Editing Metamorphic rock (section)

== Occurrence ==
{{Further|Metamorphism#Types}}
Metamorphic rocks make up a large part of the [[Crust (geology)#Earth's crust|Earth's crust]] and form 12% of the Earth's land surface.<ref name="Wilkinson2008"/> The lower [[continental crust]] is mostly metamafic-rock and pelite which have reached the [[granulite facies]]. The middle continental crust is dominated by metamorphic rock that has reached the amphibolite facies.<ref>{{cite journal |last1=Rudnick |first1=Roberta L. |last2=Fountain |first2=David M. |title=Nature and composition of the continental crust: A lower crustal perspective |journal=Reviews of Geophysics |date=1995 |volume=33 |issue=3 |pages=267 |doi=10.1029/95RG01302|bibcode=1995RvGeo..33..267R }}</ref> Within the upper crust, which is the only part of the [[Earth's crust]] geologists can directly sample, metamorphic rock forms only from processes that can occur at shallow depth. These are ''contact (thermal) metamorphism'', ''dynamic (cataclastic) metamorphism'', ''hydrothermal metamorphism'', and ''impact metamorphism''. These processes are relatively local in occurrence and usually reach only the low-pressure facies, such as the [[Hornfels facies|hornfels]] and [[sanidinite facies]]. Most metamorphic rock is formed by ''regional metamorphism'' in the middle and lower crust, where the rock reaches the higher-pressure metamorphic facies. This rock is found at the surface only where extensive uplift and erosion has [[Exhumation (geology)|exhumed]] rock that was formerly much deeper in the crust.{{sfn|Yardley|1989|pp=12-13}}

===Orogenic belts===
{{Further|Orogeny}}
Metamorphic rock is extensively exposed in [[Orogeny|orogenic belts]] produced by the collision of tectonic plates at [[convergent boundaries]]. Here formerly deeply buried rock has been brought to the surface by uplift and erosion.{{sfn|Levin|2010|pp=76-77, 82-83}} The metamorphic rock exposed in orogenic belts may have been metamorphosed simply by being at great depths below the Earth's surface, subjected to high temperatures and the great pressure caused by the immense weight of the rock layers above. This kind of regional metamorphism is known as ''burial metamorphism''. This tends to produce low-grade metamorphic rock.<ref>{{cite journal |last1=Robinson |first1=D. |last2=Bevins |first2=R. E. |last3=Aguirre |first3=L. |last4=Vergara |first4=M. |title=A reappraisal of episodic burial metamorphism in the Andes of central Chile |journal=Contributions to Mineralogy and Petrology |date=1 January 2004 |volume=146 |issue=4 |pages=513–528 |doi=10.1007/s00410-003-0516-4|bibcode=2004CoMP..146..513R |s2cid=140567746 }}</ref> Much more common is metamorphic rock formed during the collision process itself.{{sfn|Yardley|1989|p=12}} The collision of plates causes high temperatures, pressures and deformation in the rocks along these belts.<ref>{{cite book |last1=Kearey |first1=P. |last2=Klepeis |first2=Keith A. |last3=Vine |first3=Frederick J. |title=Global tectonics. |date=2009 |publisher=Wiley-Blackwell |location=Oxford |isbn=9781405107778 |pages=275–279 |edition=3rd}}</ref> Metamorphic rock formed in these settings tends to shown well-developed schistosity.{{sfn|Yardley|1989|p=12}}

Metamorphic rock of orogenic belts shows a variety of metamorphic facies. Where [[subduction]] is taking place, the basalt of the subducting slab is  metamorphosed to high-pressure metamorphic facies. It initially undergoes low-grade metamorphism to metabasalt of the [[Zeolite facies|zeolite]] and [[prehnite-pumpellyite facies]], but as the basalt subducts to greater depths, it is metamorphosed to the [[blueschist facies]] and then the [[eclogite facies]]. Metamorphism to the eclogite facies releases a great deal of water vapor from the rock, which drives [[volcanism]] in the overlying [[volcanic arc]]. Eclogite is also significantly denser than blueschist, which drives further subduction of the slab deep into the [[Earth's mantle]]. Metabasalt and blueschist may be preserved in blueschist metamorphic belts formed by collisions between continents. They may also be preserved by [[obduction]] onto the overriding plate as part of [[ophiolite]]s.{{sfn|Kearey|Klepeis|Vine|2009|pp=275-279}} Eclogites are occasionally found at sites of continental collision, where the subducted rock is rapidly brought back to the surface, before it can be converted to the granulite facies in the hot upper mantle. Many samples of eclogite are [[xenolith]]s brought to the surface by volcanic activity.{{sfn|Kearey|Klepeis|Vine|2009|pp=367-368}}

