of greenhouse gases in climate change.

Describe the sources of greenhouse gases.

Explain Earth’s energy budget and global temperature changes.

Explain how positive and negative feedback mechanisms can influence climate.

Explain how we know about climates in the geologic past.

Accurately describe which aspects of the environment are changing due to anthropogenic climate change.

Describe the causes of recent climate change, particularly the role of humans in the overall climate balance

This chapter describes the Earth systems involved in climate change, the geologic evidence of past climate changes, and the human role in today’s climate change. In science, a system is a group of interacting objects and processes. Earth System Science is the study of these systems: geosphere—rocks; atmosphere—gasses; hydrosphere—water; cryosphere—ice; and biosphere—living things. Earth science studies these systems and how they interact and change in response to natural cycles and human-driven, or anthropogenic forces. Changes in one Earth system affect other systems.

It is critically important for us to be aware of the geologic context of climate change processes and how these Earth systems interact, first, for us to understand how and why human activities cause present-day climate change and, secondly, to distinguish between natural processes and human processes in the geologic past’s climate record.

A significant part of this chapter introduces and discusses various processes from these Earth systems, how they influence each other, and how they impact global climate. For example, Earth’s temperature and climate largely change based on atmospheric gas composition, ocean circulation, and the land-surface characteristics of rocks, glaciers, and plants.

Also necessary to understanding climate change is to distinguish between climate and weather. Weather is the short-term temperature and precipitation patterns that occur in days and weeks. Climate is the variable range of temperature and precipitation patterns averaged over the long-term for a particular region (see Chapter 13.1). Thus, a single cold winter does not mean that the entire globe is cooling—indeed, the United States’ cold winters of 2013 and 2014 occurred while the rest of the Earth was experiencing record warm-winter temperatures. To avoid these generalizations, many scientists use a 30-year average as a good baseline. Therefore, climate change refers to slow temperature and precipitation changes and trends over the long term for a particular area or the Earth as a whole.

15.1 Earth’s Temperature

Without an atmosphere, Earth would have huge temperature fluctuations between day and night, like the moon. Daytime temperatures would be hundreds of degrees Celsius above normal, and nighttime temperatures would be hundreds of degrees below normal. Because the Moon doesn’t have much of an atmosphere, its daytime temperatures are around 106 °C (224℉) and nighttime temperatures are around -183°C (-298℉). That is an astonishing 272°C (522°F) degree range between the Moon’s light side and dark side. This section describes how Earth’s atmosphere is involved in regulating the Earth’s temperature.

15.1.1 Composition of Atmosphere

The atmosphere’s composition is a key component in regulating the planet’s temperature. The atmosphere is 78 percent nitrogen (N2), 21 percent oxygen (O2), one percent argon (Ar), and less than one percent trace components, which are all other gases. Trace components include carbon dioxide (CO2), water vapor (H2O), neon, helium, and methane. Water vapor is highly variable, mostly based on region, and composes about one percent of the atmosphere. Trace component gasses include several important greenhouse gases, which are the gases responsible for warming and cooling the plant. On a geologic scale, volcanoes and the weathering process, which bury CO2 in sediments, are the atmosphere’s CO2 sources. Biological processes both add and subtract CO2 from the atmosphere.

Greenhouse gases trap heat in the atmosphere and warm the planet by absorbing some of the longer-wave outgoing infrared radiation that is emitted from Earth, thus keeping heat from being lost to space. More greenhouse gases in the atmosphere absorb more longwave heat and make the planet warmer. Greenhouse gasses have little effect on shorter-wave incoming solar radiation.

The most common greenhouse gases are water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Water vapor is the most abundant greenhouse gas, but its atmospheric abundance does not change much over time. Carbon dioxide is much less abundant than water vapor, but carbon dioxide is being added to the atmosphere by human activities such as burning fossil fuels, land-use changes, and deforestation. Further, natural processes such as volcanic eruptions add carbon dioxide, but at an insignificant rate compared to human-caused contributions.

There are two important reasons why carbon dioxide is the most important greenhouse gas. First, carbon dioxide stays in the atmosphere and does not go away for hundreds of years. Second, most of the additional carbon dioxide is “fossil” in origin, which means that it is released by burning fossil fuels. For example, coal and petroleum are fossil fuels. Coal and oil are made from long-dead plant material, which was originally created by photosynthesis millions of years ago and stored in the ground. Photosynthesis takes sunlight plus carbon dioxide and creates the substances of plants. This transformation occurs over millions of years as a slow process, accumulating fossil carbon in rocks and sediments. So, when we burn coal and oil, we instantaneously release the stored solar energy and fossil carbon dioxide that took millions of years to accumulate in the first place. The rate of release is critical to comprehend current climate change.

15.1.2 Carbon Cycle

Critical to understanding global climate change is to understand the carbon cycle and how Earth’s own carbon-balancing system is being rapidly thrown off balance by human-driven activities. Earth has two important carbon cycles: the biological and the geological. In the biological cycle, living organisms—mostly plants—consume carbon dioxide from the atmosphere to make their tissues and substances through photosynthesis. Then, after the organisms die, and when they decay over years or decades, that carbon is released back into the atmosphere. The following is the general equation for photosynthesis.

CO2 + H2O + sunlight → sugars + O2

In the geological carbon cycle, a portion of the biological-cycle carbon becomes part of the geological carbon cycle: plant materials into coal and petroleum, tiny fragments and molecules into organic-rich shale, and the carbonate bearing calcareous shells and other parts of marine organisms into limestone. Such materials become buried and become part of the slow geologic formation of coal and other sedimentary materials. This cycle actually involves most of Earth’s carbon and operates very slowly.

The following are geological carbon-cycle storage reservoirs:

Organic matter from plants is stored in peat, coal, and permafrost for thousands to millions of years.

Silicate–mineral weathering converts atmospheric carbon dioxide to dissolved bicarbonate, which is stored in the oceans for thousands to tens of thousands of years.

Marine organisms convert dissolved bicarbonate to forms of calcite, which is stored in carbonate rocks for tens to hundreds of millions of years.

Carbon compounds are directly stored in sediments for tens to hundreds of millions of years; some end up in petroleum deposits.

Carbon-bearing sediments are transferred by subduction to the mantle, where the carbon may be stored for tens of millions to billions of years.

Carbon dioxide from within the Earth is released back to the atmosphere during volcanic eruptions, where it is stored for years to decades.

During much of Earth’s history, the geological carbon cycle has been balanced by volcanos releasing carbon at approximately the same rate that carbon is stored by the other processes. Under these conditions, Earth’s climate has remained relatively stable. However, in Earth’s history, there have been times when that balance has been upset. This can happen during prolonged stretches of above-average volcanic activity. One example is the Siberian Traps eruption around 250 million years ago, which contributed to strong climate warming over a few million years.

A carbon imbalance is also associated with significant mountain-building events. For example, the Himalayan Range has been forming for about 40 million years, and over that time — and still today — the rate of weathering on Earth has been enhanced because those mountains are so huge and the range is so extensive that they present a greater surface area on which weathering takes place. The weathering of these rocks — most importantly the hydrolysis of feldspar — has resulted in consumption of atmospheric carbon dioxide and transfer of the carbon to the oceans and to ocean-floor carbonate-rich sediments. The steady drop in carbon dioxide levels over the past 40 million years, which contributed to the Pliocene-Pleistocene glaciations, is partly attributable to the formation of the Himalayan Range.

Another, nongeological form of carbon-cycle imbalance is happening today on a very rapid time scale. In just a few decades, humans have extracted volumes of fossil fuels, such as coal, oil, and gas, which were stored in rocks over the past several hundred million years, and converted these fuels to energy and carbon dioxide. By doing so, we are changing the climate faster than has ever happened in the past. Remember, carbon dioxide stays in the atmosphere and does not go away for hundreds of years. The more greenhouse gases in the atmosphere, the more heat is trapped and the warmer the planet becomes.

15.1.3 Greenhouse Effect

The greenhouse effect is the reason our global temperature is rising, but it’s important to understand what this effect is and how it occurs. The greenhouse effect occurs because greenhouse gases are present in the atmosphere. The greenhouse effect is named after a similar process that warms a greenhouse or a car on a hot summer day. Sunlight passes through the glass of the greenhouse or car, reaches the interior, and changes into heat. The heat radiates upward and gets trapped by the glass windows. The greenhouse effect for the Earth can be explained in three steps.

Step 1:Solar radiation from the sun is composed of mostly ultraviolet (UV), visible light, and infrared (IR) radiation. Components of solar radiation include parts with a shorter wavelength than visible light, like ultraviolet light, and parts of the spectrum with longer wavelengths, like IR and others. Some of the radiation gets absorbed, scattered, or reflected by the atmospheric gases but about half of the solar radiation eventually reaches the Earth’s surface.

Step 2:The visible, UV, and IR radiation, that reaches the surface converts to heat energy. Most students have experienced sunlight warming a surface such as pavement, a patio, or deck. When this occurs, the warmer surface then emits thermal radiation, which is a type of IR radiation. So, there is a conversion from visible, UV, and IR to just thermal IR. This thermal IR is what we experience as heat. If you have ever felt heat radiating from a fire or a hot stove top, then you have experienced thermal IR.

