Granite Gneiss
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Gneiss ( ) is a common and widely distributed type of
metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of
igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
or
sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s. Gneiss forms at higher temperatures and pressures than
schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...
. 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

Orthogneiss from the Czech Republic In traditional English and North American usage, a gneiss is a coarse-grained
metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
showing compositional banding (
gneissic banding Gneiss ( ) 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 an ...
) but poorly developed
schistosity Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...
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 Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
-poor, high-grade metamorphic rock. The
British Geological Survey The British Geological Survey (BGS) is a partly publicly funded body which aims to advance geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research. The BGS h ...
(BGS) and the
International Union of Geological Sciences The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to international cooperation in the field of geology. About The IUGS was founded in 1961 and is a Scientific Union member of the Inte ...
(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 and tending to split into plates over thick. Neither definition depends on composition or origin, though rocks poor in platy
minerals In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
are more likely to produce gneissose texture. Gneissose rocks thus are largely recrystallized but do not carry large quantities of micas,
chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...
or other platy minerals. Metamorphic rock showing stronger schistosity is classified as schist, while metamorphic rock devoid of schistosity is called a
granofels Granofels is a term referring to medium- to coarse-grained metamorphic rocks which have a ''granoblastic'' texture (having poorly formed, equi-granular crystal grains visible to the naked eye) and lack distinct foliation or lineation. The granofels ...
. Gneisses that are metamorphosed igneous rocks or their equivalent are termed
granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...
gneisses,
diorite Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-silic ...
gneisses, and so forth. Gneiss rocks may also be named after a characteristic component such as
garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...
gneiss,
biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...
gneiss,
albite Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate. I ...
gneiss, and so forth. Orthogneiss designates a gneiss derived from an
igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
, and paragneiss is one from a
sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
. Both the BGS and the IUGS use ''gneissose'' to describe rocks with the texture of gneiss, though ''gneissic'' also remains in common use. 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. The darker bands have relatively more
mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
minerals (those containing more
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
and
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
). The lighter bands contain relatively more
felsic In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, whi ...
minerals (minerals such as feldspar or
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
, which contain more of the lighter elements, such as
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
,
sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...
, and
potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...
). 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 A protolith () is the original, unmetamorphosed rock from which a given metamorphic rock is formed. For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and thu ...
(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. These forces stretch out the rock like a
plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
, and the original material is spread out into sheets. Per the
polar decomposition theorem In mathematics, the polar decomposition of a square real or complex matrix A is a factorization of the form A = U P, where U is an orthogonal matrix and P is a positive semi-definite symmetric matrix (U is a unitary matrix and P is a positive se ...
, the deformation produced by such shearing force is equivalent to rotation of the rock combined with shortening in one direction and extension in another. 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 Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...
(lighter) and
shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
(darker), which is metamorphosed into bands of
quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...
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 german: Augen , meaning "eyes", is a gneiss resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear-bound grains (
porphyroclast 350px, A mylonite showing a number of (rotated) porphyroclasts: a clear red garnet left in the picture while smaller white feldspar porphyroclasts can be found all over. ''Location'': the tectonics, tectonic contact between the wiktionary:autochth ...
s), normally
feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
, 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 Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form ''Igneous intrusion, intrusions'', such as batholiths, dike (geology), dikes, Sill (geology), sills, laccoliths, and volcanic necks.I ...
such
pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...
,
aplite Aplite () is an intrusive igneous rock in which the mineral composition is the same as granite, but in which the grains are much finer, under 1 mm across. Quartz and feldspar are the dominant minerals. The term ''aplite'' or ''aplitic'' ...
, 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 Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...
that reaches the middle
amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky ...
to
granulite Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated w ...
metamorphic facies A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures.Essentials of Geology, 3rd Edition, Stephen Marshak The assemblage is typical of what is formed in conditions corresponding ...
. In other words, the rock was metamorphosed at a temperature in excess of at pressures between about 2 to 24
kbar The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000  Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea lev ...
. 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 The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth Earth ...
age (over 2500 million years old), mostly belong to granite-greenstone belts. The
greenstone belt Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies. The name comes from the green ...
s contain
metavolcanic Metavolcanic rock is volcanic rock that shows signs of having experienced metamorphism. In other words, the rock was originally produced by a volcano, either as lava or tephra. The rock was then subjected to high pressure, high temperature or both ...
and
metasedimentary rock In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment Sediment is a naturally occurring material that is broken down by processes of weathering and er ...
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
cratons A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and ...
.


Gneiss domes

Gneiss domes are common in
orogenic belts Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...
(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 The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sand ...
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 Metamorphic core complexes are exposures of deep crust exhumed in association with largely amagmatic extension. They form, and are exhumed, through relatively fast transport of middle and lower continental crust to the Earth's surface. During th ...
es, regions of the deep crust brought to the surface and exposed during extension of the
Earth's crust Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...
.


