Introduction

Igneous rocks are formed from molten material known as magma, which usually consists of a solution of the Earth's most abundant elements: oxygen and silicon with smaller amounts of aluminum, calcium, magnesium, iron, sodium, and potassium. Robert Bunsen (1851), of “Bunsen burner” fame, was the first to recognize that magmas were actually “solutions” no different from those of salts in water, except that they were hotter. Most magmas are silicate melts, but rarer ones contain little or no silica and, instead, are composed essentially of calcium carbonate, sulfide, or iron oxide. The major elements of common magmas combine, on cooling, to form the so-called rock-forming minerals: quartz, feldspars, feldspathoids, pyroxenes, olivine, and, when water is present, amphiboles, and mica. In addition, common accessory minerals, such as iron–titanium oxides, apatite, zircon, and sulfides, form from minor magmatic constituents (Fe3+–Ti, P, Zr, and S, respectively) that do not readily enter the structures of the major rock-forming minerals.

The history of an igneous rock begins with the formation of magma at some depth in the Earth. The composition of the magma is determined by the chemical and mineralogical composition of the rock in the source region and by the process of melting. When sufficient melt has formed and coalesced, buoyancy causes it to rise. Further chemical modifications may occur during this period of transport.