Unlike some igneous and sedimentary processes that occur in surface or near-surface environments, metamorphism most often occurs deep within Earth, beyond our direct observation. Notwithstanding this significant obstacle, geologists have developed techniques that allow them to learn about the conditions under which metamorphic rocks form. In turn, the study of metamorphic rocks provides important insights into how tectonic processes operate to alter the structure and composition of Earth’s crust.
Metamorphic rocks are produced from preexisting sedimentary and igneous rocks, as well as from other metamorphic rocks. Thus, every metamorphic rock has a parent rock—the rock from which it was formed. Metamorphism, which means to “change form,” is a process that transforms the mineralogy, texture, and sometimes the chemical composition of the parent rock. The mineralogy (mineral constituents of a rock) changes because the rock is subjected to new conditions, usually elevated temperatures and pressures, which are significantly different from those in which it initially formed. For example, clay minerals, which are the most common minerals in sedimentary rocks, are stable only at Earth’s surface.
When clay minerals are buried to a depth where temperatures exceed 200°C (nearly 400°F), they are transformed into the minerals chlorite and/or muscovite mica. (Chlorite is a mica-like mineral formed by the metamorphism of dark iron- and magnesiumrich silicate minerals.) Under more extreme conditions, chlorite becomes biotite mica. Metamorphism also alters a rock’s texture, producing larger crystals and sometimes a distinct layered or banded appearance. The degree to which a parent rock changes during metamorphism is called its metamorphic grade, and it varies from low grade (low temperatures and pressures) to high grade (high temperatures and pressures). For example, in low-grade metamorphic environments, the common sedimentary rock shale becomes the more compact metamorphic rock slate. During this transformation, the clay minerals in shale are transformed into tiny chlorite and muscovite mica flakes. Hand samples of these rocks are sometimes difficult to distinguish from one another, illustrating that the transition from sedimentary to metamorphic rock is often gradual and the change subtle (Figure 1A).
In environments where temperatures and pressures are more extreme, metamorphism causes a transformation so complete that the identity of the parent rock cannot be easily determined. In high-grade metamorphism, such features as bedding planes, fossils, and vesicles that existed in the parent rock are obliterated. Further, when rocks deep in the crust are subjected to compressional stress (like being placed in a giant vise), the entire mass may be deformed, usually by folding (Figure 1B). Figure 1AB illustrates the relationships among metamorphic, sedimentary, and igneous environments.
Figure 2 illustrates the relationships among metamorphic, sedimentary, and igneous environments. Metamorphism occurs over a range of temperatures that lie between those experienced during the formation of sedimentary rocks (up to about 200°C [400°F]) and temperatures approaching those at which rocks begin to melt (about 700°C [1300°F]). However, during metamorphism, the rock remains essentially solid. If complete melting occurs, the rock has entered the realm of igneous activity. Pressure also plays an important role in metamorphism. Although confining pressure acts to compact sediment to form sedimentary rocks, the pressures involved in metamorphism are even greater—sufficient to convert mineral matter into denser forms having more compact crystalline structures. Thus, metamorphism involves the formation of new minerals from preexisting ones.