Step 3:Thermal IR radiates from the earth’s surface back into the atmosphere. But since it is thermal IR instead of UV, visible, or regular IR, this thermal IR gets trapped by greenhouse gases. In other words, the sun’s energy leaves the Earth at a different wavelength than it enters, so the sun’s energy is not absorbed in the lower atmosphere when energy is coming in, but rather when the energy is going out. The gases that are mostly responsible for this energy blocking on Earth include carbon dioxide, water vapor, methane, and nitrous oxide. More greenhouse gases in the atmosphere results in more thermal IR being trapped. Explore this external link to an interactive animation on the greenhouse effectfrom the National Academy of Sciences.

15.1.4 Earth’s Energy Budget

The solar radiation that reaches Earth is relatively uniform over time. Earth is warmed, and energy or heat radiates from the Earth’s surface and lower atmosphere back to space. This flow of incoming and outgoing energy is Earth’s energy budget. For Earth’s temperature to be stable over long stretches of time, incoming energy and outgoing energy have to be equal on average so that the energy budget at the top of the atmosphere balances. About 29 percent of the incoming solar energy arriving at the top of the atmosphere is reflected back to space by clouds, atmospheric particles, or reflective ground surfaces like sea ice and snow. About 23 percent of incoming solar energy is absorbed in the atmosphere by water vapor, dust, and ozone. The remaining 48 percent passes through the atmosphere and is absorbed at the surface. Thus, about 71 percent of the total incoming solar energy is absorbed by the Earth system.

When this energy reaches Earth, the atoms and molecules that makeup the atmosphere and surface absorb the energy, and Earth’s temperature increases. If this material only absorbed energy, then the temperature of the Earth would continue to increase and eventually overheat. For example, if you continuously run a faucet in a stopped-up sink, the water level rises and eventually overflows. However, temperature does not infinitely rise because the Earth is not just absorbing sunlight; it is also radiating thermal energy or heat back into the atmosphere. If the temperature of the Earth rises, the planet emits an increasing amount of heat to space, and this is the primary mechanism that prevents Earth from continually heating.

Some of the thermal infrared heat radiating from the surface is absorbed and trapped by greenhouse gasses in the atmosphere, which act like a giant canopy over Earth. The more greenhouse gases in the atmosphere, the more outgoing heat Earth retains, and the less thermal infrared heat dissipates to space.

Factors that can affect the Earth’s energy budget are not limited to greenhouse gases. Increasing solar energy can increase the energy Earth receives. However, these increases are very small over time. In addition, land and water will absorb more sunlight when there is less ice and snow to reflect the sunlight back to the atmosphere. For example, the ice covering the Arctic Sea reflects sunlight back to the atmosphere; this reflectivity is called albedo. Furthermore, aerosols (dust particles) produced from burning coal, diesel engines, and volcanic eruptions can reflect incoming solar radiation and actually cool the planet. While the effect of anthropogenic aerosols on the climate’s system is weak, the effect of human-produced greenhouse gases is not weak. Thus, the net effect of human activity is warming due to more anthropogenic greenhouse gases associated with fossil fuel combustion.

An effect that changes the planet can trigger feedback mechanisms that amplify or suppress the original effect. A positive feedback mechanism occurs when the output or effect of a process enhances the original stimulus or cause. Thus, it increases the ongoing effect. For example, the loss of sea ice at the North Pole makes that area less reflective, reducing albedo. This allows the surface air and ocean to absorb more energy in an area that was once covered by sea ice. Another example is melting permafrost. Permafrost is permanently frozen soil located in the high latitudes, mostly in the Northern Hemisphere. As the climate warms, more permafrost thaws, and the thick deposits of organic matter are exposed to oxygen and begin to decay. This oxidation process releases carbon dioxide and methane, which in turn causes more warming, which melts more permafrost, and so on and on.

A negative feedback mechanism occurs when the output or effect reduces the original stimulus or cause. For example, in the short term, more carbon dioxide (CO2) is expected to cause forest canopies to grow, which absorb more CO2. Another example for the long term is that increased carbon dioxide in the atmosphere will cause more carbonic acid and chemical weathering, which results in transporting dissolved bicarbonate and other ions to the oceans, which are then stored in sediment.

Global warming is evidence that Earth’s energy budget is not balanced. Positive effects on Earth’s temperature are now greater than negative effects.

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15.2 Evidence of Recent Climate Change

While climate has changed often in the past due to natural causes (see chapter 14.5.1 and chapter 15.3), the scientific consensus is that human activity is causing current very rapid climate change. While this seems like a new idea, it was suggested more than 75 years ago. This section describes the evidence of what most scientists agree is anthropogenic or human-caused climate change. For more information, watch this six-minute video on climate change by two professors at the North Carolina State University.

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15.2.1 Global Temperature Rise

The land-ocean temperature index, 1880 to present, compared to a base reference time of 1951-1980, shows ocean temperatures steadily rising. The solid black line is the global annual mean, and the solid red line is the five-year Lowess smoothing. The blue uncertainty bars (95 percent confidence limit) account only for incomplete spatial sampling.

Since 1880, Earth’s surface-temperature average has trended upward with most of that warming occurring since 1970 (see this NASA animation). Surface temperatures include both land and ocean because water absorbs much additional trapped heat. Changes in land-surface or ocean-surface temperatures compared to a reference period from 1951 to 1980, where the long-term average remained relatively constant, are called temperature anomalies. A temperature anomaly thus represents the difference between the measured temperature and the average value during the reference period. Climate scientists calculate long-term average temperatures over thirty years or more which identified the reference period from 1951 to 1980. Another common range is a century, for example, 1900-2000. Therefore, an anomaly of 1.25 ℃ (34.3°F) for 2015 means that the average temperature for 2015 was 1.25 ℃ (34.3°F) greater than the 1900-2000 average. In 1950, the temperature anomaly was -0.28 ℃ (31.5°F), so this is -0.28 ℃ (31.5°F) lower than the 1900-2000 average. These temperatures are annual average measured surface temperatures.

This video figure of temperature anomalies shows worldwide temperature changes since 1880. The more blue, the cooler; the more yellow and red, the warmer.

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In addition to average land-surface temperatures rising, the ocean has absorbed much heat. Because oceans cover about 70 percent of the Earth’s surface and have such a high specific heat value, they provide a large opportunity to absorb energy. The ocean has been absorbing about 80 to 90 percent of human activities’ additional heat. As a result, temperature in the ocean’s top 701.4 m (2,300 ft) has increased by -17.6°C (0.3℉) since 1969 (watch this 3 minute video by NASA JPL on the ocean’s heat capacity). The reason the ocean has warmed less than the atmosphere, while still taking on most of the heat, is due to water’s very high specific heat, which means that water can absorb a lot of heat energy with a small temperature increase. In contrast, the lower specific heat of the atmosphere means it has a higher temperature increase as it absorbs less heat energy.

Some scientists suggest that anthropogenic greenhouse gases do not cause global warming because between 1998 and 2013, Earth’s surface temperatures did not increase much, despite greenhouse gas concentrations continuing to increase. However, since the oceans are absorbing most of the heat, decade-scale circulation changes in the ocean, similar to La Niña, push warmer water deeper under the surface. Once the ocean’s absorption and circulation is accounted for, and this heat is added back into surface temperatures, then temperature increases become apparent, as shown in the figure. Also, the ocean’s heat storage is temporary, as reflected in the record-breaking warm years of 2014-2016. Indeed, with this temporary ocean-storage effect, the twenty-first century’s first 15 of its 16 years were the hottest in recorded history.

15.2.2 Carbon Dioxide

Anthropogenic greenhouse gases, mostly carbon dioxide (CO2), have increased since the industrial revolution when humans dramatically increased burning fossil fuels. These levels are unprecedented in the last 800,000-year Earth history as recorded in geologic sources such as ice cores. Carbon dioxide has increased by 40 percent since 1750, and the rate of increase has been the fastest during the last decade. For example, since 1750, 20409 tonnes (2040 gigatons) of CO2 have been added to the atmosphere; about 40 percent has remained in the atmosphere while the remaining 60 percent has been absorbed into the land by plants and soil or into the oceans. Indeed, during the lifetime of most young adults, the total atmospheric CO2 has increased by 50 ppm, or 15 percent.

Charles Keeling, an oceanographer with Scripps Institution of Oceanography in San Diego, California, was the first person to regularly measure atmospheric CO2. Using his methods, scientists at the Mauna Loa Observatory, Hawaii, have constantly measured atmospheric CO2 since 1957. NASA regularly publishes these measurements at https://scripps.ucsd.edu/programs/keelingcurve/. Go there now to see the very latest measurement. Keeling’s measured values have been posted in a curve of increasing values, called the Keeling Curve. This curve varies up and down in a regular annual cycle, from summer when the plants in the Northern Hemisphere are using CO2 to winter when the plants are dormant. But the curve shows a steady CO2 increase over the past several decades. This curve increases exponentially, not linearly, showing that the rate of CO2 increase is itself increasing!

