Recall from the discussion of the rock cycle that rocks can transform from one type into another. Metamorphic rocks are produced from preexisting igneous, sedimentary, or even other metamorphic rocks. Thus, every metamorphic rock has a parent rock—the rock from which it was formed.

Metamorphism means “to change form.” When describing rock, metamorphism is a process that leads to changes in mineralogy, texture (for example, grain size), and sometimes chemical composition (Figure 26). Metamorphism occurs most often in rock when it is subjected to a significant increase in temperature and/or pressure. In response to these new conditions, the rock gradually changes until it reaches a state of equilibrium with the new environment. Most rock metamorphism occurs at the elevated temperatures and pressures that exist in the zone beginning a few kilometers below Earth’s surface and extending into the mantle.

Metamorphism often progresses incrementally, from slight changes (low-grade metamorphism) to substantial changes (high-grade metamorphism). For example, under low-grade metamorphism, the common sedimentary rock shale becomes the more compact metamorphic rock slate (Figure 27A). Hand samples of shale and slate are sometimes difficult to distinguish, illustrating that the transition from sedimentary to metamorphic rock is often gradual, and the changes can be subtle.

In more extreme environments, metamorphism causes a transformation so complete that the identity of the parent rock cannot be determined. In high-grade metamorphism, such features as bedding planes, fossils, and vesicles that may have existed in the parent rock are obliterated. Furthermore, when rocks deep in the crust (where temperatures are high) are subjected to directed pressure, the entire mass may deform, producing large-scale structures such as folds (Figure 27B).

By definition, rock undergoing metamorphism remains essentially solid. In the most extreme metamorphic environments, the temperatures approach those at which rocks melt. However, if appreciable melting occurs, the rocks have entered the realm of igneous activity.

Most metamorphism occurs in one of two settings:

• When rock is intruded by magma, contact metamorphism may take place as the magma heats the adjacent rock to temperatures that cause metamorphic changes.

• During mountain building, great quantities of rock are subjected to pressures and high temperatures associated with large-scale deformation called regional metamorphism.

Extensive areas of metamorphic rocks are exposed on every continent. Metamorphic rocks are an important component of many mountain belts, where they make up a large portion of a mountain’s crystalline core. Even the stable continental interiors, which are generally covered by sedimentary rocks, are underlain by metamorphic basement rocks. In each of these settings, the metamorphic rocks are usually highly deformed and intruded by igneous masses. Consequently, significant parts of Earth’s continental crust are composed of metamorphic and associated igneous rocks.

What Drives Metamorphism?

The agents of metamorphism include heat, confining pressure, differential stress, and chemically active fluids. During metamorphism, rocks are often subjected to all four metamorphic agents simultaneously. However, the degree of metamorphism and the contribution of each agent vary greatly from one environment to another.

Heat as a Metamorphic Agent

Thermal energy, commonly referred to as heat, is the most important factor driving metamorphism. It triggers chemical reactions that result in the recrystallization of existing minerals and the formation of new minerals. Thermal energy for metamorphism comes mainly from two sources: bodies of magma beneath Earth’s surface and the increase in temperature with depth in the Earth.

Rocks experience a rise in temperature when they are intruded by magma rising from below (contact metamorphism). In this situation, heat and fluids from the magma intrusion cause the rocks surrounding the magma to recrystallize, creating new minerals and textures. For example, sandstone intruded by magma is changed to the metamorphic rock quartzite when individual quartz grains are fused together into interlocking crystals.

By contrast, rocks that formed at Earth’s surface but are taken to deeper depths experience a gradual increase in temperature and pressure. In the upper crust, this increase in temperature averages about 25°C per kilometer. When buried to a depth of about 8 kilometers (5 miles), where temperatures are between 150° and 200°, clay minerals tend to become unstable and begin to recrystallize into other minerals, such as chlorite and muscovite, that are stable in this environment. (Chlorite is a mica-like mineral formed by the metamorphism of iron- and magnesium-rich silicates.) However, many silicate minerals, particularly those found in crystalline igneous rocks—quartz and feldspar, for example—remain stable at these temperatures. Thus, these minerals require much higher temperatures in order to metamorphose and recrystallize.

Confining Pressure and Differential Stress as Metamorphic Agents

Pressure, like temperature, increases with depth as the thickness of the overlying rock increases. Buried rocks are subjected to confining pressure—similar to water pressure in that the forces are equally applied in all directions (Figure 28A). The deeper you go in the ocean, the greater the confining pressure. The same is true for buried rock. Confining pressure causes the spaces between mineral grains to close, producing a more compact rock that has greater density. Further, at great depths, confining pressure may cause minerals to recrystallize into new minerals that display more compact crystalline forms.

