Gneiss

Gneiss (/ˈnaɪs/) is a common and widely distributed type of metamorphic rock. Gneiss is formed by high temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Orthogneiss is gneiss derived from igneous rock (such as granite). Paragneiss is gneiss derived from sedimentary rock (such as sandstone). Gneiss forms at higher temperatures and pressures than schist. Gneiss nearly all the time shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct foliation.

Gneiss
Metamorphic rock
Gneiss
Sample of gneiss exhibiting "gneissic banding".

Etymology

The word gneiss has been used in English since at least 1757. 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).[1]

Formation

Gneiss is formed from sedimentary or igneous rock exposed to temperatures greater than 320°C and relatively high pressure.

Composition

Gneissic rocks are usually medium- to coarse-foliated; they are largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. Gneiss rocks may also be named after a characteristic component such as garnet gneiss, biotite gneiss, albite gneiss, etc. Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock.

Orthogneiss Geopark
Orthogneiss from the Czech Republic

Gneissose rocks have properties similar to gneiss.

Gneissic banding

Gneiss appears to be striped in bands like parallel lines in shape, called gneissic banding.[2] The banding is developed under high temperature and pressure conditions.

The minerals are arranged into layers that appear as bands in cross section.[2] The appearance of layers, called 'compositional banding', occurs because the layers, or bands, are of different composition. The darker bands have relatively more mafic minerals (those containing more magnesium and iron). The lighter bands contain relatively more felsic minerals (silicate minerals, containing more of the lighter elements, such as silicon, oxygen, aluminium, sodium, and potassium).

A common cause of the banding 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.[2] These forces stretch out the rock like a plastic, and the original material is spread out into sheets.

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.[2]

Another cause of banding is "metamorphic differentiation", which separates different materials into different layers through chemical reactions, a process not fully understood.[2]

Not all gneiss rocks have detectable banding. In kyanite gneiss, crystals of kyanite appear as random clumps in what is mainly a plagioclase (albite) matrix.

Types

Augen gneiss

Henderson Augen Gneiss
Henderson augen gneiss

Augen gneiss, from the German: Augen [ˈaʊɡən], meaning "eyes", is a coarse-grained gneiss resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear-bound feldspar porphyroclasts, normally microcline, within the layering of the quartz, biotite and magnetite bands.

Henderson gneiss

Henderson gneiss is found in North Carolina and South Carolina, US, east of the Brevard Shear Zone. It has deformed into two sequential forms. The second, more warped, form is associated with the Brevard Fault, and the first deformation results from displacement to the southwest.[3]

Lewisian gneiss

Road Cutting - geograph.org.uk - 820828
Dark dikes (now foliated amphibolites) cutting light grey Lewisian gneiss of the Scourie complex, both deformed and cut by later (unfoliated) pink granite dikes
Yttre Ursholmen Kontakt Kosterdiabas i Nebulitisk-migmatitisk sedimentgnejs
Contact between a dark-colored diabase dike (about 1100 million years old)[4] and light-colored migmatitic paragneiss in the Kosterhavet National Park in the Koster Islands off the southwestern coast of Sweden.

Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree.[5] These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist with later intrusions of basaltic dikes and granite magma.[6]

Archean and Proterozoic gneiss

Gneisses of Archean and Proterozoic age occur in the Baltic Shield.

See also

References

Citations

  1. ^ Harper, Online Etym. Dict., "gneiss"
  2. ^ a b c d e Marshak 2013, pp. 194–95; Figs. 7.6a–c
  3. ^ Sacks & Secor (1990).
  4. ^ 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
  5. ^ Gillen (2003), p. 44.
  6. ^ McKirdy et al. (2007), p. 95.

