metamorphism

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Related to Hydrothermal metamorphism: shock metamorphism, burial metamorphism

metamorphism,

in geology, process of change in the structure, texture, or composition of rocksrock,
aggregation of solid matter composed of one or more of the minerals forming the earth's crust. The scientific study of rocks is called petrology. Rocks are commonly divided, according to their origin, into three major classes—igneous, sedimentary, and metamorphic.
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 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 tectonicsplate tectonics,
theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history.
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), are often changed by hydrothermal metamorphism (or metasomatism), which may, for example, transform granite into china claychina clay,
one of the purest of the clays, composed chiefly of the mineral kaolinite usually formed when granite is changed by hydrothermal metamorphism. Usage of the terms china clay and kaolin
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; 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 faultfault,
in geology, fracture in the earth's crust in which the rock on one side of the fracture has measurable movement in relation to the rock on the other side. Faults on other planets and satellites of the solar system also have been recognized.
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).

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

 

fundamental changes in the texture, structure, and mineral and chemical composition of rocks in the earth’s crust and mantle owing to the effect of abyssal fluids (volatile constituents), temperature, and pressure. The term “metamorphic rocks” was introduced in 1883 by the English geologist C. Lyell.

Metamorphism occurs in the crystalline (solid or plastic) state without melting of the rocks (it does not include the near-surface processes of compaction, cementation, and diagenesis of sediments or weathering); metamorphism is always associated with tectonic dislocation (folding, abyssal fractures) and sometimes also with the upwelling of magmatic masses. Dislocations penetrating into the abyssal zones of the earth stimulate the formation of ascending streams of fluids and a rise in temperature, which leads to the development of magmatism, metamorphism, and the formation of endogenic deposits. All of these phenomena are genetically related, reflecting the ascending migration of matter in the course of evolution of the earth’s crust.

The factors in metamorphism that determine the mineral composition of metamorphic rocks are temperature (T), litho-static pressure (Ps) which is determined by the depth at which the metamorphism develops, and sometimes the partial pressures or chemical potentials of the gases that are contained in fluids, including H2O, H2, CO2, CO, CH4, H2S, C12, and F2. The regions of stability for the chief minerals in metamorphic rocks (metamorphic facies) are identified in relation to these factors (primarily T, Ps, and PH2o), which is the basis for dividing all metamorphic rocks and for studying the degree of metamorphism. Unilateral pressure (stress) is not a factor in metamorphism because it does not lead to the formation of new minerals. At the same time, however, it influences the textures of metamorphic rocks, increases rock permeability to fluids, and has a catalytic effect on metamorphic reactions.

Metamorphism involving change in the content of volatile components (H2O, CO2, O2) only is arbitrarily called isochemical; metamorphism involving change in the content of other components (K2O, Na2O, CaO) is called allochemical; when there are intensive local alterations in the chemical composition of rocks and part of the components change to a completely mobile state the metamorphism is called metasomatism. In the series of processes of isochemical metamorphism, allochemical metamorphism, and metasomatism, the degree of alteration in the chemical composition of the initial rocks is greater in each process than in the preceding one.

Metamorphism may affect rocks over enormous areas (regional metamorphism) or manifest itself locally, being confined to contacts with igneous rocks (contact metamorphism) or to fractures (fracture metamorphism).

In the history of geosynclinal development a distinction is made between early (“pregranitic”) sodic metamorphism (the formation of spilites, albite-chlorite slate, glaucophane slate, eclogites) and metamorphism associated with the development of plagiogranites (plagiomigmatites, plagiogneisses, albite mica slates) or normal potassium granites (migmatites, gneisses, mica slates, phyllites). The sodic nature of metamorphism in early geosynclinal development changes in the course of evolution of metamorphic zones in the direction of a greater role for potassium in the metamorphosing solutions. In depth zones metamorphism is often combined with areas of regional development of granitoid magmatism.

Metamorphism that occurs in the course of increasing temperatures is called progressive. It is accompanied by the loss of volatile components (dehydration, decarbonatization) by the initial rocks. The opposite processes against a background of decreasing temperature are termed regressive metamorphism. Repeated regressive metamorphism is called diaphthoresis.

REFERENCES

Korzhinskii, D. S. Faktory mineraVnykh ravnovesii i mineralogicheskie fatsii glubinnosti. [Moscow, 1940.]
Eliseev, N. A. Metamorfizm. [2nd ed.] Moscow, 1963.
Priroda metamorfizma. Moscow, 1967. (Translated from English.) Winkler, H. Genezis metamorficheskikh porod. Moscow, 1969. (Translated from German.)
Fatsii metamorfizma. Moscow, 1970.
A. A. MARAKUSHEV

metamorphism

[¦med·ə¦mȯr‚fiz·əm]
(petrology)
The mineralogical and structural changes of solid rock in response to environmental conditions at depth in the earth's crust.

metamorphism

the process by which metamorphic rocks are formed