How rocks are classified Rocks 101 Click on linked terms to go to college-level rock sites. The chemical composition of the magma and its cooling rate determine the final igneous rock type. Intrusive igneous rocks are formed from magma that cools and solidifies deep beneath the Earth's surface. The insulating effect of the surrounding rock allows the magma to solidify very slowly. Slow cooling means the individual mineral grains have a long time to grow, so they grow to a relatively large size.
Intrusive rocks have a characteristically coarse grain size.
Extrusive igneous rocks are formed from magma that cools and solidifies at or near the Earth's surface. Exposure to the relatively cool temperature of the atmosphere or water makes the erupted magma solidify very quickly. Sometimes the magma is quenched so rapidly that individual minerals have no time to grow. This is how volcanic glass forms. They form from deposits that accumulate on the Earth's surface.