Bibliography

  • Blatt, Harvey and Robert J. Tracy (1996). Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed. Freeman, pp. 359–65. ISBN 0-7167-2438-3.
  • Gillen, Con (2003). Geology and landscapes of Scotland. Harpenden. Terra Publishing. ISBN 1-903544-09-2.
  • Harper, Douglas (ed.). "gneiss", Online Etymological Dictionary. Retrieved 2015-03-01.
  • Marshak, Stephen (2013). Essentials of Geology (4th ed.). W.W. Norton. ISBN 978-0-393-91939-4.
  • McKirdy, Alan, Roger Crofts and John Gordon (2007). Land of Mountain and Flood: The Geology and Landforms of Scotland. Edinburgh. Birlinn. ISBN 978-1-84158-357-0.
  • Murray, W.H. (1966). The Hebrides. London. Heinemann.
  • Sacks, Paul E. and Donald T. Secor (1990). "Kinematics of Late Paleozoic continental collision between Laurentia and Gondwana". Science, 250 (4988): 1702–05. doi:10.1126/science.250.4988.1702.

External links

Acasta Gneiss

The Acasta Gneiss is a tonalite gneiss in the Slave craton in Northwest Territories, Canada. The rock body is exposed on an island about 300 kilometres north of Yellowknife. The rock of the outcrop was metamorphosed 3.58 to 4.031 billion years ago and is the oldest known intact crustal fragment on Earth.First described in 1989, it was named for the nearby Acasta River east of Great Bear Lake. The Acasta outcrop is found in a remote area of the Tłı̨chǫ people land settlement. It is the oldest known exposed rock in the world.

Bugle Rock

Bugle Rock (Kannada: ಕಹಳೆ ಬಂಡೆ Kahale Bande) is a massive rock in the Basavanagudi area of South Bangalore, in the state of Karnataka. It is an abrupt rise above the ground of peninsular gneiss as the main rock formation and with an assessed age of about 3,000 million years. Bugle Rock has generated wide interest among the scientific community.Kempe Gowda II (who came to power in 1585), the feudal ruler of Bangalore, is credited with building four watchtowers setting limits for Bangalore's expansion, which included a tower on the Bugle Rock (on the southern boundary) as it commands a panoramic view of Bangalore city. It is said that at sunset a sentry would blow the bugle and hold a torch (Kannada:panju) which was visible from the other three watch towers (one on the southern bank of the Kempambudi tank on the west, the second near Ulsoor Lake in the east and the third tower adjoining Ramana Maharshi Ashram on Bellary Road, namely Mekhri Circle in the north). This was done to inform people that everything was safe at that location and to give a warning bugle call to alert the citizens of any intruders into the city. Most of the rocks on the Bugle Rock, next to the Bull Temple, have hollows, which were once used to light lamps. This landmark spreads over an area of 16 acres (6.5 ha). This rock is contiguous and similar to the rock at Lalbagh tower.

Foliation (geology)

Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by shearing forces (pressures pushing different sections of the rock in different directions), or differential pressure (higher pressure from one direction than in others). The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated metamorphic rocks are typically formed in the absence of significant differential pressure or shear. Foliation is common in rocks affected by the regional metamorphic compression typical of areas of mountain belt formation (orogenic belts).

More technically, foliation is any penetrative planar fabric present in metamorphic rocks. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and gneiss. The slatey cleavage typical of slate is due to the preferred orientation of microscopic phyllosilicate crystals. In gneiss, the foliation is more typically represented by compositional banding due to segregation of mineral phases. Foliated rock is also known as S-tectonite in sheared rock masses.

Examples include the bands in gneiss (gneissic banding), a preferred orientation of planar large mica flakes in schist (Schistocity), the preferred orientation of small mica flakes in phyllite (with its planes having a silky sheen, called 'phylitic luster' – the Greek word, phyllon, also means "leaf"), the extremely fine grained preferred orientation of clay flakes in slate (called "slaty cleavage"), and the layers of flattened, smeared, pancake-like clasts in metaconglomerate.

Geology of Eswatini

The geology of Eswatini formed beginning 3.6 billion years ago, in the Archean Eon of the Precambrian. Eswatini is the only country entirely underlain by the Kaapvaal Craton, one of the oldest pieces of stable continental crust and the only craton regarded as "pristine" by geologists, other than the Yilgarn Craton in Australia. As such, the country has very ancient granite, gneiss and in some cases sedimentary rocks from the Archean into the Proterozoic, overlain by sedimentary rocks and igneous rocks formed during the last 541 million years of the Phanerozoic as part of the Karoo Supergroup. Intensive weathering has created thick zones of saprolite and heavily weathered soils.

