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.
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.
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.
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.
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).
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.
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