The agents of metamorphism include heat, pressure, directional stress, and chemically active fluids.
During metamorphism, rocks may be 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
The most important factor driving metamorphism is heat because it provides the energy needed to produce the chemical reactions that result in the recrystallization of existing minerals. Recall from the discussion of igneous rocks that an increase in temperature causes the atoms within a mineral to vibrate more rapidly. Even in a crystalline solid, where atoms are strongly bonded, this elevated level of activity allows individual atoms to migrate more freely between sites in the crystalline structure.
Changes Caused by Heat
The formation of new mineral grains that tend to be larger than the original grains is called recrystallization. During this process, the mineralogy of the rock may or may not change. For example, when quartz sandstone is metamorphosed to form quartzite, the mineralogy does not change; the quartz grains remain quartz. By contrast, when shale is metamorphosed to slate, the clay minerals are recrystallized and become new minerals—usually chlorite and muscovite. Although the mineralogy changes in the transition from shale to slate, the overall chemical composition remains essentially unchanged. Instead, the existing atoms are rearranged into new crystalline structures that are more stable in the new environment. (In some environments, ions may actually migrate into or out of a rock, thereby changing its overall chemical composition.)
What Is the Source of Heat?
There are two primary sources of heat within Earth. One is the increasing temperature that occurs as we travel deeper into Earth’s interior. The second is heat being released to the surrounding rocks as a magma body cools.
Earth’s interior is extremely hot, mainly because of heat released from the repeated collision of asteroidsize bodies during the formation of our planet as well as from energy being continually released by the decay of radioactive elements. The rate of increase in temperature with depth is known as the geothermal gradient. In the upper crust, this increase in temperature averages about 25°C (77°F) per kilometer (Figure 1). Thus, rocks that formed at Earth’s surface will experience a gradual increase in temperature if they are transported to greater depths.
As described earlier, clay minerals tend to become unstable when buried to a depth of about 8 kilometers (5 miles), where temperatures are about 200°C (400°F). The clay minerals begin to recrystallize into new minerals, such as chlorite and muscovite, both of which are stable in this new environment.
However, many silicate minerals, particularly those found in crystalline igneous rocks— such as quartz and feldspar—remain stable at these temperatures. Thus, metamorphic changes in quartz and feldspar generally occur at higher temperatures. Figure 1 provides several examples of conditions in which heat drives metamorphism. Environments where rocks may be carried to great depths and heated include convergent plate boundaries where slabs of sedimentladen oceanic crust are being subducted. Rocks may also become deeply buried in large basins where gradual subsidence results in thick accumulations of sediment. These basins, exemplified by the Gulf of Mexico, are known to develop low-grade metamorphic conditions near the base of the pile. In addition, continental collisions, which result in mountain building, cause some rocks to be uplifted while others are thrust downward, where elevated temperatures and pressures trigger metamorphism.
Heat may also be transported from the mantle into the shallowest layers of the crust. Rising mantle plumes, upwelling at mid-ocean ridges, and magma generated by partial melting of mantle rock at subduction zones are three such examples (see Figure 1). When magma intrudes rocks at shallow depths, the magma cools and releases heat, which “bakes” the surrounding host rock.
Pressure, like temperature, increases with depth because the thickness of the overlying rock increases. Buried rocks are subjected to confining pressure, which is analogous to water pressure, in which the forces are applied equally in all directions (Figure 2A). For example, the deeper scuba divers go, the greater the confining pressure.
Confining pressure causes the spaces between mineral grains to close, producing more compact rocks that have greater densities.
If the pressure becomes extreme enough, it can cause the atoms in a mineral to pack more closely together to produce a new, denser mineral.
In addition to confining pressure, rocks may be subjected to directed pressure. This occurs, for example, at convergent plate boundaries where slabs of lithosphere collide. Here the forces that deform rock are unequal in different directions and are referred to as differential stress.
Unlike confining pressure, which “squeezes” rock equally in all directions, differential stresses are greater in one direction than in others.
Differential stress that squeezes a rock mass as if it were placed in a vise is termed compressional stress. As shown in Figure 2B, rocks subjected to compressional stress are shortened in the direction of greatest stress and elongated, or lengthened, in the direction perpendicular to that stress. Along convergent plate boundaries, the greatest differential stress is directed horizontally in the direction of plate motion. Consequently, in these settings, the crust is greatly shortened (horizontally) and thickened (vertically), resulting in mountainous topography. In high-temperature, high-pressure environments, rocks are ductile, which allows their mineral grains to flatten (like what happens when you step on a tennis ball) when subjected to differential stress. The metaconglomerate, also called a stretch pebble conglomerate, shown in Figure 3 illustrates this tendency.
The parent rock, a conglomerate, consisted of nearly spherical pebbles that have been flattened into elongated structures by differential stress. On a larger scale, rocks that are ductile deform by flowing rather than breaking or fracturing. As a result, deeply buried rocks will develop intricate folds when deformed by differential stress (Figure 4).
By contrast, in near-surface environments where temperatures and pressure are comparatively low, rocks are brittle and tend to fracture when subjected to differential stress. Continued deformation grinds and pulverizes the mineral grains into smaller and smaller fragments.