Gneiss

Gneis «Ollo de Sapo».

Gneiss is a metamorphic rock composed of the same minerals as granite (quartz, feldspar and mica) but with a defined orientation in bands, with alternating layers of light and dark minerals. Sometimes it presents regularly distributed feldspathic concretions, in this case called ocellated gneiss.

Gneiss receive different names depending on the components (biotite, muscovitic gneiss), origin (orthogneiss if it is the product of metamorphism of igneous rocks and paragneiss, if it is of sedimentary rocks), or texture (for example. ocellated gneiss).

Etymology

The etymology of the word gneiss is unclear. According to some sources it comes from the Middle High German verb gneist, to spark, due to the brilliance of its minerals.

Description

Ortogneis of the Czech Republic

Traditionally in the UK and US, a gneiss is a coarse-grained metamorphic rock showing compositional banding (gneiss banding) but poorly developed schistosity and indistinct cleavage. In other words, it is a metamorphic rock composed of mineral grains easily visible to the naked eye, which form obvious compositional layers, but have only a weak tendency to fracture along these layers. In Europe, the term has been more broadly applied to any coarse, mica-poor, high-grade metamorphic rock.

Both the British Geological Survey and the IUGS use the term gneiss as a broad definition of a textural category of medium- and coarse-grained metamorphic rocks with poorly developed schistosity, with compositional layers of more than 5 millimeters thick and tends to break up into plates more than 1 centimeter thick. Neither of these definitions depends on composition or origin, although rocks poor in layered minerals are more likely to produce gneissy texture. Therefore, gneissous rocks recrystallize to a large extent, but do not contain large amounts of micas, chlorite, or other layered minerals. Metamorphic rock showing stronger schistosity is classified as schists, while metamorphic rock devoid of schistosity is classified as shale. called granofels.

Gneiss that are metamorphosed igneous rocks or their equivalent are called granite gneiss, diorite gneiss, etc. Gneiss rocks can 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. Both the BGS and IUGS use gneiss to describe rocks with the texture of gneiss, although gneiss also remains in common use. For example, a gneissous metagranite or a gneissic metagranite means a granite that has metamorphosed and thus acquired a gneissous texture.

Gneissic bands

Deformation by pure clutching of the rock that produces gnesic bands. The undeformed rock is shown on the top left and the result of pure cut deformation on the top right. At the bottom left is the stretching component of the deformation, which compresses the rock in one direction and stretches it in the other, as the arrows show. The rock is rotated simultaneously to produce the final configuration, which is repeated at the bottom right..

The minerals in gneiss are arranged in layers that appear as bands in cross section. This is called gneissic bands. 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 aluminum, sodium, and potassium).

Bands develop at high temperatures when rock is more strongly compressed in one direction than in other directions (non-hydrostatic stress). The bands develop perpendicular to the direction of greatest compression, also called the direction of shortening, as layered minerals are rotated or recrystallized in parallel layers.

A common cause of nonhydrodynamic stress is the subjection of the protolith (the original rock material undergoing metamorphism) to an extreme shear force, a sliding force similar to pushing the top of a deck of cards in one direction, and the bottom of the shell in the other direction. These forces stretch the rock like plastic and the parent material spreads out into sheets. According to the polar decomposition theorem, the deformation produced by such a shear force is equivalent to the rotation of the rock combined with shortening in one direction and extension in another.

Some bands form from original rock material (protolith) that is subjected to extreme temperatures and pressures and are composed of alternating layers of sandstone (lighter) and shale (darker), which metamorphoses 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 that is not fully understood.

Gneiss Augen

Gneis augen de Leblon, Rio de Janeiro, Brazil.

Augen gneiss, from German: Augen [ˈaʊɡən], meaning "eyes", is a gneiss resulting from the metamorphism of granite, containing characteristic elliptical or lenticular grains joined by shearing (porphyroclasts), normally feldspar, surrounded by finer grained material. The finer grained material is deformed around the stronger 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 ordinary gneiss (the mesosome) and another of which has the appearance of an intrusive rock such as pegmatite, aplite, or granite (leucosome). The rock may also contain a mafic rock melanosome complementary to the leucosome. Migmatites are often interpreted as rocks that have partially melted, where the leucosome represents the silica-rich melt, the melanosome the rock residual solid remaining after partial melting and mesosome the original rock that has not yet undergone partial melting.

Locations

Dark Dikes (now foliated amphibolites) that cut Lewis' gneis of light grey from the Scourie complex, both deformed and cut by later pink granite dikes (not foliated).

Contact between a diabase dike of dark color (about 1100 million years) and a light-colored migmatite paragneis at Kosterhavet National Park in the Koster Islands off the western coast of Sweden.

Sample of gneis Sete Voltas de Bahía in Brazil, the oldest rocky outcrop in the cortex of South America, c. 3.4 billion years (Arcaico).

Gneisses are characteristic of areas of regional metamorphism reaching the medium amphibolite to granulite metamorphic facies. In other words, the rock metamorphosed at a temperature greater than 600 °C at pressures between approximately 2 and 24 kbar. Many different varieties of rock can be transformed into gneiss, so geologists are careful to add descriptions of color and mineral composition to the name of any gneiss, such as garnet-biotite paragneiss or greyish-pink orthogneiss.

Granite Green Stone Belts

Continental shields are regions of exposed ancient rock that form the stable cores of the continents. The rock exposed in the oldest regions of escudos, which is of Archean age (more than 2.5 billion years), belongs mostly to greenstone-granite belts. The greenstone belts contain metavolcanic and metasedimentary rock that has undergone a relatively mild degree of metamorphism, at temperatures of 350–500 °C and pressures of 200–500 MPa (2,000–5,000 bar). The greenstone belts are surrounded by terranes of high-grade gneiss showing highly deformed low-pressure, high-temperature (>500°C) metamorphism to amphibolite or granulite facies. These form most of the rock exposed in Archean cratons.

Gneiss Domes

Gneiss domes are common in orogenic belts (mountain-forming regions). They consist of a gneiss dome invaded by younger granite and migmatite and covered with sedimentary rock. These have been interpreted as a geologic record of two distinct mountain-building 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 cores of metamorphic core complexes, regions of the deep crust brought to the surface and exposed during the extension of the Earth's crust.

Examples

• The gneis Acasta is located in the Northwest Territories, Canada, on an island about 300 kilometres north of Yellowknife. This is one of the oldest intact bark fragments on Earth, metamorphosed 3,58 to 4,031 billion years ago.
• The gneis de Lewis is located in the Hebrides of Scotland, on the Scottish continent west of the Moine Thrust and on the islands of Coll and Tiree. These rocks are in large part of igneous origin, mixed with quartzite metamorphosed marble and myca schist with subsequent intrusions of basalt dikes and granite magma.
• The gneis de Morton is an Arcano gneis exposed in the Minnesota River Valley of Southwest Minnesota, United States. It is believed to be the oldest intact block of continental bark in the United States.
• The gneis peninsular It is a sequence of archaic gneis that are found throughout the Indian shield and have an antiquity of between 3400 and 2500 million years.

Uses

Gneiss is used in the construction of steps, paving stones, and masonry, among others. It is also used in decorative elements.