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.

Igneous petrology is the study of melts (magma) and the rocks that crystallize from such melts, encompassing an understanding of the processes involved in melting and subsequent rise, evolution, crystallization, and eruption or emplacement of the eventual rocks. Origin by crystallization from a melt seems a simple enough criterion for considering a rock to be igneous.

Igneous petrology was to some extent essentially a descriptive science until about 1960. The results were mainly obtained from field work, major element analyses, and microscopical studies. During the 1960's two simultaneous developments took place, plate tectonics became generally accepted, and the generation of magmas could now be related to the geodynamic features like convection cells and subduction zones. The other new feature was the development of new analytical apparatus which allowed high accuracy analyses of trace elements and isotopes. In addition it became possible to do experimental studies at pressures up to 100 kbar. <...>