6 Metamorphic Rocks

The painted wall is a large cliff
Painted Wall of Black Canyon of the Gunnison National Park, Colorado, made of 1.7 billion-year old gneiss intruded by younger pegmatites.

6 Metamorphic Rocks

Contributing Author: Dr. Peter Davis, Pacific Lutheran University


  • Describe the and pressure conditions of the environment
  • Identify and describe the three principal agents
  • Describe what is and how it affects crystals
  • Explain what is and how it results from directed pressure and
  • Explain the relationships among , , , and in terms of metamorphic grade
  • Define
  • Explain how relate to processes
  • Describe what a contact is and how affects surrounding rock
  • Describe the role of in forming deposits and bodies
The rock cycle shows how different rock groups are interconnected. Metamorphic rocks can come from adding heat and/or pressure to other metamorphic rock or sedimentary or igneous rocks
Rock cycle showing the five materials (such as igneous rocks and sediment) and the processes by which one changes into another (such as weathering). (Source: Peter Davis)

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

6.1 Metamorphic Processes

occurs when solid rock changes in and/or without the crystals melting, which is how is generated. , the rocks that experience the , are called the or , from proto– meaning first, and lithos- meaning rock. Most processes take place deep underground, inside the earth’s . During , chemistry is mildly changed by increased (heat), a type of pressure called pressure, and/or chemically reactive fluids. Rock is changed by heat, pressure, and a type of pressure called .

6.1.1  Temperature (Heat)  

measures a substance’s energy—an increase in represents an increase in energy. changes affect the chemical equilibrium or balance in . 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 lies in between surficial processes (as in ) and in the . Heat-driven 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 , and thus, would no longer be a process.  increases with increasing depth in the Earth along a (see Chapter 4) and records these depth-related 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 in a rock. The pressure that affects rocks can be grouped into pressure and . is a scientific term indicating a force. is the result of this , including changes within .

Confining Pressure

Pressure is a state where all stresses on a body are equal. The magnitude of these balanced stresses increases with increasing depth within the earth. These stresses can not deform rocks other than to decrease their volume. Pressure is the term used becuase the concept of pressure is used in chemistry, which it the discipline of science used to understand the mineral reactions that occur within the rock. DIRECTED STRESSES s, s, One or more directions of stress are not equal in magnitude and or not in line with each other (non-coaxial). Unlike balanced stresses, the difference in these stresses can deform rocks within the earth.
Difference between pressure and stress and how they deform rocks. Pressure (or confining pressure) has equal stress (forces) in all directions and increases with depth under the Earth’s surface. Under directed stress, some stress directions (forces) are stronger than others, and this can deform rocks. (Source: Peter Davis)

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 or pressure. 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 to form. 

pressure is measured in bars and ranges from 1 bar at sea level to around 10,000 bars at the base of the .  For 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

Pebbles in quartzite deformed by directed stress
Pebbles (that used to be spherical or close to spherical) in quartzite deformed by directed stress

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

Two rocks with very similar colors. One is a granite and another is a gneiss that has aligned dark minerals.
An igneous rock granite (left) and foliated high-temperature and high-pressure metamorphic rock gneiss (right) illustrating a metamorphic texture. (Source: Peter Davis)

Directed produce rock textures in many ways. Crystals are rotated, changing their orientation in space. Crystals can get fractured, reducing their . Conversely, they may grow larger as atoms migrate. Crystal shapes also become deformed. These mechanical changes occur via , which is when from an area of rock experiencing high and or regrow in a location having lower . For example, increases much like adjacent soap bubbles coalesce to form larger ones. rearranges 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 .

6.1.3 Fluids

A third agent is chemically reactive fluids that are expelled by crystallizing and created by 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 in the , changing its chemical equilibrium and , in a process similar to the reactions driven by heat and pressure. In addition to using found in the , the chemical reaction may incorporate substances contributed by the fluids to create new . In general, this style of , in which fluids play an important role, is called or alteration. Water actively participates in chemical reactions and allows extra mobility of the components in alteration.

