This chapter deals with the classification of igneous rocks. This reduces the thousands of rock names found in the literature down to a manageable number and links them to a logical classification based on their mineral content and chemical composition. The chapter presents the classification adopted by the International Union of Geological Sciences (IUGS), which uses the abundance of the major rock-forming minerals (the mode) to place rocks in compositional fields for which there are commonly accepted names. Some rocks are too fine-grained, or even glassy, for this modal classification to be applied.

This chapter outlines what is known about the pressures and temperatures in the Earth. We start by discussing pressure and see that although rocks near the surface are strong, they become weak and flow plastically at depth. As a result, reasonable pressures can be calculated by treating them as extremely viscous liquids; we refer to this pressure as lithostatic. Exceptions to this approximation occur if fluids are released by metamorphic reactions or melt is generated at rates that exceed the rate at which surrounding rock can deform.

This chapter introduces rocks and petrology, the science that attempts to explain the origin and distribution of rocks. Rocks are the solid material constituting the Earth, and it is essential to know what composition the planet has. We begin with a brief review of the planetary formation process and how it determined the Earth’s composition and major divisions into core and mantle.

The mineralogical composition of a metamorphic rock can be used to determine pressures, temperatures, and fluid compositions during metamorphism. However, this is only part of the record these rocks preserve. Their textures are also a source of valuable information and can be diagnostic of a particular type of metamorphism – contact versus regional, for example.

In this chapter we discuss common igneous rock associations. Since the end of the Archean, most, but not all, igneous rock associations can be related to their plate tectonic settings. The Earth’s largest igneous-rock factory has been at divergent plate boundaries in oceanic regions, where MORBs and associated intrusive rocks have been generated as a result of decompression melting.

Most prograde metamorphic reactions involve dehydration or decarbonation. The large increase in entropy that accompanies the liberation of a fluid phase from a mineral ensures that rising metamorphic temperatures will favor reactions that produce a separate fluid phase.

Metamorphism is the sum of all the changes that take place in a rock as a result of changes in the rock’s environment; that is, changes in temperature, pressure (directed as well as lithostatic), and composition of fluids. The changes in the rock may be textural, mineralogical, chemical, or isotopic. These changes proceed at varying rates, so time is an important factor in metamorphism.

In this chapter we discuss how magmas differentiate to produce the wide range of igneous rocks. Many processes have been invoked, but fractional crystallization is undoubtedly the most important of these. After dealing with the chemical evidence for fractional crystallization, we discuss the actual mechanisms by which crystals can be segregated from liquid in magmas. Historically this was thought to be due to gravitative crystal settling.

The determination of metamorphic temperatures and pressures is fundamental for understanding the petrotectonic histories of mountain belts. Quantifying metamorphic fluid compositions, rates, and processes of heat transfer through the crust, rock deformation mechanisms, and rates of tectonic burial and exhumation are just a few examples of applications that require pressure–temperature estimates.

This chapter deals with isotope geochemistry and its role in igneous and metamorphic petrology. Isotopes are of two types: those that are radioactive and decay to other isotopes of a different element, and those that are stable and do not change with time.

Metamorphic rocks have mineral assemblages that crystallized at elevated pressures and temperatures. With certain assemblages, these conditions can be estimated quite closely using thermodynamic, kinetic, and experimental data, as we have seen in Chapters 19–22.

In this chapter we discuss the forms taken by bodies of igneous rock, starting with volcanic forms because they are the best understood and following with intrusive bodies.

In this chapter we deal with two important diffusion processes: the transfer of heat (conduction) and the migration of chemical constituents (diffusion). They typically operate at very different scales; heat moves, for example, through the lithosphere, whereas chemical diffusion operates over distances of microns to a few meters at the most. The laws governing their transport, however, are similar.