metamorphism in geology, process of change in the structure, texture, or composition of rocks caused by agents of heat, deforming pressure, shearing stress, hot, chemically active fluids, or a combination of these, acting while the rock being changed remains essentially in the solid state. Theoretically, rocks are formed when their constituents are in equilibrium with ambient physical conditions. If the conditions are changed by movements in the earth's crust or by igneous activity, metamorphism occurs to reestablish equilibrium and changes the physical character of the rock mass.

Characteristics of Metamorphism

In general, a metamorphic rock is coarser and has a higher density and lower porosity than the rock from which it was formed. Under low grade metamorphic conditions, the original rocks may only compact, as in the formation of slate from shale. High grade metamorphism changes the rock so completely that the source rock often cannot be readily identified.

Foliation

Alteration of rock texture by metamorphism commonly results in a rearrangement of mineral particles into a parallel alignment, called foliation, as a result of directed stress. Foliation, called banding or layering, is probably the single most characteristic property of metamorphic rocks. For example, slate is a metamorphic rock in which there has been little recrystallization of fine-grained sedimentary shale, but mineral realignment gives the rock a tendency to break along smooth planes termed slaty cleavage. Further higher-grade metamorphic conditions lead to a foliation called schistosity, resulting in schists, formed when tabular minerals, such as hornblende, graphite, mica, or talc are aligned and tightly packed in a parallel fashion. High grade metamorphism can segregate minerals, thereby forming bands. This foliation is called gneissic layering and forms gneiss from such rock as granite. Foliation does not always occur during metamorphism.

Changes in Chemical Constituents

Chemical changes occurring during metamorphism also can rearrange the chemical constituents into assemblages stable in their new environment, thus often forming new minerals of essentially the same chemical composition as those occurring in the rock prior to metamorphism. For example, hornblende can be changed into garnet or pyroxene. The mineral composition of rocks may also be altered by the addition of new elements or by the removal of elements formerly present through the action of circulating liquids or gases or by recrystallization under pressure.

Types of Metamorphism

Local Metamorphism

Contact metamorphism occurs when local rocks are metamorphosed by the heat from an igneous intrusion, such as limestone turning to marble along the contact zone. Some of the changes that occur in the older rock are due simply to the heat radiated from the igneous mass and to the pressures it creates. More extensive alterations are produced by the fluids and gases given off by the igneous mass; metamorphism of this type rarely causes foliation. Rocks around hot springs, or mineral-rich water, both of which are common along active plate boundary ridges (see plate tectonics ), are often changed by hydrothermal metamorphism (or metasomatism), which may, for example, transform granite into china clay ; black smokers, which occur along mid-ocean ridges, are the exit vents for extensive hydrothermal systems that alter basalts and can deposit mounds of metalliferous sediments on the seafloor. Metamorphic rocks that develop by shearing and crushing of the rock at low temperature are called cataclastic and are usually associated with the mechanical forces, especially pressure, involved in faulting (see fault ).

Regional Metamorphism

Metamorphism on a grander scale, called regional metamorphism, accompanies mountain-building activity. These metamorphic rocks pervade regions that have been subjected to intense pressures and temperatures during the development of mountain chains along boundaries between crustal plates. Large scale, intense regional metamorphism is particularly great in the "roots" of these mountains, which were at considerable depths when the pressures forming the mountains were active. These kinds of metamorphic rocks are most commonly exposed in old mountain chains, like the Blue Ridge Mts., that have substantially eroded away over time, leaving only disturbed structure and regional metamorphic rocks.

Metamorphism

Metamorphism refers to the physical and chemical changes that rocks undergo when exposed to conditions of high temperature , high pressure, or some combination thereof. Rocks that have undergone metamorphism exhibit chemical and structural changes that result from the partial or complete recrystallization of minerals within them. These transformations occur while the rock is in the solid state, i.e., no melting occurs during metamorphism. The conditions of high temperature and pressure under which metamorphism occurs are typically the result of processes such as mountain building, plate convergence, volcanism, and sedimentation .

Any type of rock may be metamorphosed and several agents can be involved in altering a parent rock into its metamorphic product. The composition of the parent rock limits the mineral composition of the product, although subsurface gases and fluids may contribute new elements. Thermal energy at depth, either from the geothermal gradient or from plutonic activity, may provide the energy for recrystallization of the rock. As the temperature increases, volatile components such as water and carbon dioxide can be released causing chemical changes to the minerals within the parent rock. In addition, the temperature increase may cause the rock to behave plastically in response to stresses acting on it, frequently resulting in a contorted appearance. Pressure on the parent rock may be a result of the overlying rock, known as lithostatic or confining pressure, or may be due to forces acting in a particular direction due to tectonic activity, known as directed pressure. Pressures within the rock may cause the instability of certain minerals in favor of those that are more stable under the new conditions. The pressure may also be localized on irregularities on the boundaries of individual grains. Recrystallization of a rock undergoing directed pressure typically results in the development of a foliated rock fabric, in which the axes of the minerals are aligned with the differential pressures based on the stability of the crystal lattice to those pressures. The development of such crystals during metamorphism may be heavily influenced by amount of time that the rock is exposed to the conditions. The mobilization of ions that supports crystal growth within the rock can require extensive periods of time to produce larger mineral grains.

Metamorphism may occur in a number of forms, each having different results and areal extent. Contact metamorphism is the baking of country rock immediately adjacent to an intruded magma body. This type of metamorphism, also known as thermal metamorphism, is caused by the high temperatures associated with an igneous intrusion. The rock is altered only in a zone, called an aureole, which can range from a few centimeters to several hundred meters in width. These zones may occur very near the surface and pressure plays an insignificant role in the process. In the case of cataclastic or dynamic metamorphism, rocks in a localized zone undergo mechanical disruption without significant mineralogical change. This is a near-surface phenomenon that is often associated with faulting and occurs at low temperature. Regional metamorphism, as the name suggests, encompasses large areas and is associated with large mountain building and plutonic events. Relatively high temperature and intense, directed pressures are common in this process. The differential stress associated with regional, or dynamothermal, metamorphism frequently yields foliated rock.

See also Metamorphic rock; Shock metamorphism

metamorphism The process of changing the characteristics of a rock in response to changes in temperature, pressure, or volatile content. Most metamorphic changes do not include bulk chemical changes, but merely the crystallization of new mineral phases. These isochemical changes cause major textural changes. Compare METASOMATISM. See also BARROVIAN-TYPE METAMORPHISM; BARROW'S ZONES; BURIAL METAMORPHISM; DYNAMIC METAMORPHISM; REGIONAL METAMORPHISM; THERMAL METAMORPHISM; and METAMORPHIC GRADE.

metamorphism The process of changing the characteristics of a rock, usually by the crystallization of new mineral phases. See metamorphic rock.