The following Atmospheric CO2 video shows how atmospheric CO2 has varied recently and over the last 800,000 years, as determined by an increasing number of CO2 monitoring stations as shown on the insert map. It is also instructive to watch the video’s Keeling portion of how CO2 varies by latitude. This shows that most human CO2 sources are in the Northern Hemisphere where most of the land is and where most of the developed nations are.

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15.2.3 Melting Glaciers and Shrinking Sea Ice

Glaciers are large ice accumulations that exist year round on the land’s surface. In contrast, icebergs are masses of floating sea ice, although they may have had their origin in glaciers (see Chapter 14). Alpine glaciers, ice sheets, and sea ice are all melting. Explore melting glaciers at NASA’s interactive Global Ice Viewer). Satellites have recorded that Antarctica is melting at 1189 tonnes (118 gigatons) per year, and Greenland is melting at 2819 tonnes (281 gigatons) per year; 1 metric tonne is 1000 kilograms (1 gigaton is over 2 trillion pounds). Almost all major alpine glaciers are shrinking, deflating, and retreating. The ice-mass loss rate is unprecedented—never observed before—since the 1940’s when quality records for glaciers began.

Before anthropogenic warming, glacial activity was variable with some retreating and some advancing. Now, spring snow cover is decreasing, and sea ice is shrinking. Most sea ice is at the North Pole, which is only occupied by the Arctic Ocean and sea ice. The NOAA animation shows how perennial sea ice has declined from 1987 to 2015. The oldest ice is white, and the youngest, seasonal ice is dark blue. The amount of old ice has declined from 20 percent in 1985 to 3 percent in 2015.

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15.2.4 Rising Sea-Level

Sea level is rising 3.4 millimeters (0.13 inches) per year and has risen 0.19 meters (7.4 inches) from 1901 to 2010. This is thought largely to be from both glaciers melting and thermal expansion of sea water. Thermal expansion means that as objects such as solids, liquids, and gases heat up, they expand in volume.

Classic video demonstration (30 second) on thermal expansion with brass ball and ring (North Carolina School of Science and Mathematics).

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15.2.5 Ocean Acidification

Since 1750, about 40 percent of new anthropogenic carbon dioxide has remained in the atmosphere. The remaining 60 percent gets absorbed by the ocean and vegetation. The ocean has absorbed about 30 percent of that carbon dioxide. When carbon dioxide gets absorbed in the ocean, it creates carbonic acid. This makes the ocean more acidic, which then has an impact on marine organisms that secrete calcium carbonate shells. Recall that hydrochloric acid reacts by effervescing with limestone rock made of calcite, which is calcium carbonate. A more acidic ocean is associated with climate change and is linked to some sea snails (pteropods) and small protozoan zooplanktons’ (foraminifera) thinning carbonate shells and to ocean coral reefs’ declining growth rates. Small animals like protozoan zooplankton are an important component at the base of the marine ecosystem. Acidification combined with warmer temperature and lower oxygen levels is expected to have severe impacts on marine ecosystems and human-harvested fisheries, possibly affecting our ocean-derived food sources.

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15.2.6 Extreme Weather Events

Extreme weather events such as hurricanes, precipitation, and heatwaves are increasing and becoming more intense. Since the 1980’s, hurricanes, which are generated from warm ocean water, have increased in frequency, intensity, and duration and are likely connected to a warmer climate. Since 1910, average precipitation has increased by 10 percent in the contiguous United States, and much of this increase is associated with heavy precipitation events. However, the distribution is not even, and more precipitation is projected for the northern United States while less precipitation is projected for the already dry southwest. Also, heatwaves have increased, and rising temperatures are already affecting crop yields in northern latitudes. Increased heat allows for greater moisture capacity in the atmosphere, increasing the potential for more extreme events.

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15.3 Prehistoric Climate Change

Over Earth’s history, the climate has changed a lot. For example, during the Mesozoic Era, the Age of Dinosaurs, the climate was much warmer, and carbon dioxide was abundant in the atmosphere. However, throughout the Cenozoic Era, 65 million years ago to today, the climate has been gradually cooling. This section summarizes some of these major past climate changes.

15.3.1 Past Glaciations

Through geologic history, climate has changed slowly over millions of years. Before the most recent Pliocene-Quaternary glaciation, there were other major glaciations. The oldest, known as the Huronian, occurred toward the end of the Archean Eon-early Proterozoic Eon, about 2.5 billion years ago. The event of that time, the Great Oxygenation Event, was a major happening (see Chapter 8) most commonly associated with causing that glaciation. The increased oxygen is thought to have reacted with the potent greenhouse gas methane, causing cooling.

The end of the Proterozoic Eon, about 700 million years ago, had other glaciations. These ancient Precambrian glaciations are included in the Snowball Earth hypothesis. Widespread global rock sequences from these ancient times contain evidence that glaciers existed even in low-latitudes. Two examples are limestone rock—usually formed in tropical marine environments—and glacial deposits—usually formed in cold climates—have been found together from this time in many regions around the world. One example is in Utah. Evidence of continental glaciation is seen in interbedded limestone and glacial deposits (diamictites) on Antelope Island in the Great Salt Lake.

The controversial Snowball Earth hypothesis suggests that a runaway albedo effect—where ice and snow reflect solar radiation and increasingly spread from polar regions toward the equator—caused land and ocean surfaces to completely freeze and biological activity to collapse. Thinking is that because carbon dioxide could not enter the then-frozen ocean, the ice covering Earth could only melt when volcanoes emitted high enough carbon dioxide into the atmosphere to cause greenhouse heating. Some studies estimate that because of the frozen ocean surface, carbon dioxide 350 times higher than today’s concentration was required. Because biological activity did survive, the complete freezing and its extent in the snowball earth hypothesis are controversial. A competing hypothesis is the Slushball Earth hypothesis in which some regions of the equatorial ocean remained open. Differing scientific conclusions about the stability of Earth’s magnetic poles, impacts on ancient rock evidence from subsequent metamorphism, and alternate interpretations of existing evidence keep the idea of Snowball Earth controversial.

Glaciations also occurred in the PaleozoicEra, notably the Andean-Saharan glaciation in the late Ordovician, about 440–460 million years ago, which coincided with a major extinction event, and the Karoo Ice Age during the Pennsylvanian Period, 323 to 300 million years ago. This glaciation was one of the evidences cited by Wegener for his Continental Drift hypothesis as his proposed Pangea drifted into south polar latitudes. The Karoo glaciation was associated with an increase of oxygen and a subsequent drop in carbon dioxide, most likely produced by the evolution and rise of land plants.

During the Cenozoic Era—the last 65 million years, climate started out warm and gradually cooled to today. This warm time is called the Paleocene-Eocene Thermal Maximum, and Antarctica and Greenland were ice free during this time. Since the Eocene, tectonic events during the Cenozoic Era caused the planet to persistently and significantly cool. For example, the Indian Plate and Asian Plate collided, creating the Himalaya Mountains, which increased the rate of weathering and erosion of silicate minerals, especially feldspar. Increased weathering consumes carbon dioxide from the atmosphere, which reduces the greenhouse effect, resulting in long-term cooling.

About 40 million years ago, the narrow gap between the South American Plate and the Antarctica Plate widened, which opened the Drake Passage. This opening allowed the water around Antarctica—the Antarctic Circumpolar Current—to flow unrestrictedly west-to-east, which effectively isolated the southern ocean from the warmer waters of the Pacific, Atlantic, and Indian Oceans. The region cooled significantly, and by 35 million years ago, during the Oligocene Epoch, glaciers had started to form on Antarctica.

Around 15 million years ago, subduction-related volcanos between Central and South America created the Isthmus of Panama, which connected North and South America. This prevented water from flowing between the Pacific and Atlantic Oceans and reduced heat transfer from the tropics to the poles. This reduced heat transfer created a cooler Antarctica and larger Antarctic glaciers. As a result, the ice sheet expanded on land and water, increased Earth’s reflectivity and enhanced the albedo effect, which created a positive feedback loop: more reflective glacial ice, more cooling, more ice, more cooling, and so on.

By 5 million years ago, during the Pliocene Epoch, ice sheets had started to grow in North America and northern Europe. The most intense part of the current glaciation is the Pleistocene Epoch’s last 1 million years. The Pleistocene’s temperature varies significantly through a range of almost 10°C (18°F) on time scales of 40,000 to 100,000 years, and ice sheets expand and contract correspondingly. These variations are attributed to subtle changes in Earth’s orbital parameters, called Milankovitch cycles (see Chapter 14). Over the past million years, the glaciation cycles occurred approximately every 100,000 years, with many glacial advances occurring in the last 2 million years (Lisiecki and Raymo, 2005).

During an ice age, periods of warming climate are called interglacials; during interglacials, very brief periods of even warmer climate are called interstadials. These warming upticks are related to Earth’s climate variations, like Milankovitch cycles, which are changes to the Earth’s orbit that can fluctuate climate (see Chapter 14). In the last 500,000 years, there have been five or six interglacials, with the most recent belonging to our current time, the Holocene Epoch.