During episodes of mountain building, large rock bodies become highly crumpled and metamorphosed (Figure 28B). Unlike confining pressure, which “squeezes” rock equally in all directions, the forces that generate mountains are unequal in different directions and are called differential stress. As shown in Figure 28B, rocks subjected to differential stress are shortened in the direction of greatest stress, and they are elongated, or lengthened, in the direction perpendicular to that stress. The deformation caused by differential stresses plays a major role in developing metamorphic textures.

In surface environments where temperatures are relatively low, rocks are brittle and tend to fracture when subjected to differential stress. (Think of a heavy boot crushing a piece of fine crystal.) Continued deformation grinds and pulverizes the mineral grains into small fragments. By contrast, in high-temperature, high-pressure environments deep in Earth’s crust, rocks are ductile and tend to flow rather than break. (Think of a heavy boot crushing a soda can.) When rocks exhibit ductile behavior, their mineral grains tend to flatten and elongate when subjected to differential stress. This accounts for their ability to generate intricate folds (refer to Figure 26).

Chemically Active Fluids as Metamorphic Agents

Ion-rich fluids, composed mainly of water and other volatiles (materials that readily change to gases at surface conditions), are believed to play an important role in some types of metamorphism. Fluids that surround mineral grains act as catalysts that promote recrystallization by enhancing ion migration. In progressively hotter environments, these ion-rich fluids become correspondingly more reactive. Chemically active fluids can produce two types of metamorphism, explained below. The first type changes the arrangement and shape of mineral grains within a rock; the second type changes the rock’s chemical composition.

When two mineral grains are squeezed together, the parts of their crystalline structures that touch are the most highly stressed. Atoms at these sites are readily dissolved by hot fluids and, when heated, move to fill the voids between individual grains. Thus, hot fluids aid in the recrystallization of mineral grains by dissolving material from regions of high stress and then precipitating (depositing) this material in areas of low stress. As a result, minerals tend to recrystallize and grow longer in a direction perpendicular to compressional stresses.

When hot fluids circulate freely through rocks, ionic exchange may occur between adjacent rock layers, or ions may migrate great distances before they are finally deposited. The latter situation is particularly common when we consider hot fluids that escape during the crystallization of an intrusive mass of magma. If the rocks surrounding the magma differ markedly in composition from the invading fluids, there may be a substantial exchange of ions between the fluids and host rocks. When this occurs, the overall composition of the surrounding rock changes.

Metamorphic Textures

The degree of metamorphism is reflected in a rock’s texture and mineralogy. (Recall that the term texture is used to describe the size, shape, and arrangement of grains within a rock.) When rocks are subjected to low-grade metamorphism, they become more compact and thus denser. A common example is slate, a metamorphic rock which forms when shale is subjected to temperatures and pressures only slightly greater than those associated with the compaction that lithifies sediment. In this case, differential stress causes the microscopic clay minerals in shale to align into the more compact arrangement found in slate.

Under more extreme temperature and pressure, stress causes certain minerals to recrystallize. In general, recrystallization encourages the growth of larger crystals. Consequently, many metamorphic rocks consist of visible crystals, much like coarse-grained igneous rocks.

Foliation

The term foliation refers to any nearly flat arrangement of mineral grains or structural features within a rock. Although foliation may occur in some sedimentary and even a few types of igneous rocks, it is a fundamental characteristic of regionally metamorphosed rocks—that is, rock units (bodies of rock with distinctive characteristics) that have been strongly deformed, mainly during folding. As shown in Figure 29, foliation in metamorphic environments is ultimately driven by compressional stresses that shorten rock units, causing mineral grains in preexisting rocks to develop parallel, or nearly parallel, alignments. Examples of foliation include the parallel alignment of platy (flat and disk-like) minerals, such as the micas; elongated or flattened pebbles that are characteristic of metaconglomerates; compositional banding, in which dark and light minerals separate, generating a layered appearance; and rock cleavage, in which rocks can be easily split into tabular slabs. It is important to note that rock cleavage is not related to mineral cleavage.

Nonfoliated Textures

Not all metamorphic rocks exhibit a foliated texture. Those that do not are referred to as nonfoliated and typically develop in environments where deformation is minimal and the parent rocks are composed of minerals, such as quartz or calcite, that have a relatively simple chemical composition. For example, when a fine-grained limestone (made of calcite) is metamorphosed by the intrusion of a hot magma body (contact metamorphism), the small calcite grains recrystallize and form larger interlocking crystals. The resulting rock, marble, exhibits large, equidimensional grains that are randomly oriented, similar to those in a coarse-grained igneous rock.

Common Metamorphic Rocks

Figure 30 depicts the common rocks produced by metamorphic processes, which are described next.

