Igneous rocks are rocks formed from the crystallization of a liquid (molten rock). Igneous rocks may be divided into two categories. Intrusive or plutonic rocks crystallize from magma beneath the earth's surface. Extrusive or volcanic rocks crystallize from lava at the earth's surface. Rock becomes buried deep underground. Intense heat and pressure melt the rock into magma. The magma cools and solidifies into Igneous Rock.