Gneiss (pronounced ) 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 . 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.
Gneisses are common in the ancient crust of continental shields. Some of the oldest rocks on Earth are gneisses, such as the Acasta Gneiss.
Description
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. 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.
The British Geological Survey (BGS) and the IUGS (IUGS) both use gneiss as a broad textural category for medium- to coarse-grained metamorphic rock that shows poorly developed schistosity, with compositional layering over thick
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.
Gneissic banding
The minerals in gneiss are arranged into layers that appear as bands in cross section. This is called gneissic banding.
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).
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.
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.
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.
Another cause of banding is "metamorphic differentiation", which separates different materials into different layers through chemical reactions, a process not fully understood.
Augen gneiss
Augen gneiss, from the , meaning "eyes", is a gneiss resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear-bound grains (
), normally
feldspar, surrounded by finer grained material. The finer grained material deforms around the more resistant feldspar grains to produce this texture.
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. 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.
Occurrences
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 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.
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
contain metavolcanic and metasedimentary rock that has undergone a relatively mild grade of metamorphism, at temperatures of and pressures of . The greenstone belts are surrounded by high-grade gneiss terrains showing highly deformed low-pressure, high-temperature (over ) metamorphism to the amphibolite or granulite facies. These form most of the exposed rock in Archean
.
Gneiss domes
Gneiss domes are common in
orogenic belts (regions of mountain formation).
They consist of a dome of gneiss intruded by younger granite and migmatite and mantled with sedimentary rock.
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 complexes, regions of the deep crust brought to the surface and exposed during extension of the Earth's crust.
Examples
-
The Acasta Gneiss is found in the Northwest Territories, Canada, on an island about north of Yellowknife. This is one of the most ancient intact crustal fragments on Earth, metamorphosed 3.58 to 4.031 billion years ago.
[ 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]
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The Lewisian complex is found throughout the Outer Hebrides of Scotland, on the Scottish mainland west of the Moine Thrust, and on the islands of Coll and Tiree.
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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.
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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.
Etymology
The word
gneiss has been used in English since at least 1757.
[ From p. 308: " … to which we may add this conjecture, that the black vein-stone, or rock, usually called kneiss, at Friberg, … "] It is
Loanword from the German word italic=no, formerly also spelled italic=no, which is probably derived from the Middle High German noun gneist "spark" (so called because the rock glitters).
Uses
Gneiss is used as a building material, such as the Facoidal gneiss. It's used extensively in Rio de Janeiro.
Gneiss has also been used as construction aggregate for
asphalt pavement.
See also
-
List of rock types
-
Glossary of geology
Citations
Further reading
External links