Foliated Rocks

Slate is a very fine-grained foliated rock composed of minute mica flakes that are too small to be visible to the unaided eye (refer to Figure 30). A noteworthy characteristic of slate is its excellent rock cleavage, or tendency to break into flat slabs. This property has made slate a useful rock for roof and floor tile, as well as billiard tables (Figure 31). Slate is usually generated by the low-grade metamorphism of shale. Less frequently, it is produced when volcanic ash is metamorphosed. Slate’s color is variable. Black slate contains organic material, red slate gets its color from iron oxide, and green slate is usually composed of chlorite, a greenish mica-like mineral.

Phyllite represents a degree of metamorphism between slate and schist. Its constituent platy minerals, mainly muscovite and chlorite, are larger than those in slate but not large enough to be readily identifiable with the unaided eye. Although phyllite appears similar to slate, it can be easily distinguished from slate by its glossy sheen and wavy surface (refer to Figure 30).

Schists are moderately to strongly foliated rocks formed by regional metamorphism (refer to Figure 30). They are platy and can be readily split into thin flakes or slabs. Many schists originate from shale parent rock. The term schist describes the texture of a rock, regardless of composition. For example, schists composed primarily of muscovite and biotite are called mica schists.

Gneiss (pronounced “nice”) is the term applied to banded metamorphic rocks in which elongated and granular (as opposed to platy) minerals predominate (refer to Figure 30). The most common minerals in gneisses are quartz and feldspar, with lesser amounts of muscovite, biotite, and hornblende. Gneisses exhibit strong segregation of light and dark silicates, giving them a characteristic banded texture. While still deep below the surface where temperatures and pressures are great, banded gneisses can be deformed into intricate folds.

Nonfoliated Rocks

Marble is a coarse, crystalline rock whose parent rock is limestone (refer to Figure 30). Marble is composed of large interlocking calcite crystals formed from the recrystallization of smaller grains in the parent rock. Because of its color and relative softness (hardness of only 3 on the Mohs scale), marble is a popular building stone. White marble is particularly prized as a stone from which to carve monuments and statues, such as the Lincoln Memorial in Washington, DC, and the Taj Mahal in India. Marble can also be other colors—pink, gray, green, or even black—if the parent rocks from which it formed contain impurities that color the stone.

Quartzite is a very hard metamorphic rock most often formed from quartz sandstone (refer to Figure 30). Under moderate to high-grade metamorphism, the quartz grains in sandstone fuse. Pure quartzite is white, but iron oxide may produce reddish or pinkish stains, and dark minerals may impart a gray color.

Other Metamorphic Rocks

During intermediate- to high-grade metamorphism, recrystallization of existing minerals often produces new minerals that are mainly associated with metamorphic rocks, such as the mineral garnet. These newly formed minerals, commonly referred to as accessory minerals, tend to form large crystals that are surrounded by smaller crystals of other minerals, such as muscovite and biotite. When naming a metamorphic rock that contains one or more easily recognizable accessory minerals, geologists add a prefix to the appropriate rock name. For example, Figure 32 shows a mica schist that contains large dark red garnet crystals embedded in a matrix of fine-grained micas; consequently, this rock is called a garnet-mica schist. The metamorphic rock gneiss also frequently contains accessory minerals, including garnet and staurolite. These rocks would be called garnet gneiss and staurolite gneiss, respectively.

• When rocks are subjected to elevated temperatures and pressures, they can change form, producing metamorphic rocks. Every metamorphic rock has a parent rock—the rock it used to be prior to metamorphism. When the minerals in parent rocks are subjected to heat and pressure, new minerals can form. Depending on the intensity of alteration, metamorphism ranges from low grade to high grade.

• Heat, confining pressure, differential stress, and chemically active fluids are four agents that drive metamorphic reactions. Any one alone may trigger metamorphism, or all four may act simultaneously.

• Confining pressure results from burial. The force it exerts is the same in all directions, like the pressure exerted by water on a diver. An increase in confining pressure causes rocks to compact into more dense configurations.

• Differential stresses, which occur during mountain building, are greater in one direction than in others. Rocks subjected to differential stress under ductile conditions deep in the crust tend to shorten in the direction of greatest stress and elongate in the direction(s) of least stress, producing flattened or stretched grains. In the shallow crust, most rocks respond to differential stress with brittle deformation, breaking into pieces.

• A common kind of texture is foliation, the planar arrangement of mineral grains. Common foliated metamorphic rocks include (in order of increasing metamorphic grade) slate, phyllite, schist, and gneiss.

• Common nonfoliated metamorphic rocks include quartzite and marble, recrystallized rocks that form from quartz sandstone and limestone, respectively.