Petrology: the branch of geology that studies the origin, composition, distribution and structure of rocks. It utilizes the classical fields of mineralogy, petrography, and chemical analyses to describe the structure and composition of rocks. In addition, modern petrologists include the principles of geochemistry and geophysics to better understand the origins of rocks. There are three branches of petrology, corresponding to the three main types of rocks: igneous, metamorphic, and sedimentary. The study of rocks provides us with important information about the nature of the Earth's crust and mantle. In addition, it enables us to gain a sense of the Earth's history, including tectonic processes that occurred over the long course of geological time.