3.3 Internal Structure of Earth
Layers of the Earth
To understand the details of plate tectonics, one must first understand the layers of the Earth. Humankind has insufficient first-hand information regarding what is below; most of what we know is pieced together from models, seismic waves, and assumptions based on meteorite material. In general, the Earth can be divided into layers based on chemical composition and physical characteristics. (2 Plate Tectonics – An Introduction to Geology, n.d.)
The Earth has three main divisions based on their chemical composition, which means chemical makeup. Indeed, there are countless variations in composition throughout the Earth, but only two significant changes take place, leading to three distinct chemical layers.
The outermost chemical layer and the layer humans currently reside on is known as the crust. The crust has two types: continental crust, which is relatively low density and has a composition similar to granite, and oceanic crust, which is relatively high density (especially when it is cold and old) and has a composition similar to basalt. In the lower part of the crust, rocks start to be more ductile and less brittle, because of added heat. Earthquakes, therefore, generally occur in the upper crust.
At the base of the crust is a substantial change in seismic velocity called the Mohorovičić Discontinuity, or Moho for short, discovered by Andrija Mohorovičić (pronounced mo-ho-ro-vee-cheech) in 1909 by studying earthquake wave paths in his native Croatia. It is caused by the dramatic change in composition that occurs between the mantle and the crust. Underneath the oceans, the Moho is about 5 km down. Under continents, the average is about 30-40 km, except near a sizeable mountain-building event, known as an orogeny, where that thickness is about doubled.
The mantle is the layer below the crust and above the core. It is the most substantial 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 waves, though some direct information can be gathered from parts of the ocean floor that are brought to the surface, known as ophiolites. Also, carried within magma are xenoliths, which are small chunks of lower rock carried to the surface by eruptions. These xenoliths are made of the rock peridotite, which on the scale of igneous rocks is ultramafic. We assume the majority of the mantle is made of peridotite.
The core of the Earth, which has both liquid and solid components, is made mostly of iron, nickel, and oxygen. First discovered in 1906 by looking into seismic data, it took the union of modeling, astronomical insight, and seismic data to arrive at the idea that the core is mostly metallic iron. Meteorites contain much more iron than typical surface rocks, and if meteoric material is what made the Earth, the core would have formed as dense material (including iron and nickel) sank to the center of the Earth via its weight as the planet formed, heating the Earth intensely.
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, precisely the core-mantle boundary, there are significant differences between the two systems. (2 Plate Tectonics – An Introduction to Geology, n.d.)
The lithosphere, with ‘litho’ meaning rock, is the outermost physical layer of the Earth. Including the crust, it has both an oceanic component and a continental component. Oceanic lithosphere, ranging from a thickness of zero (at the forming of new plates on the mid-ocean ridge) to 140 km, is thin and rigid. The continental lithosphere is more plastic (especially with depth) and is overall thicker, from 40 to 280 km thick. Most importantly, the lithosphere is not continuous. It is broken into several segments that geologists call plates. A plate boundary is where two plates meet and move relative to each other. It is at and near plate boundaries where plate tectonics’ real action is seen, including mountain building, earthquakes, and volcanism.
The asthenosphere, with ‘astheno’ meaning weak, is the layer below the lithosphere. The most distinctive property of the asthenosphere is movement. While still solid, over geologic time scales, it will flow and move because it is mechanically weak. In this layer, partly driven by convection of intense interior heat, movement allows the lithospheric plates to move. Since certain types of seismic waves pass through the asthenosphere, we know that it is solid, at least at the short time scales of the passage of seismic waves. The depth and occurrence of the asthenosphere are dependent on heat and can be very shallow at mid-ocean ridges and very deep in plate interiors and beneath mountains.
The mesosphere, or lower mantle as it is sometimes called, is more rigid and immobile than the asthenosphere, though still hot. This can be attributed to increased pressure with depth. Between approximately 410 and 660 km depth, the mantle is in a state of transition, as minerals with the same composition are changed to various forms, dictated by increasing pressure conditions. Changes in seismic velocity show this, and this zone also can be a physical barrier to movement. Below this zone, the mantle is uniform and homogeneous, as no significant changes occur until the core is reached.
The outer core is the only liquid layer found within Earth. It starts at 2,890 km (1,795 mi) depth and extends to 5,150 km (3,200 mi). Inge Lehmann, a Danish geophysicist, in 1936, was the first to prove that there was an inner core that was solid within the liquid outer core based on analyzing seismic data. The solid inner core is about 1,220 km (758 mi) thick, and the outer core is about 2,300 km (1,429 mi) thick.
It seems like a contradiction that the hottest part of the Earth is substantial, as hot temperatures usually lead to melting or boiling. The solid inner core can be explained by understanding that the immense pressure inhibits melting, though as the Earth cools by heat flowing outward, the inner core grows slightly larger over time. As the liquid iron and nickel in the outer core moves and convects, it becomes the most likely source for Earth’s magnetic field. This is critically important to maintaining the atmosphere and conditions on Earth that make it favor-able to life. Loss of outer core convection and the Earth’s magnetic field could strip the atmosphere of most of the gases essential to life and dry out the planet, much like what has happened to Mars.
Structure of Earth’s Crust
The fundamental unifying principle of geology and the rock cycle is the theory of Plate Tectonics. Plate tectonics describes how the layers of the Earth move relative to each other. Specifically, the outer layer divided into tectonic or lithospheric plates. As the tectonic plates float on a mobile layer beneath called the asthenosphere, they collide, slide past each other, and split apart. Significant landforms are created at these plate boundaries, and rocks making up the tectonic plates move through the rock cycle.
The following is a summary of the Earth’s layers based on chemical composition (or the chemical makeup of the layers). Earth has three main geological layers based on chemical composition – crust, mantle, and core. The outermost layer is the crust and is composed of mostly silicon, oxygen, aluminum, iron, and magnesium. There are two types of crust, continental and oceanic crust. Continental crust is about 50 kilometers (30 miles) thick, represents most of the continents, and is composed of low-density igneous and sedimentary rocks. Oceanic crust is approximately 10 kilometers (6 miles) thick, makes up most of the ocean floor, and covers about 70 percent of the planet. Oceanic crust is high-density igneous basalt-type rocks. The moving tectonic plates are made of crust, and some of the next layers within the earth called the mantle. The crust and this portion of the upper mantle are rigid and called the lithosphere and make up the tectonic plates.
The oldest continental rocks are billions of years old, so the continents have had much time to happen to them. Constructive forces cause physical features on Earth’s surface known as landforms to grow. Crustal deformation – when crust compresses, pulls apart, or slides past other crust – results in hills, valleys, and other landforms. Mountains rise when continents collide when one slab of ocean crust plunges beneath another or a slab of continental crust to create a chain of volcanoes. Sediments are deposited to form landforms, such as deltas. Volcanic eruptions can also be destructive forces that blow landforms apart. The destructive forces of weathering and erosion modify landforms. Water, wind, ice, and gravity are essential forces of erosion.
The ocean basins are all younger than 180 million years. Although the ocean basins begin where the ocean meets the land, the continent extends downward to the seafloor, so the continental margin is made of continental crust.
The ocean floor itself is not flat. The most distinctive feature is the mountain range that runs through much of the ocean basin, known as the mid-ocean ridge. The ocean trenches are the deepest places of the ocean, many of which are found around the edge of the Pacific Ocean. Chains of volcanoes are also found in the center of the oceans, such as around Hawaii. Flat plains are found on the ocean floor with their features covered by mud.