How Is Granite Formed?

Granite is an igneous rock that forms when a pocket of magma rises into the upper levels of the Earth's crust and slowly cools. Cooling slowly gives granite, which is composed mainly of feldspar and quartz, enough time to develop large crystals that make the stone very hard and strong. Some granite, known as gneiss, can form from sedimentary rock that has gone through a long metamorphosis.

In places where a dense section of continental crust meets another, lighter section of continental crust, the denser one is driven under, or "subducted," into a hot region under the crust known as the "melt zone." Here, in the presence of a heat-conductive compounds such as water or carbon dioxide, it melts into a form of magma known as rhyolite. If the rhyolite later erupts to the surface, it cools into a basalt. In places where the rhyolite remains underground, it forms a large, slowly cooling mass. This mass, known as a pluton, cools slowly enough to allow extensive crystal formation in the rock. These crystals give granite its characteristic light color and the coarse, granular texture that gives the rock its name. The large, interlocked lattice they form gives granite much of its strength.