Granite is a common and widely-occurring group of intrusive felsicigneous rocks that form at great depths and pressures under continents. Granite consists of orthoclase and plagioclase feldspars, quartz, hornblende, biotite, muscovite and minor accessory minerals such as magnetite, garnet, zircon and apatite. Rarely, a pyroxene is present. Ordinary granite always carries a small amount of plagioclase, but when this is absent the rock is referred to as alkali granite. An increasing proportion of plagioclase feldspar causes granite to pass into granodiorite. A rock consisting of equal proportions of orthoclase and plagioclase plus quartz may be considered a quartz monzonite. A granite containing both muscovite and biotite micas is called a binary granite. The average density is 2.75 g/cm3 with a range of 1.74 to 2.80.
The word granite comes from the Latingranum, a grain, in reference to the coarse-grained structure of such a crystalline rock.
Granite occurs as relatively small, less than 100 km2 stock-like masses and as large batholiths often associated with orogenicmountain ranges and is frequently of great extent. Small dikes of granitic composition called aplites are associated with granite margins. In some locations very coarse-grained pegmatite masses occur with granite. Granite has been intruded into the crust of the Earth during all geologic periods, except perhaps the most recent; much of it is of Precambrian age. Granite is widely distributed throughout the continental crust of the Earth and is the most abundant basement rock that underlies the relatively thin sedimentary rock veneer of the continents.
Origin
There are two theories for the origin of granite. The magmatic theory states that granite is derived by the crystalfractionation of magma. Thus granite bodies are the result of intrusion of liquid magma into the existing rocks. The granitization theory states that granite is formed in place by extreme metamorphism. There is evidence to support both theories, and both are useful to explain different observed features. The two may actually merge: as metamorphic conditions increase to the melting point of the metamorphosed granite, it will melt and become a liquid magma.
Uses
Granite has been extensively used a dimension stone and as flooring tiles in public and commercial buildings and monuments.
Granite may occur in dikes or sills (tabular bodies injected in fissures and inserted between other rocks), but more characteristically it forms irregular masses of extremely variable size, ranging from less than a few square miles to larger masses (batholiths) that are often hundreds or thousands of square miles in area.
Granites in which plagioclase greatly exceeds potassiumfeldspar are common in large regions of the western United States and are thought to be characteristic of the great series of batholiths stretching from Alaska and British Columbia southward through Idaho and California into Mexico.
Granite is widely distributed throughout the continental crust of the Earth and is the most abundant basement rock that underlies the relatively thin sedimentaryveneer of the continents.
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