The two more recent climate swings, the Younger Dryas and the Holocene Climatic Optimum, demonstrate complex changes. These events are more recent, yet have conflicting information. The Younger Dryas’ cooling is widely recognized in the Northern Hemisphere, though the event’s timing, about 12,000 years ago, does not appear to be equal everywhere. Also, it is difficult to find in the Southern Hemisphere. The Holocene Climatic Optimum is a warming around 6,000 years ago; it was not universally warmer, nor as warm as current warming, and not warm at the same time everywhere.

15.3.2 Proxy Indicators of Past Climates

How do we know about past climates? Geologists use proxy indicators to understand past climate. A proxy indicator is a biological, chemical, or physical signature preserved in the rock, sediment, or ice record that acts like a fingerprint of something in the past. Thus, they are an indirect indicator of climate. An indirect indicator of ancient glaciations from the Proterozoic Eon and Paleozoic Era is the Mineral Fork Formation in Utah, which contains rock formations of glacial sediments such as diamictite (tillite). This dark rock has many fine-grained components plus some large out-sized clasts like a modern glacial till.

Deep-sea sediment is an indirect indicator of climate change during the Cenozoic Era, about the last 65 million years. Researchers from the Ocean Drilling Program, an international research collaboration, collect deep-sea sediment cores that record continuous sediment accumulation. The sediment provides detailed chemical records of stable carbon and oxygen isotopes obtained from deep-sea benthic foraminifera shells that accumulated on the ocean floor over millions of years. The oxygen isotopes are a proxy indicator of deep-sea temperatures and continental ice volume.

Sediment Cores – Stable Oxygen Isotopes

How do oxygen isotopes indicate past climate? The two main stable oxygen isotopes are 16O and 18O. They both occur in water (H2O) and in the calcium carbonate (CaCO3) shells of foraminifera as both of those substances’ oxygen component. The most abundant and lighter isotope is 16O. Since it is lighter, it evaporates more readily from the ocean’s surface as water vapor, which later turns to clouds and precipitation on the ocean and land. This evaporation is enhanced in warmer sea water and slightly increases the concentration of 18O in the surface seawater from which the plankton derives the carbonate for its shells. Thus the ratio of 16O and 18O in the fossilized shells in seafloor sediment is a proxy indicator of the temperature and evaporation of seawater.

Keep in mind, it is harder to evaporate the heavier water and easier to condense it. As evaporated water vapor drifts toward the poles and tiny droplets form clouds and precipitation, droplets of water with 18O tend to form more readily than droplets of the lighter form and precipitate out, leaving the drifting vapor depleted in 18O. During geologic times when the climate is cooler, more of this lighter precipitation that falls on land is locked in the form of glacial ice. Consider that the giant ice sheets were more than a mile thick and covered a large part of North America during the last ice age only 14,000 years ago. During glaciation, the glaciers thus effectively lock away more 16O, thus the ocean water and foraminifera shells become enriched in 18O. Therefore, the ratio of 18O to 16O (𝛿18O) in calcium carbonate shells of foraminifera is a proxy indicator of past climate. The sediment cores from the Ocean Drilling Program record a continuous accumulation of these fossils in the sediment and provide a record of glacials, interglacials and interstadials.

Sediment Cores – Boron-Isotopes and Acidity

Ocean acidity is affected by carbonic acid and is a proxy for past atmospheric CO2 concentrations. To estimate the ocean’s pH (acidity) over the past 60 million years, researchers collected deep-sea sediment cores and examined the ancient planktonic foraminifera shells’ boron-isotope ratios. Boron has two isotopes: 11B and 10B. In aqueous compounds of boron, the relative abundance of these two isotopes is sensitive to pH (acidity), hence CO2 concentrations. In the early Cenozoic, around 60 million years ago, CO2 concentrations were over 2,000 ppm, higher pH, and started falling around 55 to 40 million years ago, with noticeable drop in pH, indicated by boron isotope ratios. The drop was possibly due to reduced CO2 outgassing from ocean ridges, volcanoes and metamorphic belts, and increased carbon burial due to subduction and the Himalaya Mountains uplift. By the Miocene Epoch, about 24 million years ago, CO2 levels were below 500 ppm, and by 800,000 years ago, CO2 levels didn’t exceed 300 ppm.

Carbon Dioxide Concentrations in Ice Cores

For the recent Pleistocene Epoch’s climate, researchers get a more detailed and direct chemical record of the last 800,000 years by extracting and analyzing ice cores from the Antarctic and Greenland ice sheets. Snow accumulates on these ice sheets and creates yearly layers. Oxygen isotopes are collected from these annual layers, and the ratio of 18O to 16O (𝛿18O) is used to determine temperature as discussed above. In addition, the ice contains small bubbles of atmospheric gas as the snow turns to ice. Analysis of these bubbles reveals the composition of the atmosphere at these previous times.

Small pieces of this ice are crushed, and the ancient air is extracted into a mass spectrometer that can detect the ancient atmosphere’s chemistry. Carbon dioxide levels are recreated from these measurements. Over the last 800,000 years, the maximum carbon dioxide concentration during warm times was about 300 parts per million (ppm), and the minimum was about 170 ppm during cold stretches. Currently, the earth’s atmospheric carbon dioxide content is over 410 ppm.

Oceanic Microfossils

Microfossils, like foraminifera, diatoms, and radiolarians can be used as a proxy to interpret past climate record. Different species of microfossils are found in the sediment core’s different layers. Microfossil groups are called assemblages and their composition differs depending on the climatic conditions when they lived. One assemblage consists of species that lived in cooler ocean water, such as in glacial times, and at a different level in the same sediment core, another assemblage consists of species that lived in warmer waters.

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Tree Rings

Tree rings, which form every year as a tree grows, are another past climate indicator. Rings that are thicker indicate wetter years, and rings that are thinner and closer together indicate dryer years. Every year, a tree will grow one ring with a light section and a dark section. The rings vary in width. Since trees need much water to survive, narrower rings indicate colder and drier climates. Since some trees are several thousand years old, scientists can use their rings for regional paleoclimatic reconstructions, for example, to reconstruct past temperature, precipitation, vegetation, streamflow, sea-surface temperature, and other climate-dependent conditions. Paleoclimatic study means relating to a distinct past geologic climate. Also, dead trees, such as those found in Puebloan ruins, can be used to extend this proxy indicator by showing long-term droughts in the region and possibly explain why villages were abandoned.

Pollen

Pollen is also a proxy climate indicator. Flowering plants produce pollen grains. Pollen grains are distinctive when viewed under a microscope. Sometimes, pollen is preserved in lake sediments that accumulate in layers every year. Lake-sediment cores can reveal ancient pollen. Fossil-pollen assemblages are pollen groups from multiple species, such as spruce, pine, and oak. Through time, via the sediment cores and radiometric age-dating techniques, the pollen assemblages change, revealing the plants that lived in the area at the time. Thus, the pollen assemblages are a past climate indicator, since different plants will prefer different climates. For example, in the Pacific Northwest, east of the Cascades in a region close to grassland and forest borders, scientists tracked pollen over the last 125,000 years, covering the last two glaciations. As shown in the figure (Fig. 2 from reference Whitlock and Bartlein 1997), pollen assemblages with more pine tree pollen are found during glaciations and pollen assemblages with less pine tree pollen are found during interglacial times.

Other Proxy Indicators

Paleoclimatologists study many other phenomena to understand past climates, such as human historical accounts, human instrument records from the recent past, lake sediments, cave deposits, and corals.

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15.4 Anthropogenic Causes of Climate Change

As shown in the previous section, prehistoric climate changes occur slowly over many millions of years. The climate changes observed today are rapid and largely human caused. Evidence shows that climate is changing, but what is causing that change? Since the late 1800s, scientists have suspected that human-produced, i.e. anthropogenic changes in atmospheric greenhouse gases would likely cause climate change because changes in these gases have been the case every time in the geologic past. By the middle 1900s, scientists began conducting systematic measurements, which confirmed that human-produced carbon dioxide was accumulating in the atmosphere and other earth systems, such as forests and oceans. By the end of the 1900s and into the early 2000s, scientists solidified the Theory of Anthropogenic Climate Change when evidence from thousands of ground-based studies and continuous land and ocean satellite measurements mounted, revealing the expected temperature increase. The Theory of Anthropogenic Climate Change is that humans are causing most of the current climate changes by burning fossil fuels such as coal, oil, and natural gas. Theories evolve and transform as new data and new techniques become available, and they represent a particular field’s state of thinking. This section summarizes the scientific consensus of anthropogenic climate change.

15.4.1 Scientific Consensus

The overwhelming majority of climate studies indicate that human activity is causing rapid changes to the climate, which will cause severe environmental damage. There is strong scientific consensus on the issue. Studies published in peer-reviewed scientific journals show that 97 percent of climate scientists agree that climate warming is caused from human activities. There is no alternative explanation for the observed link between human-produced greenhouse gas emissions and changing modern climate. Most leading scientific organizations endorse this position, including the U.S. National Academy of Science, which was established in 1863 by an act of Congress under President Lincoln. Congress charged the National Academy of Science “with providing independent, objective advice to the nation on matters related to science and technology.” Therefore, the National Academy of Science is the leading authority when it comes to policy advice related to scientific issues.