Geology of Greenland

Greenland is the largest island on Earth. Only one-fifth of its surface area is exposed bedrock, the rest being covered by ice. The exposed surface is approximately 410,000 km2.

The geology of Greenland is dominated by crystalline rocks of the Precambrian Shield. The crystalline rocks of the Nuuk/Qeqertarsuatsiaat area comprise some of the oldest bedrock in Greenland which covers most of western Greenland. The surface has been altered several times and has an appearance as though it were shaped billions of years ago. This is one of the reasons why the Nuuk area is extraordinary and also because the particular climate zone for the area limits the vegetation which makes it possible to observe impressive km-scale megascopic textures. The bedrock around Nuuk consists of two major lithologic packages, the dark melanocratic Amitsoq gneiss, which is intruded by and complexly folded into the younger leucocratic Nuuk gneiss. This western gneiss complex is approximately 3600 million years old.

The Isua Greenstone Belt in the Isukasia area, southwest Greenland, is extraordinary in that it contains some of the oldest bedrock on the planet, approximately 3800 million years old. The bedrock is not nearly as metamorphosed as the surrounding gneiss bedrock and is therefore of interest for answering how the earth's surface appeared billions of years ago. There is a massive magnetite resource in this area.

There are large deposits of rare-earth oxides at Kvanefjeld.

Greenland's first gold mine is the Nalunaq mine, which opened in 2004. Nalunaq is located 33 km northeast of Nanortalik, in the Ketilidian Orogenic Belt of southern Greenland (60° 21′ 29″ N, 44° 50′ 11″ W). Gold-quartz mineralization occurs along a shallowly-dipping fault believed to be a thrust fault in which the hanging wall consists of Paleoproterozoic amphibolite-facies metavolcanic rocks, and the footwall consists of variably altered and mineralized volcanic rocks (i.e., volcanogenic massive sulfides). Quartz-gold mineralization has been dated to 1.77 to 1.80 billion years ago (late Paleoproterozoic), during the Ketilidian Orogeny.The Skaergaard intrusion is a layered mafic intrusion in eastern Greenland formed 55 million years ago during the opening of the North Atlantic Ocean. Skaergaard is one of the world's foremost examples of a layered mafic intrusion which exhibits exceptionally well-developed cumulate layering.

Geology of Guernsey

Guernsey has a geological history stretching further back into the past than most of Europe. The majority of rock exposures on the Island may be found along the coastlines, with inland exposures scarce and usually highly weathered. There is a broad geological division between the north and south of the Island. The Southern Metamorphic Complex is elevated above the geologically younger, lower lying Northern Igneous Complex. Guernsey has experienced a complex geological evolution (especially the rocks of the southern complex) with multiple phases of intrusion and deformation recognisable.

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 hue imparted by the colour of the metamorphic minerals within the mafic rocks: the typical green minerals are chlorite, actinolite, and other green amphiboles.

A greenstone belt is typically several dozens to several thousand kilometres long and although composed of a great variety of individual rock units, is considered a 'stratigraphic grouping' in its own right, at least on continental scales.

Typically, a greenstone belt within the greater volume of otherwise homogeneous granite-gneiss within a craton contains a significantly larger degree of heterogeneity and complications and forms a tectonic marker far more distinct than the much more voluminous and homogeneous granites. Additionally, a greenstone belt contains far more information on tectonic and metamorphic events, deformations and palaeogeologic conditions than the granite and gneiss events, because the vast majority of greenstones are interpreted as altered basalts and other volcanic or sedimentary rocks. As such, understanding the nature and origin of greenstone belts is the most fruitful way of studying Archaean geological history.

