DEFINITION OF GRANITE

A coarse-grained igneous plutonic rock with visible

quartz, potassium, and plagioclase feldspar, and dark minerals

such as biotite or amphibole, is generally known as granite,

but the IUGS (International Union of Geological Scientists)

define granite more exactly as a plutonic rock with 10–50 percent

quartz, and the ratio of alkali to total feldspar in the

range of 65–90 percent.

Granites and related rocks are abundant in the continental

crust and may be generated either by the melting of preexisting

rocks, or, in lesser quantities, by the differentiation by

fractional crystallization of basaltic magma. Many granites

are associated with convergent margin or Andean-style magmatic

arcs and include such large plutons and batholiths as

those of the Sierra Nevada batholith, Coast Range batholith,

and many others along the American Cordillera. Granites are

also a major component of Archean cratons and granite

greenstone terranes.

Many building stones are granitic, since they tend to be

strong, durable, nonporous, and exhibit many color and textural

varieties. Granite often forms rounded hills with large round

or oblong boulders scattered over the hillside. Many of these

forms are related to weathering along several typically perpendicular

joint sets, where water infiltrates and reacts with the

rock along the joint planes. As the joints define cubes in three

dimensions, large blocks get weathered out and eventually get

rounded as the corners weather faster than the other parts of

the joint surface. Granite also commonly forms exfoliation

domes, in which large sheets of rock weather off and slide

down mountainsides and inselbergs, isolated steep-sided hills

that remain on a more weathered plain. Many granites weather

ultimately to flat or gently rolling plains covered by erosional

detritus including cobbles, boulders, and granitic gravels.

See also CRATONS; EXFOLIATION; IGNEOUS ROCKS; PLUTON.

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