One way we know that the increased greenhouse gas emissions are from human activities is with isotopic fingerprints. For example, fossil fuels, representing plants that lived millions of years ago, have a stable carbon-13 to carbon-12 (13C/12C) ratio that is different from today’s atmospheric stable-carbon ratio (radioactive 14C is unstable). Isotopic carbon signatures have been used to identify anthropogenic carbon in the atmosphere since the 1980s. Isotopic records from the Antarctic Ice Sheet show stable isotopic signatures from ~1000 AD to ~1800 AD and a steady isotopic signature gradually changing since 1800, followed by a more rapid change after 1950 as burning of fossil fuels dilutes the CO2 in the atmosphere. These changes show the atmosphere as having a carbon isotopic signature increasingly more similar to that of fossil fuels.

15.4.2 Anthropogenic Sources of Greenhouse Gases

Anthropogenic emissions of greenhouse gases have increased since pre-industrial times due to global economic growth and population growth. Atmospheric concentrations of the leading greenhouse gas, carbon dioxide, are at unprecedented levels that haven’t been observed in at least the last 800,000 years. Pre-industrial level of carbon dioxide was at about 278 parts per million (ppm). As of 2016, carbon dioxide was, for the first time, above 400 ppm for the entirety of the year. Measurements of atmospheric carbon at the Mauna Loa Carbon Dioxide Observatory show a continuous increase since 1957 when the observatory was established from 315 ppm to over 417 ppm in 2022. The daily reading today can be seen at Daily CO2. Based on the ice core record over the past 800,000 years, carbon dioxide ranged from about 185 ppm during ice ages to 300 ppm during warm times. View the data-accurate NOAA animation below of carbon dioxide trends over the last 800,000 years.

What is the source of these anthropogenic greenhouse gas emissions? Fossil fuel combustion and industrial processes contributed 78 percent of all emissions since 1970. The economic sectors responsible for most of this include electricity and heat production (25 percent); agriculture, forestry, and land use (24 percent); industry (21 percent); transportation, including automobiles (14 percent); other energy production (9.6 percent); and buildings (6.4 percent). More than half of greenhouse gas emissions have occurred in the last 40 years, and 40 percent of these emissions have stayed in the atmosphere. Unfortunately, despite scientific consensus, efforts to mitigate climate change require political action. Despite growing climate change concern, mitigation efforts, legislation, and international agreements have reduced emissions in some places, yet the less developed world’s continual economic growth has increased global greenhouse gas emissions. In fact, the years 2000 to 2010 saw the largest increases since 1970.

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Summary

Included in Earth Science is the study of the system of processes that affect surface environments and atmosphere of the Earth. Recent changes in atmospheric temperature and climate over intervals of decades have been observed. For Earth’s climate to be stable, incoming radiation from the sun and outgoing radiation from the sun-warmed Earth must be in balance. Gases in the atmosphere called greenhouse gases absorb the infrared thermal radiation from the Earth’s surface, trapping that heat and warming the atmosphere, a process called the Greenhouse Effect. Thus the energy budget is not now in balance and the Earth is warming. Human activity produces many greenhouse gases that have accelerated climate change. CO2 from fossil fuel burning is one of the major ones. While atmospheric composition is mostly nitrogen and oxygen, trace components including the greenhouse gases (CO2 and methane are the major ones and there are others) have the greatest effect on global warming.

A number of Positive Feedback Mechanisms, processes whose results reinforce the original process, take place in the Earth system. An example of a PFM of great concern is permafrost melting which causes decay of melting organic material that produces CO2 and methane (both powerful greenhouse gases) that warm the atmosphere and promote more permafrost melting. Two carbon cycles affect Earth’s atmospheric CO2 composition, the biologic carbon cycle and the geologic carbon cycle. In the biologic cycle, organisms (mostly plants and also animals that eat them) remove CO2 from the atmosphere for energy and to build their body tissues and return it to the atmosphere when they die and decay. The biologic cycle is a rapid cycle. In the geologic cycle, some organic matter is preserved in the form of petroleum and coal while more is dissolved in seawater and captured in carbonate sediments, some of which is subducted into the mantle and returned by volcanic activity. The geologic carbon cycle is slow over geologic time.

Measurements of increasing atmospheric temperature have been made since the nineteenth century but the upward temperature trend itself increased in the mid twentieth century showing the current trend is exponential. Because of the high specific heat of water, the oceans have absorbed most of the added heat. That this is temporary storage is revealed by the record-breaking warm years of the recent decade and the increase in intense storms and hurricanes. In 1957 the Mauna Loa CO2 Observatory was established in Hawaii providing constant measurements of atmospheric CO2 since 1958. The initial value was 315 ppm. The Keeling curve, named for the observatory founder, shows that value has steadily increased, exponentially, to over 417 ppm now. Compared to proxy data from atmospheric gases trapped in ice cores that show a maximum value for CO2 of about 300 ppm over the last 800,000 years, the Keeling increase of over 100 ppm in 50 years is dramatic evidence of human caused CO2 increase and climate change! As Earth’s temperature rises, glaciers and ice sheets are shrinking resulting in sea level rise. Atmospheric CO2 is also absorbed in sea water producing increased concentrations of carbonic acid which is raising the pH of the oceans making it harder for marine life to extract carbonate for their skeletal materials.

Earth’s climate has changed over geologic time with periods of major glaciations. There was a high temperature period in the Mesozoic shown by fossils in high latitudes and the Western Interior Seaway covering what is now the Midwest. However, climate has been cooling during the Cenozoic culminating in the Ice Age. Since the Ice Age, several proxy indicators of ancient climate show that the rate and amount of current climate change is unique in geologic history and can only be attributed to human activity. Those who ignore the consequences of increasing global warming for our planet’s future do so at the peril of our posterity!

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References

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16 Energy and Mineral Resources

16 Energy and Mineral Resources

KEY CONCEPTS

Describe how a renewable resource is different from a nonrenewable resource.

Compare the pros and cons of extracting and using fossil fuels and conventional and unconventional petroleum sources.

Describe how metallic minerals are formed and extracted.

Understand how society uses nonmetallic mineral resources.

This text has previously discussed geology’s pioneers, such as scientists James Hutton and Charles Lyell, but the first real “geologists” were the hominids who picked up stones and began the stone age. Maybe stones were first used as curiosity pieces, maybe as weapons, but ultimately, they were used as tools. This was the Paleolithic Period, the beginning of geologic study, and it dates back 2.6 million years to east Africa.

In modern times, geologic knowledge is important for locating economically valuable materials for society’s use. In fact, all things we use come from only three sources: they are farmed, hunted or fished, or mined. At the turn of the twentieth century, speculation was rampant that food supplies would not keep pace with world demand, suggesting the need to develop artificial fertilizers. Sources of fertilizer ingredients are: nitrogen is processed from the atmosphere, using the Haber process for the manufacture of ammonia from atmospheric nitrogen and hydrogen; potassium comes from the hydrosphere, such as lakes or ocean evaporation; and phosphorus is mined from the lithosphere, such as minerals like apatite from phosphorite rock, which is found in Florida, North Carolina, Idaho, Utah, and around the world. Thus, without mining and processing of natural materials, modern civilization would not exist. Indeed, geologists are essential in this process.

16.1 Mining

Mining is defined as extracting valuable materials from the Earth for society’s use. Usually, these include solid materials such as gold, iron, coal, diamond, sand, and gravel, but materials can also include fluid resources such as oil and natural gas. Modern mining has a long relationship with modern society. The oldest mine dates back 40,000 years to the Lion Cavern in Swaziland where there is evidence of concentrated digging into the Earth for hematite, an important iron ore used as red dye. Resources extracted by mining are generally considered to be nonrenewable.

16.1.1. Renewable vs. nonrenewable resources

Resources generally come in two major categories: renewable and nonrenewable. Renewable resources can be reused over and over or their availability replicated over a short human life span; nonrenewable resources cannot.

Renewable resources are materials present in our environment that can be exploited and replenished. Some common renewable energy sources are linked with green energy sources because they are associated with relatively small or easily remediated environmental impact. For example, solar energy comes from fusion within the Sun, which radiates electromagnetic energy. This energy reaches the Earth constantly and consistently and should continue to do so for about five billion more years. Wind energy, also related to solar energy, is maybe the oldest renewable energy and is used to sail ships and power windmills. Both solar and wind-generated energy are variable on Earth’s surface. These limitations are offset because we can use energy storing devices, such as batteries or electricity exchanges between producing sites. The Earth’s heat, known as geothermal energy, can be viable anywhere that geologists drill deeply enough. In practice, geothermal energy is more useful where heat flow is great, such as volcanic zones or regions with a thinner crust. Hydroelectric dams provide energy by allowing water to fall through the dam under gravity, which activates turbines that produce the energy. Ocean tides are also a reliable energy source. All of these renewable resources provide energy that powers society. Other renewable resources are plant and animal matter, which are used for food, clothing, and other necessities, but are being researched as possible energy sources.

