The aim of this chapter is to introduce the main types of igneous rock and their modes of occurrence. We classify igneous rocks according to their mineral content as well as by their texture, which varies according to whether the magma solidifies slowly beneath the surface or rapidly on the surface. We accordingly start the chapter by discussing where magmas cool and solidify and the types of igneous bodies formed. We then discuss the classification of igneous rocks using the scheme proposed by the International Union of Geological Sciences. Although many igneous rocks have been found and named, the vast majority are described by only a few names, and these rocks form common associations that can be related to their plate tectonic settings. We discuss and illustrate examples of the main rock types in each of these associations and explain why each forms in its particular setting. We end the chapter by discussing three unusual rock associations, two were formed only in the Precambrian, and the third was formed by large meteorite impacts.
So far in this book we have classified rocks into igneous, sedimentary, and metamorphic, and we have discussed the origin of magmas and igneous rocks using only a few common rock names, such as basalt, andesite, and granite. In this chapter, we consider the classification of igneous rocks in more detail, and we see, for example, that some rock types commonly occur together, whereas others are never associated. In Chapter 9, we learned about igneous rock-forming processes – why rocks melt, where they melt, and how they melt. We saw that the melting process controls the composition of the liquid, which determines the composition of magmas that ascend into the crust. We also learned about the physical properties of magma and of processes that can change its composition and, hence, the spectrum of igneous rocks it can form. We are now in a position to use this knowledge, first, to describe where igneous rocks form; second, to learn how igneous rocks are classified; and finally, to relate igneous rocks to plate tectonics.
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