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For other uses, see Metamorphism (disambiguation).

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. In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere e without melting. There will be mineralogical, chemical and crystallographic changes. A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition a highly ordered atomic structure and specific Crystallography is the experimental science of determining the arrangement of Atoms in Solids In older usage it is the scientific study of Crystals The

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

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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. In Geology and Oceanography, diagenesis is any chemical physical or biological change undergone by a Sediment after its initial deposition and during Sedimentary rock is one of the three main rock types (the others being igneous and Metamorphic rock) 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 temperature interval is between 700 - 900°C, with pressures that depend on the composition of the rock. Migmatites are rocks formed on this borderline. Migmatite is a rock at the frontier between igneous and Metamorphic rocks They can also be known as diatexite. They present both melting and solid-state features.

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. The continental crust is the layer of granitic, Sedimentary, and Metamorphic rocks which form the Continents and the areas of shallow seabed Erosion is the carrying away or displacement of solids ( Sediment, Soil, rock and other particles usually by the agents of currents such as wind Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and 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. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere A volcanic arc is a chain of volcanic islands or Mountains formed by Plate tectonics as an oceanic Tectonic plate subducts under 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 mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other The techniques of structural geology are used to unravel the collisional history and determine the forces involved. Structural geology is the study of the three dimensional distribution of rock bodies and their planar or folded surfaces and their internal fabrics Regional metamorphism can be described and classified into metamorphic facies or zones of temperature/pressure conditions throughout the orogenic terrane. Historic definition The name facies was first used for specific Sedimentary environments in Sedimentary rocks by Swiss geologist Amanz

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. Historic definition The name facies was first used for specific Sedimentary environments in Sedimentary rocks by Swiss geologist Amanz The facies are named after the metamorphic rock formed under those facies conditions from basalt. Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. Facies relationships were first described by Eskola (1920).

Facies:


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. Zeolite facies describes the mineral assemblage resulting from the pressure and temperature conditions of low-grade Metamorphism. The prehnite-pumpellyite facies is a metamorphic facies typical of subseafloor alteration of the Oceanic crust around Mid-ocean ridge spreading centres Blueschist (ˈbluːʃɪst is a rock that forms by the Metamorphism of Basalt and rocks with similar composition at high Pressures and low Greenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered Mafic Volcanic rock Amphibolite (æmˈfɪbəlaɪt is the name given to a rock consisting mainly of hornblende Amphibole, the use of the term being restricted however to Metamorphic Granulites are fine to medium–grained Metamorphic rocks that have experienced high Temperatures of metamorphism composed mainly of Feldspars sometimes associated Eclogite (ˈɛklədʒaɪt is a coarse-grained Mafic ( Basaltic in composition Metamorphic rock. Slate is a fine-grained foliated homogeneous, Metamorphic rock derived from an original Shale -type Sedimentary rock composed of Clay Phyllite is a type of foliated Metamorphic rock primarily composed of Quartz, Sericite Mica, and chlorite; the rock represents The schists form a group of medium-grade Metamorphic rocks chiefly notable for the preponderance of lamellar Minerals such as Micas chlorite Gneiss (ˈnaɪs is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from preexisting formations that were originally Migmatite is a rock at the frontier between igneous and Metamorphic rocks They can also be known as diatexite. The chlorites are a group of phyllosilicate Minerals Chlorites can be described by the following four endmembers based on their chemistry via substitution Biotite is a common phyllosilicate Mineral within the Mica group with the approximate chemical formula K(Mg Fe3AlSi3O10(F The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives Staurolite is a red brown to black mostly opaque nesosilicate Mineral with a white streak Kyanite, whose name derives from the Greek word kyanos, meaning blue is a typically blue Silicate Mineral, commonly found in Aluminium -rich Sillimanite also called Bucholzite is an alumino-silicate Mineral with the Chemical formula Al2SiO5 In Geology, an intrusion is a body of Igneous rock that has Crystallized from molten Magma below the surface of the Earth. Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet Country rock is a geological term meaning the rock native to an area 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. Hornfels ( German, meaning "hornstone" is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat 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. A dike or dyke in Geology is a type of Sheet intrusion referring to any geologic body that cuts Discordantly ' across planar Ultramafic to mafic layered intrusions are found in typically ancient Cratons and are rare but worldwide in distribution