Many orogenic belts contain higher-temperature, lower-pressure metamorphic belts. These may form through heating of the rock by ascending magmas of volcanic arcs, but on a regional scale. Deformation and crustal thickening in an orogenic belt may also produce these kinds of metamorphic rocks. These rocks reach the [[Greenschist facies|greenschist]], amphibolite, or granulite facies and are the most common of metamorphic rocks produced by regional metamorphosis. The association of an outer high-pressure, low-temperature metamorphic zone with an inner zone of low-pressure, high-temperature metamorphic rocks is called a ''[[paired metamorphic belts|paired metamorphic belt]]''. The main islands of Japan show three distinct paired metamorphic belts, corresponding to different episodes of subduction.<ref>{{cite book |last1=Miyashiro |first1=Akiho |title=Metamorphism and Metamorphic Belts |date=1973 |publisher=Springer Netherlands |location=Dordrecht |isbn=9789401168366}}</ref>{{sfn|Kearey|Klepeis|Vine|2009|pp=368-369}}

===Metamorphic core complexes===
{{main|Metamorphic core complex}}
Metamorphic rock is also exposed in [[metamorphic core complex]]es, which form in region of crustal extension. They are characterized by low-angle faulting that exposes domes of middle or lower crust metamorphic rock. These were first recognized and studied in the [[Basin and Range Province]] of southwestern North America,<ref>{{cite book |editor-last1=Crittenden |editor-first1=M.D. |editor-last2=Coney |editor-first2=P.J. |editor-last3=Davis |editor-first3=G.H. |editor-last4=Davis |editor-first4=G.H. |year=1980 |title=Cordilleran metamorphic core complexes (Memoir 153) |publisher=Geological Society of America |isbn=978-0813711539}}</ref> but are also found in southern [[Aegean Sea]], in the [[D'Entrecasteaux Islands]], and in other areas of extension.{{sfn|Kearey|Klepeis|Vine|2009|p=169}}

===Granite-greenstone belts===
[[Shield (geology)|Continental shields]] are regions of exposed ancient rock that make up the stable cores of continents. The rock exposed in the oldest regions of shields, which is of [[Archean]] age (over 2500 million years old), mostly belong to granite-greenstone belts. The [[greenstone belt]]s contain metavolcanic and metasedimentary rock that has undergone a relatively mild grade of metamorphism, at temperatures of {{cvt|350-500|C||}} and pressures of {{cvt|200-500|MPa|bar}}. They can be divided into a lower group of metabasalts, including rare meta[[komatiite]]s; a middle group of meta-intermediate-rock and meta-felsic-rock; and an upper group of metasedimentary rock.{{sfn|Kearey|Klepeis|Vine|2009|p=350}}

The greenstone belts are surrounded by high-grade gneiss terrains showing highly deformed low-pressure, high-temperature (over {{cvt|500|C||}}) metamorphism to the amphibolite or granulite facies. These form most of the exposed rock in Archean cratons.{{sfn|Kearey|Klepeis|Vine|2009|p=350}}

The granite-greenstone belts are intruded by a distinctive group of granitic rocks called the [[tonalite]]-[[trondhjemite]]-[[granodiorite]] or TTG suite. These are the most voluminous rocks in the craton and may represent an important early phase in the formation of continental crust.{{sfn|Kearey|Klepeis|Vine|2009|p=350}}

=== Mid-ocean ridges ===
[[Mid-ocean ridges]] are where new [[oceanic crust]] is formed as tectonic plates move apart. Hydrothermal metamorphism is extensive here. This is characterized by metasomatism by hot fluids circulating through the rock. This produces metamorphic rock of the greenschist facies. The metamorphic rock, [[serpentinite]], is particularly characteristic of these settings, and represents chemical transformation of olivine and pyroxene in ultramafic rock to [[serpentine group]] minerals.{{sfn|Kearey|Klepeis|Vine|2009|pp=28-29, 129-131}}{{Sfn|Yardley|1989|p=12}}