Nonrenewable resources cannot be replenished at a sustainable rate. They are finite within human time frames. Many nonrenewable resources come from planetary, tectonic, or long-term biologic processes and include materials such as gold, lead, copper, diamonds, marble, sand, natural gas, oil, and coal. Most nonrenewable resources include specific concentrated elements listed on the periodic table; some are compounds of those elements. For example, if society needs iron (Fe) sources, then an exploration geologist will search for iron-rich deposits that can be economically extracted. Nonrenewable resources may be abandoned when other materials become cheaper or serve a better purpose. For example, coal is abundantly available in England and other nations, but because oil and natural gas are available at a lower cost and lower environmental impact, coal use has decreased. Economic competition among nonrenewable resources is shifting use away from coal in many developed countries.

16.1.2. Ore

Earth’s materials include the periodic table elements. However, it is rare that these elements are concentrated to the point where it is profitable to extract and process the material into usable products. Any place where a valuable material is concentrated is a geologic and geochemical anomaly. A body of material from which one or more valuable substances can be mined at a profit, is called an ore deposit. Typically, the term ore is used for only metal-bearing minerals, but it can be applied to valuable nonrenewable resource concentrations such as fossil fuels, building stones, and other nonmetal deposits, even groundwater. If a metal-bearing resource is not profitable to mine, it is referred to as a mineral deposit. The term natural resource is more common than the term ore for non-metal-bearing materials.

It is implicit that the technology to mine is available, economic conditions are suitable, and political, social and environmental considerations are satisfied in order to classify a natural resource deposit as ore. Depending on the substance, it can be concentrated in a narrow vein or distributed over a large area as a low-concentration ore. Some materials are mined directly from bodies of water (e.g. sylvite for potassium; water through desalination) and the atmosphere (e.g. nitrogen for fertilizers). These differences lead to various methods of mining, and differences in terminology depending on the certainty. Ore mineral resourceis used for an indication of ore that is potentially extractable, and the term ore mineral reserveis used for a well defined (proven), profitable amount of extractable ore.

16.1.3. Mining Techniques

The mining style is determined by technology, social license, and economics. It is in the best interest of the company extracting the resources to do so in a cost-effective way. Fluid resources, such as oil and gas, are extracted by drilling wells and pumping. Over the years, drilling has evolved into a complex discipline in which directional drilling can produce multiple bifurcations and curves originating from a single drill collar at the surface. Using geophysical tools like seismic imaging, geologists can pinpoint resources and extract efficiently.

Solid resources are extracted by two principal methods of which there are many variants. Surface mining is used to remove material from the outermost part of the Earth. Open pit mining is used to target shallow, broadly disseminated resources.

Open pit mining requires careful study of the ore body through surface mapping and drilling exploratory cores. The pit is progressively deepened through additional mining cuts to extract the ore. Typically, the pit’s walls are as steep as can be safely managed. Once the pit is deepened, widening the top is very expensive. A steep wall is thus an engineering balance between efficient and profitable mining (from the company’s point of view) and mass wasting (angle of repose from a safety p0int of view) so that there is less waste to remove. The waste is called non-valuable rock or overburden and moving it is costly. Occasionally, landslides do occur, such as the very large landslide in the Kennecott Bingham Canyon mine, Utah, in 2013. These events are costly and dangerous. The job of engineering geologists is to carefully monitor the mine; when company management heeds their warnings, there is ample time and action to avoid or prepare for any slide.

Strip mining and mountaintop mining are surface mining techniques that are used to mine resources that cover large areas, especially layered resources, such as coal. In this method, an entire mountaintop or rock layer is removed to access the ore below. Surface mining’s environmental impacts are usually much greater due to the large surface footprint that’s disturbed.

Underground mining is a method often used to mine higher-grade, more localized, or very concentrated resources. For one example, geologists mine some underground ore minerals by introducing chemical agents, which dissolve the target mineral. Then, they bring the solution to the surface where precipitation extracts the material. But more often, a mining shaft tunnel or a large network of these shafts and tunnels is dug to access the material. The decision to mine underground or from Earth’s surface is dictated by the ore deposit’s concentration, depth, geometry, land-use policies, economics, surrounding rock strength, and physical access to the ore. For example, to use surface mining techniques for deeper deposits might require removing too much material, or the necessary method may be too dangerous or impractical, or removing the entire overburden may be too expensive, or the mining footprint would be too large. These factors may prevent geologists from surface mining materials and cause a project to be mined underground. The mining method and its feasibility depends on the commodity’s price and the cost of the technology needed to remove it and deliver it to market. Thus, mines and the towns that support them come and go as the commodity price varies. And, conversely, technological advances and market demands may reopen mines and revive ghost towns.

16.1.4. Concentrating and Refining

All ore minerals occur mixed with less desirable components called gangue. The process of physically separating gangue minerals from ore bearing minerals is called concentrating. Separating a desired element from a host mineral by chemical means, including heating, is called smelting. Finally, taking a metal such as copper and removing other trace metals such as gold or silver is done through the refining process. Typically, refining is done one of three ways: 1. Materials can either be mechanically separated and processed based on the ore mineral’s unique physical properties, such as recovering placer gold based on its high density. 2. Materials can be heated to chemically separate desired components, such as refining crude oil into gasoline. 3. Materials can be smelted, in which controlled chemical reactions unbind metals from the minerals they are contained in, such as when copper is taken out of chalcopyrite (CuFeS2). Mining, concentrating, smelting, and refining processes require enormous energy. Continual advances in metallurgy- and mining-practice strive to develop ever more energy efficient and environmentally benign processes and practices.

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16.2. Fossil Fuels

Fossils fuels are extractable sources of stored energy that were created by ancient ecosystems. The natural resources that typically fall under this category are coal, oil, petroleum, and natural gas. These resources were originally formed via photosynthesis by living organisms such as plants, phytoplankton, algae, and cyanobacteria. This energy is actually fossil solar energy, since the sun’s ancient energy was converted by ancient organisms into tissues that preserved the chemical energy within the fossil fuel. Of course, as the energy is used, just like photosynthetic respiration that occurs today, carbon enters the atmosphere as CO2, causing climate consequences (see Chapter 15). Today humanity uses fossil fuels for most of the world’s energy.

Converting solar energy by living organisms into hydrocarbon fossil fuels is a complex process. As organisms die, they decompose slowly, usually due to being buried rapidly, and the chemical energy stored within the organisms’ tissues is buried within surrounding geologic materials. All fossil fuels contain carbon that was produced in an ancient environment. In environments rich with organic matter such as swamps, coral reefs, and planktonic blooms, there is a higher potential for fossil fuels to accumulate. Indeed, there is some evidence that over geologic time, organic hydrocarbon fossil fuel material was highly produced globally. Lack of oxygen and moderate temperatures in the environment seem to help preserve these organic substances. Also, the heat and pressure applied to organic material after it is buried contribute to transforming it into higher quality materials, such as brown coal to anthracite and oil to gas. Heat and pressure can also cause mobile materials to migrate to conditions suitable for extraction.

16.2.1. Fossil Fuels

OIL AND GAS

Petroleum is principally derived from organic-rich shallow marine sedimentary deposits where the remains of micro-organisms like plankton accumulated in fine grained sediments. Petroleum’s liquid component is called oil, and its gas component is called natural gas, which is mostly made up of methane (CH4). As rocks such as shale, mudstone, or limestone lithify, increasing pressure and temperature cause the oil and gas to be squeezed out and migrate from the source rock to a different rock unit higher in the rock column. Similar to the discussion of good aquifers in Chapter 11, if that rock is a sandstone, limestone, or other porous and permeable rock, and involved in a suitable stratigraphic or structural trapping process, then that rock can act as an oil and gas reservoir.

A trap is a combination of a subsurface geologic structure, a porous and permeable rock, and an impervious layer that helps block oil and gas from moving further, which concentrates it for humans to extract later. A trap develops due to many different geologic situations. Examples include an anticline or domal structure, an impermeable salt dome, or a fault bounded stratigraphic block, which is porous rock next to nonporous rock. The different traps have one thing in common: they pool fluid fossil fuels into a configuration in which extracting it is more likely to be profitable. Oil or gas in strata outside of a trap renders it less viable to extract.

Sequence stratigraphy is a branch of geology that studies sedimentary facies both horizontally and vertically and is devoted to understanding how sea level changes create organic-rich shallow marine muds, carbonates, and sands in areas that are close to each other. For example, shoreline environments may have beaches, lagoons, reefs, nearshore and offshore deposits, all next to each other. Beach sand, lagoonal and nearshore muds, and coral reef layers accumulate into sediments that include sandstones—good reservoir rocks— next to mudstones, next to limestones, both of which are potential source rocks. As sea level either rises or falls, the shoreline’s location changes, and the sand, mud, and reef locations shift with it (see the figure). This places oil and gas producing rocks, such as mudstones and limestones next to oil and gas reservoirs, such as sandstones and some limestones. Understanding how the lithology and the facies/stratigraphic relationships interplay is very important in finding new petroleum resources. Using sequence stratigraphy as a model allows geologists to predict favorable locations of the source rock and reservoir.

