Crystallization
Igneous rocks form when molten material cools and hardens. They may form either below
or above Earth’s surface. They make up most of the rocks on Earth. Most igneous rock is
buried below the surface and covered with sedimentary rock, and so we do not often see just
how much igneous rock there is on Earth. In some places, however, large areas of igneous
rocks can be seen at Earth’s surface. Figure4.11shows a landscape in California’s Sierra
Nevada that consists entirely of granite, an igneous rock.
Figure 4.11: This landscape high in California’s Sierra Nevada is completely made up of
granite exposed at Earth’s surface. ( 25 )
Igneous rocks are calledintrusivewhen they cool and solidify beneath the surface. Because
they form within the Earth, cooling can proceed slowly, as discussed in the chapter ”Earth’s
Minerals.” Because such slow cooling allows time for large crystals to form, intrusive igneous
rock has relatively large mineral crystals that are easy to see. Granite is the most common
intrusive igneous rock (Figure4.12).
Igneous rocks are calledextrusivewhen they form above the surface. Theysolidifyafter
molten material pours out onto the surface through an opening such as a volcano (Figure
4.13). Extrusive igneous rocks cool much more rapidly than intrusive rocks. They have
smaller crystals, since the rapid cooling time does not allow time for large crystals to form.
Some extrusive igneous rocks cool so rapidly that crystals do not develop at all. These form