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metamorphism

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.

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Metamorphism

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

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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.

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metamorphism

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

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Metamorphism

Metamorphism

Types of metamorphism

Current research in metamorphism

Resources

Metamorphism is the process by which the structure and mineral content of rocks transform in response to changes in temperature, pressure, fluid content (gas or water), or a combination of these. Because the minerals that make up rocks are stable only within certain ranges of temperature and pressure, large changes in these conditions cause minerals to change chemically or to change shape, or both. Minerals that form during metamorphism include varieties of garnet, mica, amphibole, and serpentine. Metamorphism produces characteristic textures in metamorphic rocks, the type of rocks that have undergone metamorphism, such as alignment of elongate crystals or differentiation of different minerals into layers. Distinctive minerals and textures are keys to distinguishing rocks that have experienced metamorphism from unmetamorphosed sedimentary and igneous rocks.

Metamorphism has captured the interest of geologists for many years. James Dana, a noted American geologist, wrote about the alteration of rocks by metamorphism in his Manual of Geology, first published in 1862. By the 1920, the Finnish geologist Pentii Eskola began to note differences in the degree of metamorphism of rocks in different areas on the basis of the different groups of minerals that typically occur together in metamorphic rocks.

The minerals that typically occur in distinctive groups in metamorphic rocks are known as metamorphic facies. Metamorphic facies reflect different conditions during metamorphism. For example, ongoing metamorphism of shale at continuously increasing temperature and pressure initially produces slate, then phyllite, schist, and gneiss. As the pressure and temperature change, the structures and minerals in rocks change to forms that are stable in those conditions. Thus, by studying the minerals present in an area, scientists can estimate the pressure and temperature at which metamorphism occurred.

Matching metamorphic rocks to their unmetamorphosed precursors is not always easy. However, some metamorphic rocks typically form from certain precursor rocks. For example, marble is a metamorphic rock that forms during metamorphism of limestone, a sedimentary rock. Metamorphism of other sedimentary rock, such as shale, can produce slate. Sandstone metamorphoses into quartzite. Granite, an igneous rock, can become gneiss during metamorphism.

Products of metamorphism occur worldwide. Familiar examples include slate roofs and marble floors, garnet jewelry, and asbestos insulation made from serpentine and amphibole minerals. Because of their unusual minerals and structures, as well as their association with majestic mountain ranges, metamorphic rocks are among the most beautiful.

Types of metamorphism

Scientists recognize several types of metamorphism: regional metamorphism, contact metamorphism, dynamic metamorphism, and hydrothermal metamorphism. These occur between the low-temperature process of diagenesis (temperature above 392°F [200°C] and pressure greater than 1,000 bars) and the high temperatures at which rocks melt and later cool to form igneous rocks (approximately 1,1121,472°F [600800°C] in temperature and more than 10,000 bars pressure).

Regional metamorphism, the wide-scale alteration of rocks during major tectonic events, can produce spectacular textures and structures in rocks, including folds of layers of rocks, folds of individual minerals (mica, for example), and rotated garnet crystals. Examples of regional metamorphism abound, from Acadia National Park in Maine and the Appalachian Mountains in the eastern part of the United States, to the Llano Uplift of central Texas, and the Precambrian rocks of the Grand Canyon. The Alps of Europe and the Himalayas of Asia also show effects of regional metamorphism.

Contact metamorphism, or thermal metamorphism, occurs when heat from igneous intrusions, melted rocks that move upward, come in contact with cooler rocks above. The cooler rocks do not melt, but recrystallize as a result of heating. The Palisades sill, an igneous intrusion, produced contact

KEY TERMS

Diagenesis Compaction, cementation, and other processes that transform sediments into sedimentary rock at low temperatures.

Intrusion Movement of melted rock into solid rock. The heat from the melted material can cause contact metamorphism of the solid rock.

Isotopes Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties. Uranium has three naturally occurring isotopes, uranium-238, uranium-235, and uranium-234.

Metamorphic facies A group of metamorphic minerals typically found together. Different metamorphic facies form at different temperatures and pressures.

Mineral A naturally occurring substance with a distinct chemical composition and structure. Quartz, magnetite, calcite, and garnet are minerals.

Regional metamorphism Widespread change in temperature and pressure that alters rock, usually associated with tectonic events.

Rock A naturally occurring solid mixture of minerals.

Tectonic event Episode of movement or deformation of the large plates of oceanic and continental crust that cover Earth. Mountain building and regional metamorphism can result from tectonic events.

metamorphism in the rocks into which it intruded, and is well exposed beneath the George Washington Bridge near New York City.