Tar Sands

Conventional oil and gas, which is pumped from a reservoir, is not the only way to obtain hydrocarbons. There are a few fuel sources known as unconventional petroleum sources. However, they are becoming more important as conventional sources become scarce. Tar sands, or oil sands, are sandstones that contain petroleum products that are highly viscous, like tar, and thus cannot be drilled and pumped out of the ground readily like conventional oil. This unconventional fossil fuel is bitumen, which can be pumped as a fluid only at very low recovery rates and only when heated or mixed with solvents. So, using steam and solvent injections or directly mining tar sands to process later are ways to extract the tar from the sands. Alberta, Canada is known to have the largest tar sand reserves in the world. Note: as with ores, an energy resource becomes uneconomic if the total extraction and processing costs exceed the extracted material’s sales revenue. Environmental costs may also contribute to a resource becoming uneconomic.

Oil Shale

Oil shale, or tight oil, is a fine-grained sedimentary rock that has significant petroleum or natural gas quantities locked tightly in the sediment. Shale has high porosity but very low permeability and is a common fossil fuel source rock. To extract the oil directly from the shale, the material has to be mined and heated, which, like with tar sands, is expensive and typically has a negative environmental impact.

Fracking

Another process used to extract the oil and gas from shale and other unconventional tight resources is called hydraulic fracturing, better known as fracking. In this method, high-pressure water, sand grains, and added chemicals are injected and pumped underground. Under high pressure, this creates and holds open fractures in the rocks, which help release the hard-to-access mostly natural gas fluids. Fracking is more useful in tighter sediments, especially shale, which has a high porosity to store the hydrocarbons but low permeability to allow transmission of the hydrocarbons. Fracking has become controversial because its methods contaminate groundwater and induce seismic activity. This has created much controversy between public concerns, political concerns, and energy value.

16.2.2. Coal

Coal comes from fossilized swamps, though some older coal deposits that predate terrestrial plants are presumed to come from algal buildups. Coal is chiefly carbon, hydrogen, nitrogen, sulfur, and oxygen, with minor amounts of other elements. As plant material is incorporated into sediments, heat and pressure cause several changes that concentrate the fixed carbon, which is the coal’s combustible portion. So, the more heat and pressure that coal undergoes, the greater is its carbon concentration and fuel value and the more desirable is the coal.

This is the general sequence of a swamp progressing through the various stages of coal formation and becoming more concentrated in carbon: Swamp => Peat => Lignite => Sub-bituminous => Bituminous => Anthracite => Graphite. As swamp materials collect on the swamp floor and are buried under accumulating materials, they first turn to peat.

Peat itself is an economic fuel in some locations like the British Isles and Scandinavia. As lithification occurs, peat turns to lignite. With increasing heat and pressure, lignite turns to sub-bituminous coal, bituminous coal, and then, in a process like metamorphism, anthracite. Anthracite is the highest metamorphic grade and most desirable coal since it provides the highest energy output. With even more heat and pressure driving out all the volatiles and leaving pure carbon, anthracite can become graphite.

Humans have used coal for at least 6,000 years, mainly as a fuel source. Coal resources in Wales are often cited as a primary reason for Britain’s rise, and later, for the United States’ rise during the Industrial Revolution. According to the US Energy Information Administration, US coal production has decreased due to competing energy sources’ cheaper prices and due to society recognizing its negative environmental impacts, including increased very fine-grained particulate matter as an air pollutant, greenhouse gases, acid rain, and heavy metal pollution. Seen from this perspective, the coal industry as a source of fossil energy is unlikely to revive.

As the world transitions away from fossil fuels including coal, and manufacturing seeks strong, flexible, and lighter materials than steel including carbon fiber for many applications, current research is exploring coal as a source of this carbon.

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16.3 Mineral Resources

Mineral resources, while principally nonrenewable, are generally placed in two main categories: metallic, which contain metals, and nonmetallic, which contain other useful materials. Most mining has been traditionally focused on extracting metallic minerals. Human society has advanced significantly because we’ve developed the knowledge and technologies to yield metal from the Earth. This knowledge has allowed humans to build the machines, buildings, and monetary systems that dominate our world today. Locating and recovering these metals has been a key facet of geologic study since its inception. Every element across the periodic table has specific applications in human civilization. Metallic mineral mining is the source of many of these elements.

16.3.1. Types of Metallic Mineral Deposits

The various ways in which minerals and their associated elements concentrate to form ore deposits are too complex and numerous to fully review in this text. However, entire careers are built around them. In the following section, we describe some of the more common deposit types along with their associated elemental concentrations and world class occurrences.

Magmatic Processes

When a magmatic body crystallizes and differentiates (see Chapter 4), it can cause certain minerals and elements to concentrate. Layered intrusions, typically ultramafic to mafic, can host deposits that contain copper, nickel, platinum, palladium, rhodium, and chromium. The Stillwater Complex in Montana is an example of economic quantities of layered mafic intrusion. Associated deposit types can contain chromium or titanium-vanadium. The largest magmatic deposits in the world are the chromite deposits in the Bushveld Igneous Complex in South Africa. These rocks have an areal extent larger than the state of Utah. The chromite occurs in layers, which resemble sedimentary layers, except these layers occur within a crystallizing magma chamber.

Water and other volatiles that are not incorporated into mineral crystals when a magma crystallizes can become concentrated around the crystallizing magma’s margins. Ions in these hot fluids are very mobile and can form exceptionally large crystals. Once crystallized, these large crystal masses are then called pegmatites. They form from magma fluids that are expelled from the solidifying magma when nearly the entire magma body has crystallized. In addition to minerals that are predominant in the main igneous mass, such as quartz, feldspar, and mica, pegmatite bodies may also contain very large crystals of unusual minerals that contain rare elements like beryllium, lithium, tantalum, niobium, and tin, as well as native elements like gold. Such pegmatites are ores of these metals.

An unusual magmatic process is a kimberlite pipe, which is a volcanic conduit that transports ultramafic magma from within the mantle to the surface. Diamonds, which are formed at great temperatures and pressures of depth, are transported by a Kimberlite pipe to locations where they can be mined. The process that created these kimberlite ultramafic rocks is no longer common on Earth. Most known deposits are from the Archean Eon.

Hydrothermal Processes

Fluids rising from crystallizing magmatic bodies or that are heated by the geothermal gradient cause many geochemical reactions that form various mineral deposits. The most active hydrothermal process today produces volcanogenic massive sulfide (VMS) deposits, which form from black smoker hydrothermal chimney activity near mid-ocean ridges all over the world. They commonly contain copper, zinc, lead, gold, and silver when found at the surface. Evidence from around 7000 BC in a period known as the Chalcolithic shows copper was among the earliest metals smelted by humans as means of obtaining higher temperatures were developed. The largest of these VMS deposits occur in Precambrian period rocks. The Jerome deposit in central Arizona is a good example.

Another deposit type that draws on magma-heated water is a porphyry deposit. This is not to be confused with the porphyritic igneous texture, although the name is derived from the porphyritic texture that is nearly always present in the igneous rocks associated with a porphyry deposit. Several types of porphyry deposits exist, such as porphyry copper, porphyry molybdenum, and porphyry tin. These deposits contain low-grade disseminated ore minerals closely associated with intermediate and felsic intrusive rocks that are present over a very large area. Porphyry deposits are typically the largest mines on Earth. One of the largest, richest, and possibly best studied mine in the world is Utah’s Kennecott Bingham Canyon Mine. It’s an open pit mine, which, for over 100 years, has produced several elements including copper, gold, molybdenum, and silver. Underground carbonate replacement deposits produce lead, zinc, gold, silver, and copper. In the mine’s past, the open pit predominately produced copper and gold from chalcopyrite and bornite. Gold only occurs in minor quantities in the copper-bearing minerals, but because the Kennecott Bingham Canyon Mine produces on such a large scale, it is one of the largest gold mines in the US. In the future, this mine may produce more copper and molybdenum (molybdenite) from deeper underground mines.

Most porphyry copper deposits owe their high metal content, and hence, their economic value to weathering processes called supergene enrichment which occurs when the deposit is uplifted, eroded, and exposed to oxidation. This process occurred millions of years after the initial igneous intrusion and hydrothermal expulsion ends. When the deposit’s upper pyrite-rich portion is exposed to rain, the pyrite in the oxidizing zone creates an extremely acid condition that dissolves copper out of copper minerals, such as chalcopyrite, and converts the chalcopyrite to iron oxides, such as hematite or goethite. The copper minerals are carried downward in water until they arrive at the groundwater table and an environment where the primary copper minerals are converted into secondary higher-copper content minerals. Chalcopyrite (35% Cu) is converted to bornite (63% Cu), and ultimately, chalcocite (80% Cu). Without this enriched zone, which is two to five times higher in copper content than the main deposit, most porphyry copper deposits would not be economic to mine.

