Gneiss

It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks.

[1] The British Geological Survey (BGS) and the International Union of Geological Sciences (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 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 platy minerals are more likely to produce gneissose texture.

[8] 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.

Per the polar 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.

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

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 greenstone belts contain metavolcanic and metasedimentary 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.

[16] They consist of a dome of gneiss intruded by younger granite and migmatite and mantled with sedimentary rock.

Orthogneiss from the Czech Republic
Pure shear deformation of rock producing gneissic banding. The undeformed rock is shown at upper left, and the result of pure shear deformation at upper right. At lower left is the stretching component of the deformation, which compresses the rock in one direction and stretches it in the other, as shown by the arrows. The rock is simultaneously rotated to produce the final configuration, repeated at lower right.
Augen gneiss from Leblon , Rio de Janeiro City, Brazil
Ordovician augen gneiss outcrop, Canigou massif, eastern Pyrenees, France
Dark dikes (now foliated amphibolites ) cutting light grey Lewisian gneiss of the Scourie complex, both deformed and cut by later (unfoliated) pink granite dikes
Contact between a dark-colored diabase dike (about 1100 million years old) [ 13 ] and light-colored migmatitic paragneiss in the Kosterhavet National Park in the Koster Islands off the western coast of Sweden.
Sample of Sete Voltas gneiss from Bahia in Brazil, the oldest rock outcropping in the crust of South America, c. 3.4 billion years old (Archean)