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. |
