Chapters 3 and 4 are about igneous rocks and igneous processes. (Either chapter may be read first). Chapter 4 focuses on volcanoes and igneous activity that takes place at Earth's surface. Chapter 3 describes igneous processes that take place underground. However, you will learn early in this chapter how volcanic as well as intrusive rocks are classified based on their grain size and mineral content.
We begin the chapter by introducing the rock cycle. This is a conceptual device that shows the interrelationship between igneous, sedimentary, and metamorphic rocks. We then begin focusing on igneous rocks. After the section on igneous rocks classification, we describe structural relationships between bodies of intrusive rock and other rocks in the earth's crust. This is followed by a discussion of how magmas form and are altered. We conclude by discussing various hypotheses that relate igneous activity to plate tectonic activity.