Granites

Abstract

Granites are coarse-grained igneous intrusive rocks, which, although present in most tectonic settings, are most characteristic of continental crust. While basalts are the most abundant volcanic rocks, granites are the most abundant intrusive rocks. On the scale of the Solar System, while basalts are ubiquitous, granites seem to be restricted to the Earth. The volcanic equivalent of granite is rhyolite, although rhyolites are of much lower abundance than granite, probably because such magmas are more viscous and less likely to reach the surface. In addition, many pyroclastic rocks have a granitic composition and are likely to have their origins in underlying granitic plutons. Granites may originate by differentiation of a basaltic magma, but the abundance of granites and the low abundance of intermediate rocks argues in general against this possibility and granite magmas are generally thought to have formed by melting of pre-existing rocks. Several distinct types of granite are recognized depending on their origin.