Examples

* The Acasta Gneiss is found in the
Northwest Territories The Northwest Territories (abbreviated ''NT'' or ''NWT''; french: Territoires du Nord-Ouest, formerly ''North-Western Territory'' and ''North-West Territories'' and namely shortened as ''Northwest Territory'') is a federal territory of Canada. ...
,
Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
, on an island about north of
Yellowknife Yellowknife (; Dogrib: ) is the capital, largest community, and only city in the Northwest Territories, Canada. It is on the northern shore of Great Slave Lake, about south of the Arctic Circle, on the west side of Yellowknife Bay near the ...
. This is one of the most ancient intact crustal fragments on Earth, metamorphosed 3.58 to 4.031 billion years ago. *The
Lewisian gneiss The Lewisian complex or Lewisian gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane and the North Atlantic Craton. These rocks are of Archaean and Paleoprote ...
is found throughout the
Outer Hebrides The Outer Hebrides () or Western Isles ( gd, Na h-Eileanan Siar or or ("islands of the strangers"); sco, Waster Isles), sometimes known as the Long Isle/Long Island ( gd, An t-Eilean Fada, links=no), is an island chain off the west coast ...
of Scotland, on the Scottish mainland west of the
Moine Thrust The Moine Thrust Belt or Moine Thrust Zone is a linear tectonic feature in the Scottish Highlands which runs from Loch Eriboll on the north coast south-west to the Sleat peninsula on the Isle of Skye. The thrust belt consists of a series of t ...
, and on the islands of
Coll Coll (; gd, Cola; sco, Coll)Mac an Tàilleir (2003) p. 31 is an island located west of the Isle of Mull in the Inner Hebrides of Scotland. Coll is known for its sandy beaches, which rise to form large sand dunes, for its corncrakes, and ...
and
Tiree Tiree (; gd, Tiriodh, ) is the most westerly island in the Inner Hebrides of Scotland. The low-lying island, southwest of Coll, has an area of and a population of around 650. The land is highly fertile, and crofting, alongside tourism, and ...
. These rocks are largely igneous in origin, mixed with metamorphosed
marble Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or Dolomite (mineral), dolomite. Marble is typically not Foliation (geology), foliated (layered), although there are exceptions. In geology, the ...
, quartzite and mica schist with later intrusions of
basaltic Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
dikes Dyke (UK) or dike (US) may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), ''Dikē'', the Greek goddess of moral justice * Dikes, ...
and granite magma. *The
Morton Gneiss Morton gneiss, also known as rainbow gneiss, is an Archean-age gneiss found in the Minnesota River Valley of southwestern Minnesota, United States. It is one of the oldest stones on Earth, at about 3.5 billion years old. Along with the nearby Mon ...
is an Archean-age gneiss exposed in the
Minnesota River The Minnesota River ( dak, Mnísota Wakpá) is a tributary of the Mississippi River, approximately 332 miles (534 km) long, in the U.S. state of Minnesota. It drains a watershed of in Minnesota and about in South Dakota and Iowa. It ris ...
Valley of southwestern
Minnesota Minnesota () is a state in the upper midwestern region of the United States. It is the 12th largest U.S. state in area and the 22nd most populous, with over 5.75 million residents. Minnesota is home to western prairies, now given over to ...
, United States. It is thought to be the oldest intact block of continental crust in the United States. * The
Peninsular Gneiss Peninsular Gneiss or Peninsular Gniessic Complex are the gneissic complex of the metamorphics found all over the Indian Peninsula, on top of which, the supra-crustal Dharwar System have been laid down. The term was first fashioned by W.F.Smeeth o ...
is a sequence of Archean gneisses found throughout the
Indian Shield The Indian Shield is part of the Indian Craton and occupies two-thirds of the southern Indian peninsula. The shield has remained relatively stable since its formation, over 3500 million years ago. It is delimited on the west, south and east by the ...
and ranging in age from 3400 to 2500 million years old.


Etymology

The word ''gneiss'' has been used in English since at least 1757. It is borrowed from the German word , formerly also spelled , which is probably derived from the
Middle High German Middle High German (MHG; german: Mittelhochdeutsch (Mhd.)) is the term for the form of German spoken in the High Middle Ages. It is conventionally dated between 1050 and 1350, developing from Old High German and into Early New High German. High ...
noun "spark" (so called because the rock glitters).


Uses

Gneiss has been used as a building material, such as the Facoidal gneiss, used extensively in
Rio de Janeiro Rio de Janeiro ( , , ; literally 'River of January'), or simply Rio, is the capital of the state of the same name, Brazil's third-most populous state, and the second-most populous city in Brazil, after São Paulo. Listed by the GaWC as a b ...
. Gneiss has also been used as
construction aggregate Construction aggregate, or simply aggregate, is a broad category of coarse- to medium-grained particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates ...
for
asphalt pavement A road surface (British English), or pavement (American English), is the durable surface material laid down on an area intended to sustain vehicular or foot traffic, such as a road or walkway. In the past, gravel road surfaces, hoggin, cobble ...
.


See also

*
List of rock types The following is a list of rock types recognized by geologists. There is no agreed number of specific types of rocks. Any unique combination of chemical composition, mineralogy, grain size, texture, or other distinguishing characteristics can desc ...
*
Glossary of geology This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences Earth science or geoscience includes all fields of natural scienc ...
*


References


Citations


Additional reading

*


External links

* * {{Authority control Metamorphic petrology Industrial minerals