Igneous rocks are the most abundant rocks in the Earth's crust, formed by the crystallization of melts that have risen from the planet's interior. The crust itself has formed over an extended period of time, in fact billions of years, by just such crystallization. The outer half of the planet is normally solid, so it is natural to wonder where magmas come from. Where does this melting take place, and why do normally solid rocks become molten? What composition do rocks have in the source region? Is melting partial or total? What magma compositions are formed when these rocks melt? What makes magma rise toward the surface, and how fast does it travel? As magma ascends and begins to cool, it becomes a mixture of liquid and crystals that can separate from one another and change the composition of the magma. Eventually, magma cools and solidifies to form igneous rock. The processes leading to the formation of this rock are of great importance because they have controlled the differentiation of our planet. The composition of the crust is very different from that of the planet as a whole, and life as we know it would be very different if these processes had not taken minor elements from the Earth's interior and concentrated them in the crust. In this chapter we look into each of these igneous rock-forming processes. We leave to Chapter 10 a discussion of the diversity and classification of igneous rocks.
Igneous rocks are those formed by the solidification of molten rock. This molten material, which we call magma, is formed at depth in the Earth and rises toward the surface, where it cools and solidifies, either beneath the surface, where it usually has time to crystallize, or on the surface as volcanic rocks, where cooling may be rapid enough to form glass. We classify igneous rocks on the basis of the minerals they contain, which reflect the composition of the magma. Magma compositions are determined in the source region by the type of rock that undergoes partial melting, but they can be modified during ascent and solidification, especially in large magma chambers, where solidification can take thousands of years. Throughout geologic time, the rise of magmas from the mantle has slowly generated the Earth's crust, whose composition is therefore determined by the composition of magmas.
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