Gneiss: Difference between revisions
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|caption= Sample of gneiss exhibiting "gneissic banding" | |caption= Sample of gneiss exhibiting "gneissic banding" | ||
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'''Gneiss''' ( | '''Gneiss''' ({{IPAc-en|n|aɪ|s}} {{respell|NYSE}}) 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 °C (572 °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]]. | 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]]. | ||
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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> | 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 [[ | The [[British Geological Survey]] (BGS) and the [[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"/> | 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"/> | ||
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===Migmatite=== | ===Migmatite=== | ||
{{Main|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> | 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 as [[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== | ==Occurrences== | ||
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*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 [[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 [[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 | * 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|archive-date=2009-03-30|access-date=2021-07-05|archive-url=https://web.archive.org/web/20090330120907/http://www.vedamsbooks.com/no24484.htm|url-status=dead}}</ref> | ||
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==Etymology== | ==Etymology== | ||