Fluids-activated is frequently involved in creating economically important deposits that are located next to intrusions or bodies. For example, the districts in the Cottonwood Canyons and of northern Utah produce valuable such as argentite (silver ), galena (lead ), and chalcopyrite (copper iron ), as well as the gold. These deposits were created from the interaction between a granitic intrusion called the Little Cottonwood Stock and consisting of mostly and dolostone. Hot, circulating fluids expelled by the crystallizing reacted with and the surrounding and dolostone, precipitating out new created by the chemical reaction. alternation of rock, such as and , creates the , a member of the serpentine subgroup of . This process happens at mid-ocean where newly formed 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 alterations remove from the rather than deposit them. This happens when seawater circulates down through in the fresh, still-hot , reacting with and removing ions from it. The are usually ions that do not fit snugly in the crystal structure, such as copper. The -laden water emerges from the sea floor via vents called , named after the dark-colored precipitates produced when the hot water meets cold seawater. (see Chapter 4, and Processes) Ancient were an important source of copper 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

is the description of the shape and orientation of grains in a . textures are , , or lineated are described below.

Table identifying the types of metamorphic rocks.
Metamorphic rock identification table. (Source: Belinda Madsen)

6.2.1 Foliation and Lineation

is a term used that describes lined up in planes. Certain , most notably the group, are mostly thin and planar by default. rocks typically appear as if the are stacked like pages of a book, thus the use of the term ‘folia’, like a leaf. Other , 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 . Linear crystals, such as hornblende, tourmaline, or stretched grains, can be arranged as part of a , a , or / together. If they lie on a plane with , but with no common or preferred direction, this is . If the line up and point in a common direction, but with no planar fabric, this is . When lie on a plane AND point in a common direction; this is both and .  

Lineation is aligned linear features in a rock. An example in the figure is a bundle of aligned straws.
Example of lineation where minerals are aligned like a stack of straws or pencils. (Source: Peter Davis)
Aligned tourmaline crystals in line with foliation. Foliation is the fine "layers" of the rock.
An example of foliation WITH lineation. (Source: Peter Davis)
Foliated surface displays non-lineated hornblende grains. A cross-section displays a cross section of foliated plagioclase and hornblende
An example of foliation WITHOUT lineation. (Source: Peter Davis)

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

is a fine-grained that exhibits a called that is the flat orientation of the small platy crystals of and chlorite forming perpendicular to the direction of The in are too small to see with the unaided eye. The thin layers in may resemble sedimentary , but they are a result of and may lie at angles to the original . In fact, original sedimentary layering may be partially or completely obscured by the . Thin slabs of 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.

Foliation is caused by metamorphism. Bedding is a result of sedimentary processes. They do not have to align.
Foliation vs. bedding. Foliation is caused by metamorphism. Bedding is a result of sedimentary processes. They do not have to align. (Source: Peter Davis)
A foliated rock with a slight sheen.
Phyllite with a small fold. (Source: Peter Davis)

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


Schist is a scalely looking foliated metamorphic rock.
Shiny foliated rock with small crystals of red faceted garnet among the foliated micas.
Garnet staurolite muscovite schist. (Source: Peter Davis)

<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 

 is a in which visible separate into dark and light or lineations. These grains tend to be coarse and often folded. A rock with this is called . Since gneisses form at the highest temperatures and pressures, some may occur. This partially melted rock is a transition between and rocks called a .

Swirling bands of light and dark minerals.
Migmatite, a rock which was partially molten. (Source: Peter Davis)

appear as dark and light that may be swirled or twisted some since some 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 . The lighter colored layers are interpreted to be the result of the separation of a melt from the adjacent highly metamorphosed darker layers, or injection of a melt from some distance away.

6.2.2 Non-foliated

pink crystallized rock with interlocking crystals
Marble (Source: Peter Davis)
Crystallized rock with interlocking crystals.
Baraboo Quartzite

textures do not have lineations, foliations, or other alignments of grains. rocks are typically of just one , and therefore, usually show the effects of with in which crystals grow together, but with no preferred direction. The two most common examples of rocks are and . is a from the . In , the grains from the original are enlarged and interlocked by . A defining characteristic for distinguishing from is that when broken with a rock hammer, the crystals break across the grains. In a , only a thin cement holds the grains together, meaning that a broken piece of will leave the grains intact. Because most are rich in , and is a mechanically and chemically durable substance, is very hard and resistant to .

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

A third rock is identified by its dense, fine grained, hard, blocky or splintery of several . Crystals in grow smaller with , and become so small that specialized study is required to identify them. These are common around bodies and are hard to identify. The of can be even harder to distinguish, which can be anything from to .

<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

Large weathered garnet crystals in a matrix of platy micas. The garnets are round-shaped with octagonal sides.
Garnet schist.

