This material, called magma, is usually a mixture of liquid rock, gases, and mineral crystals. Igneous rocks are one of the three major types of rocks. The other types are metamorphic rocks and sedimentary rocks. See Rock . Extrusive rocks form when magma flows onto the surface of the earth or floor of the ocean and then cools and hardens.
Intrusive rocks result when magma solidifies beneath the earth's surface.
This magma that flows onto the surface is called lava. It forms broad flat sheets, or it may build up a volcanic shape by repeatedly erupting from a vent. Most types of lavas cool rapidly, resulting in the formation of rocks composed mainly of microscopic crystals. Some lavas cool so quickly that they form a smooth volcanic glass called obsidian. A porous volcanic glass called pumice results when gases bubble out of the solidifying lava.