Granites and plutons

Part of Hall of Planet Earth.

Granites and Plutons AMNH/R.Mickens

Granite forms the core of many mountain chains, such as the Andes, Himalayas, and Sierra Nevada, and makes up most of the upper continental crust. The majority of granitic magmas are formed by melting near the base of the continents. The magmas slowly rise through the crust like great balloons. They solidify near the surface to form gigantic bodies of igneous rock called plutons, which are exposed later when uplift and erosion remove the overlying rock.

Cooling and crystallization
Granite and pegmatite have similar compositions, but they differ in the size and proportion of their crystals. Granite is composed of crystals large enough to be seen because the magma cools and solidifies slowly at depth, giving individual crystals time to grow. Pegmatites are rocks composed of very large crystals, sometimes as large as automobiles. These from from water-rich magmas.