Dynamic metamorphism occurs along faults that have zones of intense pressure. Rocks along faults grind past each other during faulting. The finely ground rock in the fault can recrystallize under pressure, especially if friction along the fault produces heat or if hot fluids move through the fault.

Hydrothermal metamorphism requires the presence of hot fluids derived from igneous rock nearby. The fluids react with minerals in the surrounding rock to produce different minerals. The metamorphic mineral serpentine forms when dense igneous rocks, such as those in the oceanic crust, metamorphose in the presence of hot fluids to form less dense metamorphic rock called serpentinite.

Current research in metamorphism

Current research in the field of metamorphism ranges from studies of the chemical composition of single crystals within metamorphic rocks (garnet, for example) to the mysteries of metamorphic rocks that form at extremely high pressure, presumably deep within Earth, and now exist at Earths surface without changes in the minerals in the rocks as the pressure decreased during uplift to the surface. The effects of fluids on metamorphism continue to attract the attention of researchers.

Studies of the ratios of unstable isotopes (for example, uranium, rubidium, strontium, and argon) allow scientists to determine the times at which rocks metamorphosed. Such data are valuable in studies of regional metamorphism, particularly in areas that have experienced multiple episodes of metamorphism.

Economically important metamorphic minerals such as serpentine can affect health. In the past few years, asbestos removal has had significant impact on the cost of operating schools and other public buildings. In the confusion over illness associated with asbestos made from the amphibole mineral crocidolite, citizens demanded the removal of all asbestos, unaware that a less-hazardous form of asbestos, the serpentine mineral chrysotile, also was removed at great expense.

Research in the field of metamorphism continues to include traditional geological activities such as preparing maps of surface exposures of metamorphic rocks from field studies, observing thin slices of metamorphic rocks using microscopes, and assessing the time, temperature, and pressure at which metamorphism occurs. New technology, particularly lasers and x-ray tomography, allow scientists to examine rocks and single crystals in sufficient detail to understand how crystals grow during metamorphism and at what temperatures and pressures they grow.

Resources

BOOKS

Blatt, H., R. Tracy, and B. Owens. Petrology: Igneous, Sedimentary, and Metamorphic. New York: Freeman, 2005.

Fowler, C.M.R. The Solid Earth: An Introduction to Global Geophysics. Cambridge, United Kingdon: Cambridge University Press, 2004.

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, NJ: Prentice Hall, 2004.

Gretchen M. Gillis

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Metamorphism

Metamorphism

Metamorphism is the process by which the structure and mineral content of rocks transform in response to changes in temperature , pressure , fluid content (gas or water ), or a combination of these. Because the minerals that make up rocks are stable only within certain ranges of temperature and pressure, large changes in these conditions cause minerals to change chemically or to change shape, or both. Minerals that form during metamorphism include spectacular varieties of garnet, mica, amphibole, and serpentine, to name a few. Metamorphism produces characteristic textures in metamorphic rocks, the type of rocks that have undergone metamorphism, such as alignment of elongate crystals or differentiation of different minerals into layers. Distinctive minerals and textures are keys to distinguishing rocks that have experienced metamorphism from unmetamorphosed sedimentary and igneous rocks .

Metamorphism has captured the interest of geologists for many years. James Dana, a noted American geologist, wrote about the alteration of rocks by metamorphism in his Manual of Geology, first published in 1862. By the 1920, the Finnish geologist Pentii Eskola began to note differences in the degree of metamorphism of rocks in different areas on the basis of the different groups of minerals that typically occur together in metamorphic rocks.

The minerals that typically occur in distinctive groups in metamorphic rocks are known as metamorphic facies. Metamorphic facies reflect different conditions during metamorphism. For example, ongoing metamorphism of shale at continuously increasing temperature and pressure initially produces slate, then phyllite, schist, and gneiss. As the pressure and temperature change, the structures and minerals in rocks change to forms that are stable in those conditions. Thus, by studying the minerals present in an area, scientists can estimate the pressure and temperature at which metamorphism occurred.

Matching metamorphic rocks to their unmetamorphosed precursors is not always easy. However, some metamorphic rocks typically form from certain precursor rocks. For example, marble is a metamorphic rock that forms during metamorphism of limestone, a sedimentary rock . Metamorphism of other sedimentary rock, such as shale, can produce slate. Sandstone metamorphoses into quartzite. Granite, an igneous rock, can become gneiss during metamorphism.

Products of metamorphism occur worldwide. Familiar examples include slate roofs and marble floors, garnet jewelry, and asbestos insulation made from serpentine and amphibole minerals. Because of their unusual minerals and structures, as well as their association with majestic mountain ranges, metamorphic rocks are among the most beautiful.