If limestone or other calcareous sedimentary rocks are near the magmatic body, then another type of ore deposit called a skarn deposit forms. These metamorphic rocks form as magma-derived, highly saline metalliferous fluids react with carbonate rocks to create calcium-magnesium-silicate minerals like pyroxene, amphibole, and garnet, as well as high-grade iron, copper, zinc minerals, and gold. Intrusions that are genetically related to the intrusion that made the Kennecott Bingham Canyon deposit have also produced copper-gold skarns, which were mined by the early European settlers in Utah. When iron and/or sulfide deposits undergo metamorphism, the grain size commonly increases, which makes separating the gangue from the desired sulfide or oxide minerals much easier.

Sediment-hosted disseminated gold deposits consist of low concentrations of microscopic gold as inclusions and disseminated atoms in pyrite crystals. These are formed via low-grade hydrothermal reactions, generally in the realm of diagenesis, that occur in certain rock types, namely muddy carbonates and limey mudstones. This hydrothermal alteration is generally far removed from a magma source, but can be found in rocks situated with a high geothermal gradient. The Mercur deposit in Utah’s Oquirrh Mountains was this type’s earliest locally mined deposit. There, almost a million ounces of gold was recovered between 1890 and 1917. In the 1960s, a metallurgical process using cyanide was developed for these low-grade ore types. These deposits are also called Carlin-type deposits because the disseminated deposit near Carlin, Nevada, is where the new technology was first applied and where the first definitive scientific studies were conducted. Gold was introduced into these deposits by hydrothermal fluids that reacted with silty calcareous rocks, removing carbonate, creating additional permeability, and adding silica and gold-bearing pyrite in the pore space between grains. The Betze-Post mine and the Gold Quarry mine on the Carlin Trend are two of the largest disseminated gold deposits in Nevada. Similar deposits, but not as large, have been found in China, Iran, and Macedonia.

Non-magmatic Geochemical Processes

Geochemical processes that occur at or near the surface without magma’s aid also concentrate metals, but to a lesser degree than hydrothermal processes. One of the main reactions is redox, short for reduction/oxidation chemistry, which has to do with the amount of available oxygen in a system. Places where oxygen is plentiful, as in the atmosphere today, are considered oxidizing environments, while oxygen-poor places are considered reducing environments. Uranium deposits are an example of where redox concentrated the metal. Uranium is soluble in oxidizing groundwater environments and precipitates as uraninite when encountering reducing conditions. Many of the deposits across the Colorado Plateau, such as in Moab, Utah, were formed by this method.

Redox reactions are also responsible for creating banded iron formations (BIFs), which are interbedded layers of iron oxide—hematite and magnetite, chert, and shale beds. These deposits formed early in the Earth’s history as the atmosphere was becoming oxygenated. Cycles of oxygenating iron-rich waters initiated precipitation of the iron beds. Because BIFs are generally Precambrian in age, happening at the event of atmospheric oxygenation, they are only found in some of the older exposed rocks in the United States, such as in Michigan’s upper peninsula and northeast Minnesota.

Deep, saline, connate fluids (trapped in pore spaces) within sedimentary basins may be highly metalliferous. When expelled outward and upward as basin sediments compacted, these fluids formed lead and zinc deposits in limestone by replacing or filling open spaces, such as caves and faults, and in sandstone by filling pore spaces. The most famous are called Mississippi Valley-type deposits. Also known as carbonate-hosted replacement deposits, they are large deposits of galena and sphalerite lead and zinc ores that form from hot fluids ranging from 100°C to 200°C (212°F to 392°F). Although they are named for occurring along the Mississippi River Valley in the US, they are found worldwide.

Sediment-hosted copper deposits occurring in sandstones, shales, and marls are enormous, and their contained resources are comparable to porphyry copper deposits. These deposits were most likely formed diagenetically by groundwater fluids in highly permeable rocks. Well-known examples are the Kupferschiefer in Europe, which has an areal coverage of >500,000 Km2, (310,685.596mi) and the Zambian Copper Belt in Africa.

Soils and mineral deposits that are exposed at the surface experience deep and intense weathering, which can form surficial deposits. Bauxite, an aluminum ore, is preserved in karst topography and laterites, which are soils formed in wet tropical environments. Soils containing aluminum concentrate minerals, such as feldspar, and ferromagnesian minerals in igneous and metamorphic rocks, undergo chemical weathering processes that concentrate the metals. Ultramafic rocks that undergo weathering form nickel-rich soils, and when the magnetite and hematite in banded iron formations undergo weathering, it forms goethite, a friable mineral that is easily mined for its iron content.

Surficial Physical Processes

At the Earth’s surface, mass wasting and moving water can cause hydraulic sorting, which forces high-density minerals to concentrate. When these minerals are concentrated in streams, rivers, and beaches, they are called placer deposits, and occur in modern sands and ancient lithified rocks. Native gold, native platinum, zircon, ilmenite, rutile, magnetite, diamonds, and other gemstones can be found in placers. Humans have mimicked this natural process to recover gold manually by gold panning and by mechanized means such as dredging.

16.3.2. Environmental Impacts of Metallic Mineral Mining

Metallic mineral mining’s primary impact comes from the mining itself, including disturbing the land surface, covering landscapes with tailings impoundments, and increasing mass wasting by accelerating erosion. In addition, many metal deposits contain pyrite, an uneconomic sulfide mineral, that when placed on waste dumps, generates acid rock drainage (ARD) during weathering. In oxygenated water, sulfides such as pyrite react and undergo complex reactions to release metal ions and hydrogen ions, which lowers pH to highly acidic levels. Mining and processing of mined materials typically increase the surface area to volume ratio in the material, causing chemical reactions to occur even faster than would occur naturally. If not managed properly, these reactions lead to acidic streams and groundwater plumes that carry dissolved toxic metals. In mines where limestone is a waste rock or where carbonate minerals like calcite or dolomite are present, their acid neutralizing potential helps reduce acid rock drainage. Although this is a natural process too, it is very important to isolate mine dumps and tailings from oxygenated water, both to prevent the sulfides from dissolving and subsequently percolating the sulfate-rich water into waterways. Industry has taken great strides to prevent contamination in recent decades, but earlier mining projects are still causing problems with local ecosystems.

16.3.3. Nonmetallic Mineral Deposits

While receiving much less attention, nonmetallic mineral resources, also known as industrial minerals, are just as vital to ancient and modern society as metallic minerals. The most basic is building stone. Limestone, travertine, granite, slate, and marble are common building stones and have been quarried for centuries. Even today, building stones from slate roof tiles to granite countertops are very popular. Especially pure limestone is ground up, processed, and reformed as plaster, cement, and concrete. Some nonmetallic mineral resources are not mineral specific; nearly any rock or mineral can be used. This is generally called aggregate, which is used in concrete, roads, and foundations. Gravel is one of the more common aggregates.

Evaporites

Evaporite deposits form in restricted basins where water evaporates faster than it recharges, such as the Great Salt Lake in Utah, or the Dead Sea, which borders Israel and Jordan. As the waters evaporate, soluble minerals are concentrated and become supersaturated, at which point they precipitate from the now highly-saline waters. If these conditions persist for long stretches, thick rock salt, rock gypsum, and other mineral deposits accumulate (see Chapter 5).

Evaporiteminerals, such as halite, are used in our food as common table salt. Salt was a vitally important food preservative and economic resource before refrigeration was developed. While still used in food, halite is now mainly mined as a chemical agent, water softener, or road de-icer. Gypsum is a common nonmetallic mineral used as a building material; it is the main component in dry wall. It is also used as a fertilizer. Other evaporites include sylvite—potassium chloride, and bischofite—magnesium chloride, both of which are used in agriculture, medicine, food processing, and other applications. Potash, a group of highly soluble potassium-bearing evaporite minerals, is used as a fertilizer. In hyper-arid locations, even more rare and complex evaporites, like borax, trona, ulexite, and hanksite are mined. They can be found in places such as Searles Dry Lake and Death Valley, California, and in the Green River Formation’s ancient evaporite deposits in Utah and Wyoming.

Phosphorus

Phosphorus is an essential element that occurs in the mineral apatite, which is found in trace amounts in common igneous rocks. Phosphorite rock, which is formed in sedimentary environments in the ocean, contains abundant apatite and is mined to make fertilizer. Without phosphorus, life as we know it is not possible. Phosphorous is an important component of bone and DNA. Bone ash and guano are natural sources of phosphorus.

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Summary

Energy and mineral resources are vital to modern society, and it is the role of the geologist to locate these resources for human benefit. As environmental concerns have become more prominent, the value of the geologist has not decreased, as they are still vital in locating the deposits and identifying the least intrusive methods of extraction.

Energy resources are general grouped as being renewable or nonrenewable. Geologists can aid in locating the best places to exploit renewable resources (e.g. locating a dam), but are commonly tasked with finding nonrenewable fossil fuels. Mineral resources are also grouped in two categories: metallic and nonmetallic. Minerals have a wide variety of processes that concentrate them to economic levels, and are usually mined via surface or underground methods.

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