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. Pelite ( Greek Pelos, Clay is a descriptive name for a Clastic rock with a grain size of less than 1/16mm (originally sand or silt Examples include 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. A metamorphic reaction is a Chemical reaction that takes place during the Geological process of Metamorphism in an amalgamate of Minerals that Extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks. In this case the metamorphism grades into metasomatism. Metasomatism is the chemical alteration of a rock by Hydrothermal and other fluids If the intruded rock is rich in carbonate the result is a skarn. In Chemistry, a carbonate is a salt or Ester of Carbonic acid. Skarn is a Metamorphic rock that is usually variably colored green or red occasionally grey black brown or white Fluorine-rich magmatic waters which leave a cooling granite may often form greisens within and adjacent to the contact of the granite. Fluorine, fluorum meaning "to flow" is the Chemical element with the symbol F and Atomic number 9 Greisen is a highly altered granitic rock or Pegmatite. Greisen is formed by autogenic alteration of a granite and is a class of endoskarn. Metasomatic altered aureoles can localize the deposition of metallic ore minerals and thus are of economic interest. An ore is a volume of rock containing components or Minerals in a mode of occurrence that renders it valuable for mining

Hydrothermal metamorphism

Hydrothermal metamorphism is the result of the interaction of a rock with a high-temperature fluid of variable composition. Hydrothermal circulation in its most general sense is the circulation of hot water 'hydros' in the Greek meaning water and 'thermos' meaning heat The difference in composition between existing rock and the invading fluid triggers a set of metamorphic and metasomatic reactions. Metasomatism is the chemical alteration of a rock by Hydrothermal and other fluids The hydrothermal fluid may be magmatic (originate in an intruding magma), circulating groundwater, or ocean water. Groundwater is Water located beneath the Ground surface in Soil pore spaces and in the Fractures of lithologic formations Convective circulation of water in the ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas. Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. 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. A meteorite is a natural object originating in Outer space that survives an impact with the Earth 's surface Plate tectonics and hotspots Divergent plate boundaries At the 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. Polymorphism in Materials science is the ability of a solid material to exist in more than one form or Crystal structure Coesite is a form ( polymorph) of Silicon dioxide Si[[oxygen O]]2 that is formed when very high pressure (2&ndash3 Gigapascals and Stishovite (after SM Stishov 20th-century Russian mineralogist is an extremely hard dense Tetragonal form ( polymorph) of Silicon dioxide.

Dynamic metamorphism

Dynamic metamorphism is associated with major fault planes. In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement 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. Cataclastic rocks contain angular fragments formed by cataclasis

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. Breccia (ˈbrɛtʃiə ˈbrɛʃiə breach is a rock composed of angular fragments of several Minerals or rocks in a matrix, that is a cementing material Cataclasite is a metamorphic rock that is formed by mechanical Shear stress during faulting. This generally forms a mélange. This article is about the geological usage for other uses see Melange (disambiguation. 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. Pseudotachylite is a fault rock that has the appearance of the Basaltic Glass, Tachylyte. 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. Study of geological shear is related to the study of Structural geology, Rock microstructure or rock texture and fault mechanics. Here, deformation forms mylonite, with dynamothermal metamorphism observed rarely as the growth of porphyroblasts in mylonite zones. Mylonite is a fine-grained compact rock produced by dynamic crystallization of the constituent minerals resulting in a A porphyroblast is a large Mineral Crystal in a Metamorphic rock which has grown within the finer grained groundmass.

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. A thrust fault is a type of fault, or break in the Earth's crust with resulting movement of each side against the other in which a lower stratigraphic position is pushed up 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. Radiometric dating (often called radioactive dating) is a technique used to date materials usually based on a comparison between the observed abundance of a naturally occurring

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. Paragenesis is a petrologic concept meaning an equilibrium assemblage of mineral phases. 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.

See also

References

Eskola P. Metamorphic rock is the result of the transformation of an existing rock type the protolith, in a process called Metamorphism, which means "change Metasomatism is the chemical alteration of a rock by Hydrothermal and other fluids Recrystallization (see also Crystallization) is a physical process that has meanings in Chemistry, Metallurgy and Geology. Geothermobarometry is the science of measuring the previous pressure and temperature history of a Metamorphic or intrusive Igneous rocks 1920. The mineral facies of rocks. Norsk. Geol. Tidsskr. , 6, 143-194.

Winter J. D. , 2001. An introduction to Igneous and Metamorphic Petrology. Prentice-Hall Inc. , 695 pages. ISBN 0-13-240342-0.

External links

Dictionary

metamorphism

-noun

  1. (mineralogy) The process by which rocks are changed into other forms by the application of heat and/or pressure.
  2. (zoology) The process by which insects development through life stages -- as, for example, those of embryo, larva, pupa and imago. The life cycle of the butterfly is one of complete metamorphosis, in which the embryo grows within the egg, hatches into the larval stage caterpillar, enters the pupal stage within its chrysalis, and finally emerges as an adult butterfly imago.
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