Editing Gneiss

{{short description|Common high-grade metamorphic rock}}
{{other uses}}
{{Infobox rock
|name= Gneiss
|type= Metamorphic
|coordinates=
|image= Gneiss.jpg
|alt=
|caption= Sample of gneiss exhibiting "gneissic banding"
}}
'''Gneiss''' (pronounced {{IPAc-en|n|aɪ|s}} {{respell|nice}}) is a common and widely distributed type of [[metamorphic rock]]. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of [[igneous]] or [[sedimentary rock]]s. This rock is formed under pressures ranging from 2 to 15 kbar, sometimes even more, and temperatures over 300&nbsp;°C (572&nbsp;°F). Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct [[Cleavage (geology)|cleavage]].

Gneisses are common in the ancient crust of [[Continental Shield|continental shields]]. Some of the oldest rocks on Earth are gneisses, such as the [[Acasta Gneiss]].

==Description==
[[image:Orthogneiss Geopark.jpg|thumb|Orthogneiss from the Czech Republic]]
In traditional English and North American usage, a gneiss is a coarse-grained [[metamorphic rock]] showing compositional banding ([[gneissic banding]]) but poorly developed [[schistosity]] and indistinct [[Cleavage (geology)|cleavage]]. In other words, it is a metamorphic rock composed of mineral grains easily seen with the unaided eye, which form obvious compositional layers, but which has only a weak tendency to fracture along these layers. In Europe, the term has been more widely applied to any coarse, [[mica]]-poor, high-grade metamorphic rock.<ref>{{cite book |last1=Yardley |first1=B. W. D. |title=An introduction to metamorphic petrology |date=1989 |publisher=Longman Scientific & Technical |location=Harlow, Essex, England |isbn=0582300967 |page=22}}</ref>

The [[British Geological Survey]] (BGS) and the [[IUGS|International Union of Geological Sciences]] (IUGS) both use ''gneiss'' as a broad [[Texture (geology)|textural]] category for medium- to coarse-grained metamorphic rock that shows poorly developed schistosity, with compositional layering over {{convert|5|mm||sp=us}} thick<ref name="BGS">{{cite journal |last1=Robertson |first1=S. |title=BGS Rock Classification Scheme, Volume 2: Classification of metamorphic rocks |journal=British Geological Survey Research Report |date=1999 |volume=RR 99-02 |url=http://nora.nerc.ac.uk/id/eprint/3226/1/RR99002.pdf |access-date=27 February 2021 |ref={{harvid|British Geological Survey|1999}}}}</ref> and tending to split into plates over {{convert|1|cm||sp=us}} thick.<ref name="schid-etal=2007">{{cite book |last1=Schmid |first1=R. |last2=Fettes |first2=D. |last3=Harte |first3=B. |last4=Davis |first4=E. |last5=Desmons |first5=J. |year=2007 |chapter=How to name a metamorphic rock. |title=Metamorphic Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Metamorphic Rocks |publisher=Cambridge University Press |location=Cambridge |pages=3–15 |url=https://stuff.mit.edu/afs/athena.mit.edu/course/12/12.115/www/12.114%20Papers/BGS/Metamorphic/DetailedMetamorphic/1%20How%20to%20name%20a%20metamorphic%20rock.pdf |access-date=28 February 2021}}</ref> Neither definition depends on composition or origin, though rocks poor in platy [[minerals]] are more likely to produce gneissose texture. Gneissose rocks thus are largely [[Recrystallization (geology)|recrystallized]] but do not carry large quantities of micas, [[Chlorite group|chlorite]] or other platy minerals.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology : igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0716724383 |edition=2nd |page=360}}</ref> Metamorphic rock showing stronger schistosity is classified as schist, while metamorphic rock devoid of schistosity is called a [[granofels]].<ref name="BGS"/><ref name="schid-etal=2007"/>

