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Gneiss (/ n aɪ s / nice) 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. This rock is formed under pressures ranging from 2 to 15 kbar, sometimes even more, and temperatures over 300 °C (572 °F).
Metamorphic rocks can be derived from any other kind of non-metamorphic rock and thus there is a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology . Protoliths are non-metamorphic rocks and have no protoliths themselves.
These rocks reach the greenschist, amphibolite, or granulite facies and are the most common of metamorphic rocks produced by regional metamorphosis. The association of an outer high-pressure, low-temperature metamorphic zone with an inner zone of low-pressure, high-temperature metamorphic rocks is called a paired metamorphic belt .
Hall found that this produced a material strongly resembling marble, rather than the usual quicklime produced by heating of chalk in the open air. French geologists subsequently added metasomatism, the circulation of fluids through buried rock, to the list of processes that help bring about metamorphism. However, metamorphism can take place ...
Ultra-high-pressure metamorphism refers to metamorphic processes at pressures high enough to stabilize coesite, the high-pressure polymorph of SiO 2.It is important because the processes that form and exhume ultra-high-pressure (UHP) metamorphic rocks may strongly affect plate tectonics, the composition and evolution of Earth's crust.
Outcrop of weathered Lewisian gneiss, 5 km NW of Lochinver. 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 ...
The mafic minerals in the TTG rock body, possibly biotite, were weathered, which introduced a brownish coating on the TTG rock surface. Tonalite–trondhjemite–granodiorite (TTG) rocks are intrusive rocks with typical granitic composition (quartz and feldspar) but containing only a small portion of potassium feldspar.
This diamond is a mineral from within an igneous or metamorphic rock that formed at high temperature and pressure. The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous.