Gneiss
Metamorphic rock | |
Gneiss (/naɪs/ 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 rocks. Gneiss forms at higher temperatures and pressures than schist. Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct cleavage.
Gneisses are common in the ancient crust of
Description
In traditional English and North American usage, a gneiss is a coarse-grained
The
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.[2][3] Both the BGS and the IUGS use gneissose to describe rocks with the texture of gneiss,[2][3] though gneissic also remains in common use.[5] For example, a gneissose metagranite or a gneissic metagranite both mean a granite that has been metamorphosed and thereby acquired gneissose texture.
Gneissic banding
The minerals in gneiss are arranged into layers that appear as bands in cross section. This is called gneissic banding.[6] 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).[7]
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.[8]
A common cause of nonhydrodynamic stress is the subjection of the
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.[6]
Another cause of banding is "metamorphic differentiation", which separates different materials into different layers through chemical reactions, a process not fully understood.[6]
Augen gneiss
Augen gneiss, from the German: Augen [ˈaʊɡən], meaning "eyes", is a gneiss resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear-bound grains (porphyroclasts), normally feldspar, surrounded by finer grained material. The finer grained material deforms around the more resistant feldspar grains to produce this texture.[10]
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.[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.[12]
Occurrences
Gneisses are characteristic of areas of
Granite-greenstone belts
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 belts contain metavolcanic and metasedimentary rock that has undergone a relatively mild grade of metamorphism, at temperatures of 350–500 °C (662–932 °F) and pressures of 200–500 MPa (2,000–5,000 bar). The greenstone belts are surrounded by high-grade gneiss terrains showing highly deformed low-pressure, high-temperature (over 500 °C (932 °F)) metamorphism to the amphibolite or granulite facies. These form most of the exposed rock in Archean cratons.[15]
Gneiss domes
Gneiss domes are common in
Examples
- The Acasta Gneiss is found in the Northwest Territories, Canada, on an island about 300 kilometers (190 mi) north of Yellowknife. This is one of the most ancient intact crustal fragments on Earth, metamorphosed 3.58 to 4.031 billion years ago.[19]
- The
- 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.[22]
- 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.[23][24]
Etymology
The word gneiss has been used in English since at least 1757.[25] It is borrowed from the German word Gneis, formerly also spelled Gneiss, which is probably derived from the Middle High German noun gneist "spark" (so called because the rock glitters).[26]
Uses
Gneiss has been used as a building material, such as the Facoidal gneiss, used extensively in
See also
References
Citations
- ISBN 0582300967.
- ^ a b c d Robertson, S. (1999). "BGS Rock Classification Scheme, Volume 2: Classification of metamorphic rocks" (PDF). British Geological Survey Research Report. RR 99-02. Retrieved 27 February 2021.
- ^ a b c d Schmid, R.; Fettes, D.; Harte, B.; Davis, E.; Desmons, J. (2007). "How to name a metamorphic rock.". Metamorphic Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Metamorphic Rocks (PDF). Cambridge: Cambridge University Press. pp. 3–15. Retrieved 28 February 2021.
- ISBN 0716724383.
- ISBN 0922152349.
- ^ ISBN 978-0-393-91939-4.
- ^ Yardley 1989, p. 22.
- ^ Blatt & Tracy 1996, p. 359.
- ISBN 9781107057647.
- ^ Blatt & Tracy 1996, pp. 358–359.
- ^ British Geological Survey 1999, p. 11.
- ISBN 978-0660197876.
- ^ Bjørn Hageskov (1985): 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
- ^ British Geological Survey 1999, pp. 5–6.
- ISBN 9781405107778.
- ^ Whitney, D.L; Teyssier, C.; Vanderhaeghe, O. (2004). "Gneiss domes and crustal flow". In Whitney, D.L.; Teyssier, C.; Siddoway, C.S. (eds.). Gneiss domes in orogeny: Boulder, Colorado, Geological Society of America Special Paper 380. Retrieved 5 July 2021.
- .
- ^ Yin, A. (2004). "Gneiss domes and gneiss dome systems". In Whitney, D.L.; Teyssier, C.; Siddoway, C.S. (eds.). Gneiss domes in orogeny (PDF). Boulder, Colorado: Geological Society of America. pp. 1–14. Special Paper 380. Retrieved 4 July 2021.
- ^ 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
- ISBN 1-903544-09-2.
- ISBN 978-1-84158-357-0.
- ^ Aber, James S. (2012). "Morton Gneiss, Minnesota". Emporia State University. Archived from the original on 2014-11-26. Retrieved 2019-05-22.
- ^ "Peninsular Gneiss". Geological Survey of India. Archived from the original on 21 July 2011. Retrieved 2009-02-27.
- ISSN 0254-0436.
- ^ Henckel, Johann Friedrich (1757). Pyritologia, or a History of the Pyrites …. London, England: A. Millar and A. Linde. p. 308. From p. 308: " … to which we may add this conjecture, that the black vein-stone, or rock, usually called kneiss, at Friberg, … "
- ^ Harper, Douglas (ed.). "Gneiss". Online Etymological Dictionary. Retrieved 4 July 2021.
- .
- .
Further reading
- ASIN B0000CMWPC.
External links
- Encyclopædia Britannica (11th ed.). 1911. .
- New International Encyclopedia. 1906. .