metamorphism

[met-uh-mawr-fiz-uhm]

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

Mineralogic and structural changes in solid rocks caused by physical conditions different from those under which the rocks originally formed. Changes produced by surface conditions such as compaction are usually excluded. The most important agents of metamorphism are temperature (from 300°–2,200°F, or 150°–1200°C), pressure (from 10 to several hundred kilobars, or 150,000 to several million lbs. per sq in.), and stress. Dynamic metamorphism results from mechanical deformation with little long-term temperature change. Contact metamorphism results from increases in temperature with minor differential stress, is highly localized, and may occur relatively quickly. Regional metamorphism results from the general increase, usually correlated, of temperature and pressure over a large area and a long period of time, as in mountain-building processes. Seealso metamorphic rock.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Metamorphism

Metamorphism can be defined as the solid state recrystallisation of pre-existing rocks due to changes in heat and/or pressure and/or introduction of fluids i.e without melting. There will be mineralogical, chemical and crystallographic changes.

Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.

Limits of metamorphism

The temperature lower limit of metamorphism is considered to be between 100 - 150°C, to exclude diagenetic changes, due to compaction, which result in sedimentary rocks. There is no agreement as for a pressure lower limit. Some workers argue that changes in atmospheric pressures are not metamorphic, but some types of metamorphism can occur at extremely low pressures (see below).

The upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The maximum temperature for metamorphism is typically between 700 - 900°C, depending on the pressure and on the composition of the rock. Migmatites are rocks formed at this upper limit, which contain pods and veins of material that has started to melt but has not fully segregated from the refractory residue. Since the 1980s, it has been recognized that rarely, rocks are dry enough, and of a refractory enough composition, to record without melting "ultrahigh" metamorphic temperatures of 900 - 1100°C.

Kinds of metamorphism

Regional metamorphism

Regional or Barrovian metamorphism covers large areas of continental crust typically associated with mountain ranges, particularly subduction zones or the roots of previously eroded mountains. Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism. These orogenic mountains are later eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone also produce regional metamorphic effects. The techniques of structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into metamorphic facies or zones of temperature/pressure conditions throughout the orogenic terrane.

Metamorphic facies
Metamorphic facies are recognizable terranes or zones with an equilibrium assemblage of key minerals that were in equilibrium under specific range of temperature and pressure during a metamorphic event. The facies are named after the metamorphic rock formed under those facies conditions from basalt. Facies relationships were first described by Eskola (1920).

Facies:

Low T - low P : Zeolite
Mod - high T - low P : Prehnite-Pumpellyite
High-P low T : Blueschist
Mod P - Mod to high T: Greenschist - Amphibolite - Granulite
High P - Mod - high T : Eclogite

Metamorphic grades

In the Barrovian sequence (described by George Barrow in zones of progressive metamorphism in Scotland), metamorphic grades are also classified by mineral assemblage based on the appearance of key minerals in rocks of pelitic (shaly, aluminous) origin:

Low grade ------------------- Intermediate --------------------- High grade

Greenschist ------------- Amphibolite ----------------------- Granulite

Slate --- Phyllite ---- Schist --------- Gneiss -----------------------Migmatite(partial melting) >>>melt

Chlorite zone

Biotite zone

Garnet zone

Staurolite zone

Kyanite zone

Sillimanite zone

Contact (thermal) metamorphism

Contact metamorphism occurs typically around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock. The area surrounding the intrusion (called aureoles) where the contact metamorphism effects are present is called the metamorphic aureole. Contact metamorphic rocks are usually known as hornfels. Rocks formed by contact metamorphism may not present signs of strong deformation and are often fine-grained.

Contact metamorphism is greater adjacent to the intrusion and dissipates with distance from the contact. The size of the aureole depends on the heat of the intrusive, its size, and the temperature difference with the wall rocks. Dikes generally have small aureoles with minimal metamorphism whereas large ultramafic intrusions can have significantly thick and well-developed contact metamorphism.

