Gneiss (/naɪs/nice) is a common and widely distributed type ofmetamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed ofigneous orsedimentary rocks. 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 distinctcleavage.

Gneiss

Metamorphic rock

Sample of gneiss exhibiting "gneissic banding"

Gneisses are common in the ancient crust ofcontinental shields. Some of the oldest rocks on Earth are gneisses, such as theAcasta Gneiss.

In traditional English and North American usage, a gneiss is a coarse-grainedmetamorphic rock showing compositional banding (gneissic banding) but poorly developedschistosity and indistinctcleavage. 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.^[1]

TheBritish Geological Survey (BGS) and theInternational Union of Geological Sciences (IUGS) both usegneiss as a broadtextural category for medium- to coarse-grained metamorphic rock that shows poorly developed schistosity, with compositional layering over 5 millimeters (0.20 in) thick^[2] and tending to split into plates over 1 centimeter (0.39 in) thick.^[3] Neither definition depends on composition or origin, though rocks poor in platyminerals are more likely to produce gneissose texture. Gneissose rocks thus are largelyrecrystallized but do not carry large quantities of micas,chlorite or other platy minerals.^[4] Metamorphic rock showing stronger schistosity is classified as schist, while metamorphic rock devoid of schistosity is called agranofels.^[2][3]

Gneisses that are metamorphosed igneous rocks or their equivalent are termedgranite gneisses,diorite gneisses, and so forth. Gneiss rocks may also be named after a characteristic component such asgarnet gneiss,biotite gneiss,albite gneiss, and so forth.Orthogneiss designates a gneiss derived from anigneous rock, andparagneiss is one from asedimentary rock.^[2][3]Both the BGS and the IUGS usegneissose to describe rocks with the texture of gneiss,^[2][3] thoughgneissic 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.

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 moremafic minerals (those containing moremagnesium andiron). The lighter bands contain relatively morefelsic minerals (minerals such as feldspar orquartz, which contain more of the lighter elements, such asaluminium,sodium, andpotassium).^[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 theprotolith (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.^[6] These forces stretch out the rock like aplastic, and the original material is spread out into sheets. Per thepolar 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.^[9]

Some banding is formed from original rock material (protolith) that is subjected to extreme temperature and pressure and is composed of alternating layers ofsandstone (lighter) andshale (darker), which is metamorphosed into bands ofquartzite 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, from theGerman:Augen[ˈaʊɡən], meaning "eyes", is a gneiss resulting from metamorphism of granite, which contains characteristic elliptic orlenticular shear-bound grains (porphyroclasts), normallyfeldspar, surrounded by finer grained material. The finer grained material deforms around the more resistant feldspar grains to produce this texture.^[10]

Migmatite is a gneiss consisting of two or more distinct rock types, one of which has the appearance of an ordinary gneiss (themesosome), and another of which has the appearance of anintrusive rock suchpegmatite,aplite, or granite (theleucosome). The rock may also contain amelanosome 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]

Gneisses are characteristic of areas ofregional metamorphism that reaches the middleamphibolite togranulitemetamorphic facies. In other words, the rock was metamorphosed at a temperature in excess of 600 °C (1,112 °F) at pressures between about 2 to 24kbar. 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 asgarnet-biotite paragneiss orgrayish-pink orthogneiss.^[14]

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 ofArchean age (over 2500 million years old), mostly belong to granite-greenstone belts. Thegreenstone belts containmetavolcanic andmetasedimentary 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 Archeancratons.^[15]

Gneiss domes are common inorogenic belts (regions of mountain formation).^[16] They consist of a dome of gneiss intruded by younger granite and migmatite and mantled with sedimentary rock.^[17] These have been interpreted as ageologic 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 ofmetamorphic core complexes, regions of the deep crust brought to the surface and exposed during extension of theEarth's crust.^[18]

• TheAcasta Gneiss is found in theNorthwest Territories,Canada, on an island about 300 kilometers (190 mi) north ofYellowknife. This is one of the most ancient intact crustal fragments on Earth, metamorphosed 3.58 to 4.031 billion years ago.^[19]
• TheLewisian gneiss is found throughout theOuter Hebrides of Scotland, on the Scottish mainland west of theMoine Thrust, and on the islands ofColl andTiree.^[20] These rocks are largely igneous in origin, mixed with metamorphosedmarble, quartzite and mica schist with later intrusions ofbasalticdikes and granite magma.^[21]
• TheMorton Gneiss is an Archean-age gneiss exposed in theMinnesota River Valley of southwesternMinnesota, United States. It is thought to be the oldest intact block of continental crust in the United States.^[22]
• ThePeninsular Gneiss is a sequence of Archean gneisses found throughout theIndian Shield and ranging in age from 3400 to 2500 million years old.^[23][24]

The wordgneiss has been used in English since at least 1757.^[25] It isborrowed from the German wordGneis, formerly also spelledGneiss, which is probably derived from theMiddle High German noungneist "spark" (so called because the rock glitters).^[26]

Gneiss is used as a building material, such as the Facoidal gneiss. It's used extensively inRio de Janeiro.^[27] Gneiss has also been used asconstruction aggregate forasphalt pavement.^[28]

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• Murray, W.H. (1966).The Hebrides. London: Heinemann.ASIN B0000CMWPC.

• "Gneiss" .Encyclopædia Britannica (11th ed.). 1911.
• "Gneiss" .New International Encyclopedia. 1906.