Gneisses that are metamorphosed igneous rocks or their equivalent are termed [[granite]] gneisses, [[diorite]] gneisses, and so forth. Gneiss rocks may also be named after a characteristic component such as [[garnet]] gneiss, [[biotite]] gneiss, [[albite]] gneiss, and so forth. '''Orthogneiss''' designates a gneiss derived from an [[igneous rock]], and '''paragneiss''' is one from a [[sedimentary rock]].<ref name="BGS"/><ref name="schid-etal=2007"/>
Both the BGS and the IUGS use ''gneissose'' to describe rocks with the texture of gneiss,<ref name="BGS"/><ref name="schid-etal=2007"/> though ''gneissic'' also remains in common use.<ref>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=Gneissic}}</ref> For example, a gneissose metagranite or a gneissic metagranite both mean a granite that has been metamorphosed and thereby acquired gneissose texture.

===Gneissic banding===
[[File:Pure shear.png|thumb|Pure shear deformation of rock producing gneissic banding. The undeformed rock is shown at upper left, and the result of pure shear deformation at upper right. At lower left is the stretching component of the deformation, which compresses the rock in one direction and stretches it in the other, as shown by the arrows. The rock is simultaneously rotated to produce the final configuration, repeated at lower right.]]
The minerals in gneiss are arranged into layers that appear as bands in cross section. This is called gneissic banding.<ref name="Marshak 2009">{{Cite book | last = Marshak | first = Stephen | year = 2013 | title = Essentials of Geology | edition = 4th | publisher = W.W. Norton | isbn = 978-0-393-91939-4 |pages=194–95; Figs. 7.6a–c}}</ref> The darker bands have relatively more [[mafic]] minerals (those containing more [[magnesium]] and [[iron]]). The lighter bands contain relatively more [[felsic]] minerals (minerals such as feldspar or [[quartz]], which contain more of the lighter elements, such as [[aluminium]], [[sodium]], and [[potassium]]).{{sfn|Yardley|1989|p=22}}

The banding is developed at high temperature when the rock is more strongly compressed in one direction than in other directions (''nonhydrostatic stress''). The bands develop perpendicular to the direction of greatest compression, also called the shortening direction, as platy minerals are rotated or recrystallized into parallel layers.{{sfn|Blatt|Tracy|1996|p=359}}

A common cause of nonhydrodynamic stress is the subjection of the [[protolith]] (the original rock material that undergoes metamorphism) to extreme shearing force, a sliding force similar to the pushing of the top of a deck of cards in one direction, and the bottom of the deck in the other direction.<ref name="Marshak 2009"/> These forces stretch out the rock like a [[Plasticity (physics)|plastic]], and the original material is spread out into sheets. Per the [[polar decomposition theorem]], the deformation produced by such shearing force is equivalent to rotation of the rock combined with shortening in one direction and extension in another.<ref>{{cite book |last1=Fossen |first1=Haakon |title=Structural geology |date=2016 |publisher=Cambridge University Press |location=Cambridge, United Kingdom |isbn=9781107057647 |page=38 |edition=Second}}</ref>

Some banding is formed from original rock material (protolith) that is subjected to extreme temperature and pressure and is composed of alternating layers of [[sandstone]] (lighter) and [[shale]] (darker), which is metamorphosed into bands of [[quartzite]] and mica.<ref name="Marshak 2009"/>

Another cause of banding is "metamorphic differentiation", which separates different materials into different layers through chemical reactions, a process not fully understood.<ref name="Marshak 2009"/>

===Augen gneiss===
<!-- This section is the target of a redirect from [[Augen gneiss]]. -->
{{See also|Augen}}
[[File:Augen-gneiss.jpg|thumb|Augen gneiss from {{Lang|pt|[[Leblon]]|italic=no}}, Rio de Janeiro City, Brazil]]
[[File:Orthogneiss oeillé - Ras Prat Cabrera (Canigou).jpg|thumb|Ordovician augen gneiss outcrop, [[Canigó]] massif, eastern Pyrenees, France]]
'''Augen gneiss''', from the {{langx|de|Augen}} {{IPA|de|ˈaʊɡən|}}, meaning "eyes", is a gneiss resulting from metamorphism of granite, which contains characteristic elliptic or [[Lens (geology)|lenticular]] shear-bound grains ([[porphyroclast]]s), normally [[feldspar]], surrounded by finer grained material. The finer grained material deforms around the more resistant feldspar grains to produce this texture.{{sfn|Blatt|Tracy|1996|pp=358-359}}

