Image:North america rock volcanic.jpg|Volcanic rock in North America.Image:North america rock plutonic.jpg|Plutonic rock in North America.Igneous rocks (etymology from Latin ignis, fire) are rock formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle or crust. Typically, the melting is caused by one or more of the following processes — an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust. Geologic significance Igneous rocks make up approximately ninety-five percent of the upper part of the Earth's crust, but their great abundance is hidden on th

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # TAGS