Magma Composition and Igneous Rocks

Introduction In the following chapters we will find that nearly all of the varied aspects of volcanism are either directly or indirectly controlled by the chemical composition of magma and the rate at which it cools and solidifies. If you are to understand and appreciate subsequent discussions of volcanoes and related volcanic phenomena, it is essential that you have a good grasp of the basic concepts of magma composition and the controls of magma cooling rate on magma crystallization. To provide this important background, this chapter first discusses how the chemical composition of magma (and any resultant igneous rocks) is expressed, followed by a discussion of the compositional characteristics of natural magmas. Following this, the important controls of magma cooling rate on igneous rock texture are discussed. Finally, the dual concepts of magma composition and cooling rate are combined to define a very useful classification scheme for the most common types of igneous rocks.