Metamorphism 202831 221153909 2008-06-23T07:48:25Z SundarBot 648410 robot Adding: [[ta:வளருருமாற்றம்]] {{pagenumbers|article}} {{otheruses}} '''Metamorphism''' can be defined as the solid state recrystallisation of pre-existing [[Rock (geology)|rock]]s due to changes in heat and/or pressure and/or introduction of fluids i.e without melting. There will be [[mineral]]ogical, chemical and [[Crystallography|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 [[diagenesis|diagenetic]] changes, due to compaction, which result in [[sedimentary rock]]s. 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. [[Migmatite]]s are rocks formed on this borderline. 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 [[Erosion|eroded]] mountains. Conditions producing widespread regionally metamorphosed rocks occur during an [[orogeny|orogenic event]]. The collision of two [[Tectonic plate|continental plate]]s or [[island arc]]s 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 (geology)|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'''<br> [[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 facies|Zeolite]] *Mod - high T - low P : [[Prehnite-pumpellyite facies|Prehnite-Pumpellyite]] *High-P low T : [[Blueschist]] *Mod P - Mod to high T: [[Greenschist]] - [[Amphibolite]] - [[Granulite]] *High P - Mod - high T : [[Eclogite]] <!-- hornfels is not a facies by itself as it is a rock type; Rolinator --> ===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 group|Chlorite]] zone ::::[[Biotite]] zone :::::::[[Garnet]] zone ::::::::::[[Staurolite]] zone :::::::::::::[[Kyanite]] zone ::::::::::::::::[[Sillimanite]] zone ===Contact (thermal) metamorphism=== Contact metamorphism occurs typically around [[Intrusion (geology)|intrusive]] [[igneous rock]]s as a result of the temperature increase caused by the intrusion of [[magma]] into cooler [[country rock (geology)|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. [[Dike (geology)|Dikes]] generally have small aureoles with minimal metamorphism whereas large [[ultramafic to mafic layered intrusions|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 [[Pelite|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 reaction]]s. 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 [[greisen]]s 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 [[metasomatism|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 [[basalt]]s 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 [[volcano|volcanic eruption]]. Impact metamorphism is, therefore, characterized by ultrahigh pressure conditions and low temperature. The resulting minerals (such as SiO<small><sub>2</sub></small> [[polymorphism (materials science)|polymorphs]] [[coesite]] and [[stishovite]]) and textures are characteristic of these conditions. ===Dynamic metamorphism=== Dynamic metamorphism is associated with major [[Fault (geology)|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 [[porphyroblast]]s in mylonite zones. [[Thrust fault|Overthrust]]ing 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 [[radiometric dating|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. ==See also== *[[Metamorphic rock]] *[[Metasomatism]] *[[Recrystallization]] * [[Geothermobarometry]] * [[Ultrahigh-temperature metamorphism]] ==References== Eskola P. 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== *[http://csmres.jmu.edu/geollab/Fichter/MetaRx/MetaKind.html James Madison University: Metamorphism] *[http://www.brocku.ca/earthsciences/people/gfinn/petrology/barrow1.htm BARROVIAN METAMORPHISM: Brock Univ.] *[http://www.uwgb.edu/dutchs/PETROLGY/barrcarb.HTM Metamorphism of Carbonate Rocks: University of Wisconsin - Green Bay] [[Category:Geology]] [[Category:Geological processes]] [[de:Metamorphose (Geologie)]] [[et:Moone (geoloogia)]] [[es:Metamorfismo]] [[fr:Métamorphisme]] [[it:Metamorfismo]] [[he:התמרה]] [[lt:Metamorfizmas]] [[hu:Metamorfózis (kőzettan)]] [[nl:Metamorfose (geologie)]] [[ja:変成作用]] [[pl:Metamorfizm]] [[pt:Metamorfismo]] [[ru:Метаморфизм]] [[sk:Metamorfóza (geológia)]] [[fi:Metamorfoosi (geologia)]] [[ta:வளருருமாற்றம்]] [[uk:Метаморфізм]]