Igneous rock or igneus (from the Latin ignis, "fire") is a type of rock formed from magma that cools and hardens, with or without crystallization, either below the surface as intrusive rocks (plutonic) or above the surface as a rock extrusive (volcanic). This magma can be derived from a half-molten rock or rock that already exist, either in the mantle or crust of the earth. Generally, the melting process occurs by one of the following processes: the increase of temperature, pressure drop, or change in composition. More than 700 types of igneous rocks has been described, largely formed beneath the surface of the earth's crust.
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