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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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  Distribution of Plutonic Rocks
 
  Map of Plutonic Rock Distribution in North America

PLUTONIC ROCKS

Those igneous rocks that cool within the crust are called intrusive rocks. They form from magma that flows into the upper crust, and hardens very slowly, over many years. These masses of intrusive igneous rocks are called plutons (derived from Pluto, the Roman god of the underworld), and thus the rocks are called plutonic. They are often found in the bases of old volcanoes or mountain ranges formed by volcanic activity, the rock resulting from magma that did not erupt. Since these rocks form much more gradually, crystals usually have more time to grow. As a result, the minerals in these coarse-grained plutonic rocks are typically easy to identify. For example, granite, a well-known plutonic rock, usually has evident grains of quartz, biotite and feldspar in it.

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