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Some igneous rocks form from volcanic lava.
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Courtesy of USGS

Igneous Rocks

Igneous rocks form when molten rock cools and becomes solid. Molten rock is called magma when it is below the Earth’s surface and lava when it is above.

Igneous rocks are divided into two groups, based on where the rock forms.

Igneous rocks that form below the Earth’s surface are called intrusive igneous rocks (or plutonic). They form when magma enters an underground chamber, cools very slowly, and forms rocks full of large crystals.

Igneous rocks that form above the Earth’s surface are called extrusive igneous rocks. These rocks, also called volcanic rocks, form when lava cools quickly at or above the Earth’s surface.


Last modified June 17, 2003 by Lisa Gardiner.

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The Fall 2010 issue of The Earth Scientist, focuses on rocks and minerals, including articles on minerals and mining, the use of minerals in society, and rare earth minerals, and includes 3 posters!

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