The metamorphic grade of an aureole is measured by the peak metamorphic mineral which forms in the aureole. This is usually related to the metamorphic temperatures of pelitic or alumonisilicate rocks and the minerals they form. The metamorphic grades of aureoles are andalusite hornfels, sillimanite hornfels, pyroxene hornfels.

Magmatic fluids coming from the intrusive rock may also take part in the metamorphic reactions. Extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks. In this case the metamorphism grades into metasomatism. If the intruded rock is rich in carbonate the result is a skarn. Fluorine-rich magmatic waters which leave a cooling granite may often form greisens within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic ore minerals and thus are of economic interest.

Hydrothermal metamorphism

Hydrothermal metamorphism is the result of the interaction of a rock with a high-temperature fluid of variable composition. The difference in composition between existing rock and the invading fluid triggers a set of metamorphic and metasomatic reactions. The hydrothermal fluid may be magmatic (originate in an intruding magma), circulating groundwater, or ocean water. Convective circulation of water in the ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas. The patterns of this hydrothermal alteration is used as a guide in the search for deposits of valuable metal ores.

Impact metamorphism

This kind of metamorphism occurs when either an extraterrestrial object (a meteorite for instance) collides with the Earth's surface or during an extremely violent volcanic eruption. Impact metamorphism is, therefore, characterized by ultrahigh pressure conditions and low temperature. The resulting minerals (such as SiO₂ polymorphs coesite and stishovite) and textures are characteristic of these conditions.

Dynamic metamorphism

Dynamic metamorphism is associated with major fault planes. Metamorphism is localised adjacent to the fault plane and is caused by frictional heat generated by the fault movement. Cataclasis, crushing and grinding of rocks into angular fragments, occurs in dynamic metamorphic zones, giving cataclastic texture.

The textures of dynamic metamorphic zones are dependent on the depth at which they were formed, as the confining pressure determines the deformation mechanisms which predominate. Within depths less than 5km, dynamic metamorphism is not often produced because the confining pressure is too low to produce frictional heat. Instead, a zone of breccia or cataclasite is formed, with the rock milled and broken into random fragments. This generally forms a mélange. At depth, the angular breccias transit into a ductile shear texture and into mylonite zones.

Within the depth range of 5-10km pseudotachylite is formed, as the confining pressure is enough to prevent brecciation and milling and thus energy is focused into discrete fault planes. The frictional heating in this case may melt the rock to form pseudotachylite glass or mylonite, and adjacent to these zones, result in growth of new mineral assemblages.

Within the depth range of 10-20km, deformation is governed by ductile deformation conditions and hence frictional heating is dispersed throughout shear zones, resulting in a weaker thermal imprint and distributed deformation. Here, deformation forms mylonite, with dynamothermal metamorphism observed rarely as the growth of porphyroblasts in mylonite zones.

Overthrusting may juxtapose hot lower crustal rocks against cooler mid and upper crust blocks, resulting in conductive heat transfer and localised contact metamorphism of the cooler blocks adjacent to the hotter blocks, and often retrograde metamorphism in the hotter blocks. The metamorphic assemblages in this case are diagnostic of the depth and temperature and the throw of the fault and can also be dated to give an age of the thrusting.

Prograde and retrograde metamorphism

Metamorphism is further divided into prograde and retrograde metamorphism. Prograde metamorphism involves the change of mineral assemblages (paragenesis) with increasing temperature and (usually) pressure conditions. These are solid state dehydration reactions, and involve the loss of volatiles such as water or carbon dioxide. Prograde metamorphism results in a rock representing the maximum pressure and temperature experienced. These rocks often return to the surface without undergoing retrograde metamorphism , where the mineral assemblages would become more stable under lower pressures and temperatures.

Retrograde metamorphism involves the reconstitution of a rock under decreasing temperatures (and usually pressures) where revolatisation occurs; allowing the mineral assemblages formed in prograde metamorphism to return to more stable minerals at the lower pressures. This is a relatively uncommon process, because volatiles must be present for retrograde metamorphism to occur. Most metamorphic rocks return to the surface as a representation of the maximum pressures and temperatures they have undergone.