===Migmatite===
{{Main|Migmatite}}
Migmatite is a gneiss consisting of two or more distinct rock types, one of which has the appearance of an ordinary gneiss (the ''mesosome''), and another of which has the appearance of an [[intrusive rock]] such [[pegmatite]], [[aplite]], or granite (the ''leucosome''). The rock may also contain a ''melanosome'' of mafic rock complementary to the leucosome.{{sfn|British Geological Survey|1999|p=11}} Migmatites are often interpreted as rock that has been partially melted, with the leucosome representing the silica-rich melt, the melanosome the residual solid rock left after partial melting, and the mesosome the original rock that has not yet experienced partial melting.<ref>{{cite book |last1=Sawyer |first1=E. W. |title=Atlas of migmatites |date=2008 |publisher=NRC Research Press |location=Ottawa, Ontario |isbn=978-0660197876}}</ref>

==Occurrences==
[[File:Road Cutting - geograph.org.uk - 820828.jpg|thumb|Dark dikes (now [[Foliation (geology)|foliated]] [[amphibolite]]s) cutting light grey Lewisian gneiss of the Scourie complex, both deformed and cut by later (unfoliated) pink granite dikes]]
[[File:Yttre Ursholmen Kontakt Kosterdiabas i Nebulitisk-migmatitisk sedimentgnejs.jpg|thumb|Contact between a dark-colored [[diabase]] dike (about 1100 million years old)<ref name="hageskov">Bjørn Hageskov (1985): [http://2dgf.dk/xpdf/bull34-03-04-151-197.pdf ''Constrictional deformation of the Koster dyke swarm in a ductile sinistral shear zone, Koster islands, SW Sweden.''] Bulletin of the Geological Society of Denmark 34 (3–4): 151–97</ref> and light-colored [[Migmatite|migmatitic]] paragneiss in the [[Kosterhavet National Park]] in the [[Koster Islands]] off the western coast of Sweden.]]
[[File:Gnaisse Sete Voltas - Bahia - Brasil.jpg|thumb|Sample of Sete Voltas gneiss from [[Bahia]] in Brazil, the oldest rock outcropping in the crust of South America, {{circa}} 3.4 billion years old (Archean)]]
Gneisses are characteristic of areas of [[regional metamorphism]] that reaches the middle [[amphibolite]] to [[granulite]] [[metamorphic facies]]. In other words, the rock was metamorphosed at a temperature in excess of {{convert|600|C||sp=us}} at pressures between about 2 to 24 [[kbar]]. Many different varieties of rock can be metamorphosed to gneiss, so geologists are careful to add descriptions of the color and mineral composition to the name of any gneiss, such as ''garnet-biotite paragneiss'' or ''grayish-pink orthogneiss''.{{sfn|British Geological Survey|1999|pp=5-6}}

===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 rock|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}}. 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 [[craton]]s.<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 |page=350 |edition=3rd}}</ref>

