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Geology
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Generalized Geologic Map of the Conterminous United States
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  The North American Tapestry of Time and Terrain

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Legend and Rock Ages
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  Distribution of Volcanic Rocks
 
  Map of Volcanic Rock Distribution of North America

VOLCANIC ROCKS

Igneous rocks are formed from magma (molten rock; the term igneous derives from the Latin ignis for "fire") that cools at the earth's surface or in the upper regions of the crust. The magma that reaches the surface, called lava, is exposed to much cooler water or air; and because it forms outside of the crust, it is called extrusive rock. It solidifies relatively quickly, sometimes within minutes or hours. Since magma typically reaches the surface through volcanoes, this extrusive rock is commonly labeled volcanic. Since the cooling process is so short, the chemical components don't have the time to develop into large mineral crystals, and so tend to be very small, giving the rock a fine-grained texture. In some cases lava cools so fast that there are no discernible grains and the rock instead has a glassy texture. A good example of this is obsidian, a dark rock used by some ancient cultures to make stone cutting tools. Other common or well-known volcanic rocks include basalt, rhyolite, and pumice.

To learn more about volcanic rocks, visit the USGS Photo Glossary of Volcano Terms.