Igneous rocks that form above the ground are called extrusive and are usually associated with volcanoes. Rocks that form underground are called intrusive rocks. The magma that forms intrusive rocks solidifies slowly, allowing large crystals to form. Granite is a common example of an intrusive igneous rock. Where magma solidifies below the surface, it often forms solid plugs of granite that may be several kilometres thick and just as wide. These are called plutons.