Science glossary film: Extrusion

The process of magma coming to the surface, either in steady flows or violent eruptions, and cooling to form rocks. Because the magma cools more quickly than in rocks that form underground, there's no time for large crystals to form, and the mineral grains tend to be relatively small. Basalt is the most common form of extrusive rock.

Because the magma cools more quickly than in rocks that form underground, there's no time for large crystals to form, and the mineral grains tend to be relatively small. Basalt is the most common form of extrusive rock.