Hillock

A hillock or knoll is a small hill, usually separated from a larger group of hills such as a range. Hillocks are similar in their distribution and size to small mesas or buttes. The term is largely a British one. This particular formation occurs often in Great Britain and China. A similar type of landform in the Scandinavian countries goes by the name ”kulle” or ”bakke” (depending on the country) and is contrary to the above phenomena formed when glaciers polish down hard, crystalline bedrock of gneiss or granites, leaving a rounded rocky hillock with sparse vegetation.One of the most famous knolls is the one near John F. Kennedy's point of assassination, the grassy knoll.

A "blind knoll" is either hidden or not readily apparent to those driving vehicles. There are road signs that warn of this, advising drivers to slow down.

Inks Lake State Park

Inks Lake State Park is a state park located in Burnet County, Texas, United States, next to Inks Lake on the Colorado River. The landscape of the park is hilly, with many cedar, live oak, prickly pear cacti, and yuccas. The ground is rocky, mainly consisting of gneiss rock.

Devil's Waterhole is a small extension of Inks Lake, which is almost completely surrounded by rock. A canoe tour is conducted at Devil's Waterhole, and although diving and swimming may be done at the waterhole, it is at one's own risk, without a lifeguard.

There is abundant wildlife at the park, including deer, vultures, quail, and many other types of birds.

K2

K2 (Urdu: کے ٹو‬‎, Kai Ṭū), also known as Mount Godwin-Austen or Chhogori (Balti and Urdu: چھوغوری‬‎, Chinese: 乔戈里峰), at 8,611 metres (28,251 ft) above sea level, is the second highest mountain in the world, after Mount Everest at 8,848 metres (29,029 ft). It is located on the China–Pakistan border between Baltistan in the Gilgit-Baltistan region of northern Pakistan, and the Taxkorgan Tajik Autonomous County of Xinjiang, China. K2 is the highest point of the Karakoram range and the highest point in both Pakistan and Xinjiang.

K2 is known as the Savage Mountain due to the extreme difficulty of ascent. It has the second-highest fatality rate among the eight thousanders, with around 300 successful summits and 77 fatalities; about one person dies on the mountain for every four who reach the summit. It is more difficult and hazardous to reach the peak of K2 from the Chinese side, so it is usually climbed from the Pakistani side. K2 has never been climbed during winter, unlike Annapurna, the mountain with the highest fatality-to-summit rate (191 summits and 61 fatalities), or the other eight-thousanders. Ascents have almost always been made in July and August, the warmest times of year; K2's more northern location makes it more susceptible to inclement and colder weather.

Knot Gneiss

Knot Gneiss is the 34th book of the Xanth series by Piers Anthony

Lewisian complex

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 Paleoproterozoic age, ranging from 3.0–1.7 Ga. They form the basement on which the Torridonian and Moine Supergroup sediments were deposited. The Lewisian consists mainly of granitic gneisses with a minor amount of supracrustal rocks. Rocks of the Lewisian complex were caught up in the Caledonian orogeny, appearing in the hanging walls of many of the thrust faults formed during the late stages of this tectonic event.

Mejillones Metamorphic Complex

The Mejillones Metamorphic Complex is made up of two separate outcrops of metamorphic rocks in the Mejillones Peninsula of northern Chile. Turbidites of low metamorphic grade make up the northern outcrop at Morro Mejillones. The southern outcrop lies at Morro Jorgiño and is made up of schist, gneiss, amphibolite and quartzite. Rocks at Morro Jorgiño are intruded by garnet-bearing leucogranites.

Metamorphic rock

Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (100 megapascals (1,000 bar) or more), causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock.

Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. They are classified by texture and by chemical and mineral assemblage (metamorphic facies). They may be formed simply by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above it. They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion. They are also formed when rock is heated by the intrusion of hot molten rock called magma from the Earth's interior. The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides information about the temperatures and pressures that occur at great depths within the Earth's crust.

Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite.

Narryer Gneiss Terrane

The Narryer Gneiss Terrane is a geological complex in Western Australia that is composed of a tectonically interleaved and polydeformed mixture of granite, mafic intrusions and metasedimentary rocks in excess of 3.3 billion years old, with the majority of the Narryer Gneiss Terrane in excess of 3.6 billion years old. The rocks have experienced multiple metamorphic events at amphibolite or granulite conditions, resulting in often complete destruction of original igneous or sedimentary (protolith) textures. Importantly, it contains the oldest known samples of the Earth's crust: samples of zircon from the Jack Hills portion of the Narryer Gneiss have been radiometrically dated at 4.4 billion years old, although the majority of zircon crystals are about 3.6-3.8 billion years old.