=== Contact aureoles ===
[[Image:Eozoon01.jpg|thumb|A contact metamorphic rock made of interlayered [[calcite]] and [[Serpentine group|serpentine]] from the [[Precambrian]] of Canada. Once thought to be a [[pseudofossil]] called ''[[Eozoön]] canadense''.  Scale in mm.]]
[[File:Rock contact metamorphism eng big text.jpg|thumbnail]]
Contact metamorphism takes place when magma is injected into the surrounding solid rock ([[country rock (geology)|country rock]]).{{sfn|Yardley|1989|p=12}} The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it. Around the igneous rock that forms from the cooling magma is a metamorphosed zone called a ''[[contact aureole]]''. Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed (unchanged) country rock some distance away. The formation of important [[ore]] minerals may occur by the process of [[metasomatism]] at or near the contact zone.<ref>{{cite book |last1=Marshak |first1=Stephen |year=2009 |title=Essentials of Geology |publisher=W. W. Norton & Company |edition=3rd |isbn=978-0393196566}}</ref> Contact aureoles around large [[pluton]]s may be as much as several kilometers wide.<ref>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd |page=427}}</ref>

The term ''[[hornfels]]'' is often used by geologists to signify those fine grained, compact, non-foliated products of contact metamorphism.<ref name=EB1911>{{EB1911|wstitle=Petrology |volume=21 |page=332–33 |inline=1 |first=John Smith |last=Flett}}</ref> The contact aureole typically shows little deformation, and so hornfels is usually devoid of schistosity and forms a tough, equigranular rock. If the rock was originally banded or [[Foliation (geology)|foliated]] (as, for example, a laminated sandstone or a foliated calc-[[schist]]) this character may not be obliterated, and a banded hornfels is the product.<ref name=EB1911/> Contact metamorphism close to the surface produces distinctive low-pressure metamorphic minerals,{{sfn|Yardley|1989|p=12}} such as [[spinel]], andalusite, [[vesuvianite]], or [[wollastonite]].{{sfn|Klein|Hurlbut|1993|pp=385, 456, 466, 485}}

Similar changes may be induced in shales by the burning of [[coal]] seams.<ref name=EB1911/> This produces a rock type named ''clinker''.<ref>{{cite journal |last1=Milligan |first1=Mark |title=Geosights: Colorful coal "clinker" close to Castle Gate, Carbon County |journal=Survey Notes |date=September 2007 |volume=39 |issue=3 |url=https://geology.utah.gov/map-pub/survey-notes/geosights/coal-clinker-castle-gate/ |access-date=28 February 2021}}</ref>

There is also a tendency for [[metasomatism]] between the igneous magma and sedimentary country rock, whereby the chemicals in each are exchanged or introduced into the other.  In that case, hybrid rocks called [[skarn]] arise.<ref name=EB1911/>{{sfn|Yardley|1989|p=126}}

===Other occurrences===
Dynamic (cataclastic) metamorphism takes place locally along [[fault (geology)|faults]]. Here intense shearing of the rock typically forms mylonites.{{Sfn|Yardley|1989|p=12}}

Impact metamorphism is unlike other forms of metamorphism in that it takes place during [[impact event]]s by extraterrestrial bodies. It produces rare ultrahigh pressure metamorphic minerals, such as [[coesite]] and [[stishovite]].{{sfn|Yardley|1989|p=13}} Coesite is rarely found in eclogite brought to the surface in [[kimberlite pipes]], but the presence of stishovite is unique to impact structures.<ref>{{cite journal |last1=Liu |first1=Liang |last2=Zhang |first2=Junfeng |last3=Green |first3=Harry W. |last4=Jin |first4=Zhenmin |last5=Bozhilov |first5=Krassmir N. |title=Evidence of former stishovite in metamorphosed sediments, implying subduction to >350 km |journal=Earth and Planetary Science Letters |date=November 2007 |volume=263 |issue=3–4 |page=181 |doi=10.1016/j.epsl.2007.08.010 |bibcode=2007E&PSL.263..180L |language=en}}</ref>