Types of metamorphism

Scientists recognize several types of metamorphism: regional metamorphism, contact metamorphism, dynamic metamorphism, and hydrothermal metamorphism. These occur between the low-temperature process of diagenesis (temperature above 392°F [200°C] and pressure greater than 1,000 bars) and the high temperatures at which rocks melt and later cool to form igneous rocks (approximately 1,112–1,472°F [600–800°C] in temperature and more than 10,000 bars pressure).

Regional metamorphism, the wide-scale alteration of rocks during major tectonic events, can produce spectacular textures and structures in rocks, including folds of layers of rocks, folds of individual minerals (mica, for example), and rotated garnet crystals. Examples of regional metamorphism abound, from Acadia National Park in Maine and the Appalachian Mountains in the eastern part of the United States, to the Llano Uplift of central Texas, and the Precambrian rocks of the Grand Canyon. The Alps of Europe and the Himalayas of Asia also show effects of regional metamorphism.

Contact metamorphism, or thermal metamorphism, occurs when heat from igneous intrusions, melted rocks that move upward, come in contact with cooler rocks above. The cooler rocks do not melt, but recrystallize as a result of heating. The Palisades sill, an igneous intrusion, produced contact metamorphism in the rocks into which it intruded, and is well exposed beneath the George Washington Bridge near New York City.

Dynamic metamorphism occurs along faults that have zones of intense pressure. Rocks along faults grind past each other during faulting. The finely ground rock in the fault can recrystallize under pressure, especially if friction along the fault produces heat or if hot fluids move through the fault.

Hydrothermal metamorphism requires the presence of hot fluids derived from igneous rock nearby. The fluids react with minerals in the surrounding rock to produce different minerals. The metamorphic mineral serpentine forms when dense igneous rocks, such as those in the oceanic crust, metamorphose in the presence of hot fluids to form less dense metamorphic rock called serpentinite.

Current research in metamorphism

Current research in the field of metamorphism ranges from studies of the chemical composition of single crystals within metamorphic rocks (garnet, for example) to the mysteries of metamorphic rocks that form at extremely high pressure, presumably deep within the Earth, and now exist at Earth's surface without changes in the minerals in the rocks as the pressure decreased during uplift to the surface. The effects of fluids on metamorphism continue to attract the attention of researchers.

Studies of the ratios of unstable isotopes (for example, uranium , rubidium, strontium, and argon) allow scientists to determine the times at which rocks metamorphosed. Such data are valuable in studies of regional metamorphism, particularly in areas that have experienced multiple episodes of metamorphism.

Economically important metamorphic minerals such as serpentine can affect health. In the past few years, asbestos removal has had significant impact on the cost of operating schools and other public buildings. In the confusion over illness associated with asbestos made from the amphibole mineral crocidolite, citizens demanded the removal of all asbestos, unaware that a less-hazardous form of asbestos, the serpentine mineral chrysotile, also was removed at great expense.

Research in the field of metamorphism continues to include traditional geological activities such as preparing maps of surface exposures of metamorphic rocks from field studies, observing thin slices of metamorphic rocks using microscopes, and assessing the time , temperature, and pressure at which metamorphism occurs. New technology, particularly lasers and x-ray tomography, allow scientists to examine rocks and single crystals in sufficient detail to understand how crystals grow during metamorphism and at what temperatures and pressures they grow.


Resources

periodicals

Abelson, Phillip H. "The Asbestos Removal Fiasco." Science 247 (1990): 1017.

Lang, Helen M. "Metamorphic Petrology." Geotimes 40 (1995): 44-45.


Gretchen M. Gillis

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Diagenesis

—Compaction, cementation, and other processes that transform sediments into sedimentary rock at low temperatures.

Intrusion

—Movement of melted rock into solid rock. The heat from the melted material can cause contact metamorphism of the solid rock.

Isotopes

—Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties. Uranium has three naturally occurring isotopes, uranium-238, uranium-235, and uranium-234.

Metamorphic facies

—A group of metamorphic minerals typically found together. Different metamorphic facies form at different temperatures and pressures.

Mineral

—A naturally occurring substance with a distinct chemical composition and structure. Quartz, magnetite, calcite, and garnet are minerals.

Regional metamorphism

—Widespread change in temperature and pressure that alters rock, usually associated with tectonic events.

Rock

—A naturally occurring solid mixture of minerals.

Tectonic event

—Episode of movement or deformation of the large plates of oceanic and continental crust that cover Earth. Mountain building and regional metamorphism can result from tectonic events.

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http://www.chicagomanualofstyle.org/tools_citationguide.html

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