The Narryer Gneiss Terrane is adjacent to the northernmost margin of the Yilgarn Craton and is abutted on the north by the Gascoyne Complex metasedimentary and metagranite orogen. The Narryer Gneiss Terrane also includes parts of the Yarlarweelor Gneiss which abuts to Nabberu Basin metamorphic sequences of the Bryah-Padbury Basins, where it is present as discontinuous slivers of metamorphic rocks, pelites, metaconglomerates and gneisses caught up within regional strike-slip oblique thrust faults.

The Narryer Gneiss in this far-eastern region may form the basement to the 2.0-1.8 billion year old Proterozoic rocks, and the unconformity surface may be preserved within the thrust sheets.

The Narryer Gneiss Terrane is divided into four major rock sequences (Myers 1990); the Dugel Gneiss, Meeberrie Gneiss, Manfred Complex, and unassigned polydeformed leucocratic gneisses and metasediments.

Oldest dated rocks

The oldest dated rocks on Earth, as an aggregate of minerals that have not been subsequently broken down by erosion or melted, are more than 4 billion years old, formed during the Hadean Eon of Earth's geological history. Such rocks are exposed on the Earth's surface in very few places.

Some of the oldest surface rock can be found in the Canadian Shield, Australia, Africa and in a few other old regions around the world. The ages of these felsic rocks are generally between 2.5 and 3.8 billion years. The approximate ages have a margin of error of millions of years. In 1999, the oldest known rock on Earth was dated to 4.031 ±0.003 billion years, and is part of the Acasta Gneiss of the Slave craton in northwestern Canada. Researchers at McGill University found a rock with a very old model age for extraction from the mantle (3.8 to 4.28 billion years ago) in the Nuvvuagittuq greenstone belt on the coast of Hudson Bay, in northern Quebec; the true age of these samples is still under debate, and they may actually be closer to 3.8 billion years old. Older than these rocks are crystals of the mineral zircon, which can survive the disaggregation of their parent rock and be found and dated in younger rock formations.

In January 2019, NASA scientists reported the discovery of the oldest known Earth rock – on the Moon. Apollo 14 astronauts returned several rocks from the Moon and later, scientists determined that a fragment from one of the rocks contained "a bit of Earth from about 4 billion years ago." The rock fragment contained quartz, feldspar, and zircon, all common on the Earth, but highly uncommon on the Moon.

Saint-Paul-la-Roche

Saint-Paul-la-Roche, in Occitan Sent Pau la Ròcha, is a commune in the northeast of the Dordogne department in the Nouvelle-Aquitaine region in southwestern France. The commune is integrated into the Regional Natural Parc Périgord Limousin.

Western Gneiss Region

The Western Gneiss Region (Norwegian: Gneisregionen) is a large geological unit in Norway chiefly made of gneiss rock that formed through metamorphism during the Caledonian orogeny. It makes up a tectono-stratigraphic terrane of the Scandinavian Caledonides and is also part of the Baltic shield. The region extends across western Norway from Bergen to Trondheim as a Caledonian window, outliers of the Western Gneiss Region crop out as far north as the Lofoten archipelago. The rocks of the Western Gneiss Region are made up of variouly deformed Precambrian basement, and cover rocks of autochthonous and para-autochthonous origin with evidence of medium to high grade metamorphism including ultra-high-pressure metamorphism.Rocks of the Western Gneiss Region formed 1700 to 1600 million years ago in the Late Paleoproterozoic. What was to become the Western Gneiss Region was intruded by migmatites and granites during the Sveconorwegian orogeny.

Yilgarn Craton

The Yilgarn Craton is a large craton that constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts. Zircon grains in the Jack Hills, Narryer Terrane have been dated at ~4.27 Ga, with one detrital zircon dated as old as 4.4 Ga.

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