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Granite (/ ˈ ɡ r æ n ɪ t / GRAN-it) is a coarse-grained 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 underground. It is common in the continental crust of Earth, where it is found in igneous ...
Eventually the sediments will become so dense that they would essentially form a rock. This process is known as lithification. Igneous rocks have crystallised from a melt or magma. The melt is made up of various components of pre-existing rocks which have been subjected to melting either at subduction zones or within the Earth's mantle.
Igneous and metamorphic rocks make up 90–95% of the top 16 kilometres (9.9 mi) of the Earth's crust by volume. [1] Igneous rocks form about 15% of the Earth's current land surface. [note 1] Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are also geologically important because:
Granite forms deep in the Earth's crust under conditions of high ambient or lithostatic pressure. In order for the granite to be exposed at the Earth's surface a considerable thickness of rock must be eroded. This unloading allows the granite to expand radially and sheet fractures form tangentially to the radial stress. This indicates that the ...
Exfoliated granite sheets in Texas, possibly caused by pressure release. Pressure release or unloading is a form of physical weathering seen when deeply buried rock is exhumed. Intrusive igneous rocks, such as granite, are formed deep beneath the Earth's surface.
The simpler anoxygenic form arose about 3.8 Ga, not long after the appearance of life. The timing of oxygenic photosynthesis is more controversial; it had certainly appeared by about 2.4 Ga, but some researchers put it back as far as 3.2 Ga. [ 107 ] The latter "probably increased global productivity by at least two or three orders of magnitude".
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Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. [2] The lower continental crust is mostly metamafic-rock and pelite which have reached the granulite facies. The middle continental crust is dominated by metamorphic rock that has reached the amphibolite facies. [26]