===Gneiss domes===
Gneiss domes are common in [[orogenic belts]] (regions of mountain formation).<ref>{{cite book|last1=Whitney |first1=D.L |last2=Teyssier |first2=C. |last3=Vanderhaeghe |first3=O.  |year=2004 |chapter=Gneiss domes and crustal flow |editor-last1=Whitney |editor-first1=D.L.  |editor-last2=Teyssier |editor-first2=C. |editor-last3=Siddoway |editor-first3=C.S. |editor3-link= Christine Siddoway|title=Gneiss domes in orogeny: Boulder, Colorado, Geological Society of America Special Paper 380 |url=https://www.researchgate.net/publication/309201751 |access-date=5 July 2021}}</ref> They consist of a dome of gneiss intruded by younger granite and migmatite and mantled with sedimentary rock.<ref>{{cite journal |last1=Teyssier |first1=Christian |last2=Whitney |first2=Donna L. |title=Gneiss domes and orogeny |journal=Geology |date=1 December 2002 |volume=30 |issue=12 |pages=1139–1142 |doi=10.1130/0091-7613(2002)030<1139:GDAO>2.0.CO;2|bibcode=2002Geo....30.1139T }}</ref> These have been interpreted as a [[geologic record]] of two distinct mountain-forming events, with the first producing the granite basement and the second deforming and melting this basement to produce the domes. However, some gneiss domes may actually be the cores of [[metamorphic core complex]]es, regions of the deep crust brought to the surface and exposed during extension of the [[Earth's crust]].<ref>{{cite book |last1=Yin |first1=A. |year=2004 |chapter=Gneiss domes and gneiss dome systems |editor-last1=Whitney |editor-first1=D.L. |editor-last2=Teyssier |editor-first2=C. |editor-last3=Siddoway |editor-first3=C.S.|editor3-link= Christine Siddoway|title=Gneiss domes in orogeny |publisher=Geological Society of America |id=Special Paper 380 |location=Boulder, Colorado|pages= 1–14 |url=https://faculty.epss.ucla.edu/~yin/05-Publications/papers/085-Yin-2004-GSA%20Special%20papers.pdf |access-date=4 July 2021}}</ref>

===Examples===
<!-- This is not a comprehensive list of gneisses. Include only examples that have something particularly notable about them or that show the global occurrence of gneiss. -->

* The [[Acasta Gneiss]] is found in the [[Northwest Territories]], [[Canada]], on an island about {{convert|300|km||sp=us}} north of [[Yellowknife]]. This is one of the most ancient intact crustal fragments on Earth, metamorphosed 3.58 to 4.031 billion years ago.<ref>[http://www2.ess.ucla.edu/~schauble/Isotope_geochemistry/Acasta_Concordia_Bowring.pdf Bowring, S.A., and Williams, I.S., 1999. Priscoan (4.00–4.03 Ga) orthogneisses from northwestern Canada. Contributions to Mineralogy and Petrology, v. 134, 3–16]</ref>
*The [[Lewisian complex|Lewisian gneiss]] is found throughout the [[Outer Hebrides]] of Scotland, on the Scottish mainland west of the [[Moine Thrust Belt|Moine Thrust]], and on the islands of [[Coll]] and [[Tiree]].<ref>{{cite book |last1=Gillen |first1=Con |title=Geology and landscapes of Scotland |date=2003 |publisher=Terra |location=Harpenden |isbn=1-903544-09-2|page=44}}</ref> These rocks are largely igneous in origin, mixed with metamorphosed [[marble]], quartzite and mica schist with later intrusions of [[basaltic]] [[Dike (geology)|dikes]] and granite magma.<ref>{{cite book |last1=McKirdy |first1=Alan |title=Land of mountain and flood : the geology and landforms of Scotland |date=2007 |publisher=Birlinn |location=Edinburgh |isbn=978-1-84158-357-0|page=95}}</ref>
*The [[Morton Gneiss]] is an Archean-age gneiss exposed in the [[Minnesota River]] Valley of southwestern [[Minnesota]], United States.  It is thought to be the oldest intact block of continental crust in the United States.<ref name=Aber>{{cite web |last=Aber |first=James S. |title=Morton Gneiss, Minnesota |url=http://academic.emporia.edu/aberjame/tectonic/morton_gneiss/morton.htm |publisher=Emporia State University |year=2012 |accessdate=2019-05-22 |archive-date=2014-11-26 |archive-url=https://web.archive.org/web/20141126003847/http://academic.emporia.edu/aberjame/tectonic/morton_gneiss/morton.htm |url-status=dead }}</ref>
* The [[Peninsular Gneiss]] is a sequence of Archean gneisses found throughout the [[Indian Shield]] and ranging in age from 3400 to 2500 million years old.<ref name=monument>{{cite web|url=http://www.portal.gsi.gov.in/portal/page?_pageid=127,529567&_dad=portal&_schema=PORTAL|title=Peninsular Gneiss|publisher=Geological Survey of India|accessdate=2009-02-27|archive-url=https://web.archive.org/web/20110721161351/http://www.portal.gsi.gov.in/portal/page?_pageid=127,529567&_dad=portal&_schema=PORTAL|archive-date=21 July 2011|url-status=dead}}</ref><ref name= GSI>{{cite journal |title=National Geological Monuments, pages 96, Peninsular Gneiss, page29-32|journal=Special Publication Series|publisher=Geological Survey of India,27, Jawaharlal Nehru Road, Kolkata-700016|issn=0254-0436|year=2001|url=http://www.vedamsbooks.com/no24484.htm
}}</ref>

==Etymology==
The word ''gneiss'' has been used in English since at least 1757.<ref>{{cite book |last1=Henckel |first1=Johann Friedrich |title=Pyritologia, or a History of the Pyrites … |date=1757 |publisher=A. Millar and A. Linde |location=London, England |page=308 |url=https://www.biodiversitylibrary.org/item/129945#page/336/mode/1up}}  From p. 308:  " … to which we may add this conjecture, that the black vein-stone, or rock, usually called ''kneiss'', at Friberg, … "</ref> It is [[Loanword|borrowed]] from the German word {{Lang|de|Gneis|italic=no}}, formerly also spelled {{Lang|de|Gneiss|italic=no}}, which is probably derived from the [[Middle High German]] noun {{lang|gmh|gneist}} "spark" (so called because the rock glitters).<ref name=etymonline>{{cite web |editor-last1=Harper |editor-first1=Douglas |url=http://www.etymonline.com/index.php?search=gneiss&searchmode=none |title=Gneiss |website=Online Etymological Dictionary |access-date=4 July 2021}}</ref>

==Uses==
Gneiss is used as a building material, such as the Facoidal gneiss. It's used extensively in [[Rio de Janeiro]].<ref>{{cite journal |last1=Castro |first1=Nuria Fernández |last2=Mansur |first2=Kátia Leite |last3=Frascá |first3=Maria Heloisa Barros de Oliveira |last4=Silva |first4=Rosana Elisa Coppedê |title=A heritage stone of Rio de Janeiro (Brazil): the Facoidal gneiss |journal=Episodes |date=1 March 2021 |volume=44 |issue=1 |pages=59–74 |doi=10.18814/epiiugs/2020/0200s13|doi-access=free }}</ref> Gneiss has also been used as [[construction aggregate]] for [[asphalt pavement]].<ref>{{cite journal |last1=Chen |first1=Zongwu |last2=Wu |first2=Shaopeng |last3=Wen |first3=Jin |last4=Zhao |first4=Meiling |last5=Yi |first5=Mingwei |last6=Wan |first6=Jiuming |title=Utilization of gneiss coarse aggregate and steel slag fine aggregate in asphalt mixture |journal=Construction and Building Materials |date=September 2015 |volume=93 |pages=911–918 |doi=10.1016/j.conbuildmat.2015.05.070}}</ref>

==See also==
* [[List of rock types]]
* [[Glossary of geology]]
* {{portal-inline|Geology}}

==References==

===Citations===
{{Reflist}}

==Further reading==
* {{Cite book |last1=Murray |first1=W.H. |author1-link=W. H. Murray |year=1966 |title=The Hebrides |location=London |publisher=Heinemann |asin=B0000CMWPC}}

==External links==
{{commons category|Gneiss}}
* {{Cite EB1911|wstitle=Gneiss |short=x}}
* {{Cite NIE|wstitle=Gneiss|year=1906 |short=x}}
{{Rock type}}
{{Authority control}}

[[Category:Gneiss| ]]
[[Category:Metamorphic rocks]]
[[Category:Industrial minerals]]