Chapters 3 and 4 cover igneous activity. Either may be read before the other. Chapter 3 emphasizes intrusive activity, but it also covers igneous rock classification and the origin of magmas, which are applicable both to volcanic and intrusive phenomena. Chaper 4 concentrates on volcnoes and related extrusive activity.

Volcanic eruptions, while awesome natural spectacles, also provide important information on the workings of the Earth's interior. Volcanic eruptions vary in nature and in degree of explosvie violence. A srong correlation exists between the chemical composition of magma (or lava) and the violence of an eruption. The size and shape of volcanoes and lava flows and their pattern of distribution on the Earth's surface also correspond to the composition of their lavas.

Understanding volcanism provides a background for theories relating to mountain building, the development and evolution of continental and oceanic crust, and how the crust is deformed. Our observations of volcanic activity fit nicely into plate-tectonic theory as described in chapter 3.