Igneous rocks form when molten rock (magma) cools and solidifies, 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 are geologically important because:
* their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, and the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted; * their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological strata, allowing a time sequence of events; * their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions (see plate tectonics); * in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin, and uranium are commonly associated with granites, whereas ores of chromium and platinum are commonly associated with gabbros.
Source: Wikipedia.Org
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