Volcanism

Volcanism is the movement of molten rock, called magma, toward or onto the earth’s surface. It is not just the eruption you see; the term also covers the creation of many intrusive and extrusive volcanic landforms — that is, forms built below the ground and forms built on the surface.

The book points out that many aspects of volcanism have already been dealt with in detail in two earlier places: under volcanoes in Unit II, and under igneous rocks in the preceding chapter of this unit. So this section is a connecting piece, not a fresh full treatment. …