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Igneous rock

Igneous rocks are formed 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.

Note:   The above text is excerpted from the Wikipedia article "Igneous rock", which has been released under the GNU Free Documentation License.
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July 29, 2015

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updated 12:56 pm ET