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

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

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

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6.4 Metamorphic Environments

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

Pressure-temperature graphs of various metamorphic facies. (Source: Peter Davis)

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

Metamorphic zones in Scotland show increasing metamorphic grade across a transect of a deformed mountain range.
Barrovian sequence in Scotland.

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

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

6.4.1 Burial Metamorphism

occurs when rocks are deeply buried, at depths of more than 2000 meters (1.24 miles). commonly occurs in , where rocks are buried deeply by overlying . As an of , a process that occurs during (Chapter 5), can cause clay , such as smectite, in to change to another clay illite. Or it can cause to metamorphose into the such the Big Cottonwood in the Wasatch Range of Utah. This was deposited as ancient near- sands in the late (see Chapter 7), deeply buried and metamorphosed to , folded, and later exposed at the surface in the Wasatch Range today. Increase of with depth in combination with an increase of pressure produces low- rocks with a assemblages indicative of a zeolite .

6.4.2 Contact Metamorphism

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

As with all , the and chemistry are major factors in determining the final outcome of the process, including what are present. Fine-grained and , which happen to be chemically similar, characteristically recrystallize to produce . (silica) surrounding an intrusion becomes via , and () becomes .

<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 occurs deeper in the Earth, can be seen as rings of around the intrusion, resulting in . These differences in appear as distinct surrounding the intrusion, as can be seen around the Alta Stock in Little Cottonwood Canyon, Utah. The Alta Stock is a intrusion surrounded first by rings of the (tremolite) and (forsterite), with a ring of talc (dolostone) located further away.

6.4.3 Regional Metamorphism

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

An example of an old regional 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 gradually increases from sedimentary , to low- , then higher- , and eventually the . The rock sequence is , , , , , , and . In fact, New Hampshire is nicknamed the State. The reverse sequence can be seen heading east, from eastern New Hampshire to the .

6.4.4 Subduction Zone Metamorphism

A blue rock with bands of silvery mica grains.
Blueschist (Source: Peter Davis)

metamorphism is a type of that occurs when a of is under (see Chapter 2). Because rock is a good insulator, the of the descending increases slowly relative to the more rapidly increasing pressure, creating a environment of high pressure and low . Glaucophane, which has a distinctive blue color, is an found in (see diagram). The California Range near San Francisco has rocks created by -zone , which include rocks made of , greenstone, and red . Greenstone, which is metamorphized , gets its color from the chlorite.

6.4.5 Fault Metamorphism

Layers of shears material with rotated grains.
Mylonite (Source: Peter Davis)

There are a range of rocks made along . Near the surface, rocks are involved in repeated produce a material called rock flour, which is rock ground up to the particle size of flour used for food. At lower depths, create cataclastites, chaotically-crushed mixes of rock material with little internal . At depths below , where becomes , are formed. are rocks created by dynamic through directed , generally resulting in a reduction of . When larger, stronger crystals (like , , garnet) embedded in a matrix are into an asymmetrical eye-shaped crystal, an is formed.

Rounded mineral grains from shear forces.
Examples of augens. (Source: Peter Davis)

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) is resulting from or other impacts, or from a similar high-pressure shock event. is the result of very high pressures (and higher, but less extreme temperatures) delivered relatively rapidly. produces planar features, tektites, shatter cones, and . produces planar features (shock ), which are narrow planes of glassy material with distinct orientations found in grains. Shocked has planar features

Shatter cones are cone-shaped features, that show lines that converge to cone shapes.
Shatter cone.

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

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

Shatter cones are cone-shaped features, that show lines that converge to cone shapes.

can also produce glass. Tektites are gravel-size glass grains ejected during an impact event. They resemble glass but, unlike glass, tektites contain no water or , and have a different bulk and isotopic chemistry. Tektites contain partially melted of shocked 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 can be called with the general term pseudotachylytes, a name which can also be applied to glasses created by . The term pseudo in this context means ‘false’ or ‘in the appearance of’, a called tachylite because the material observed looks like a , but is produced by significant heating.

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

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  3. Marshak, S., 2009, Essentials of Geology, 3rd or 4th Edition:
  4. Proctor, B.P., McAleer, R., Kunk, M.J., and Wintsch, R.P., 2013, Post-Taconic tilting and Acadian structural overprint of the classic Barrovian in Dutchess County, New York: Am. J. Sci., v. 313, no. 7, p. 649–682.
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