Petrology, the study of rocks. Petrology is a major subdivision of geology and is divided into two main branches: petrography and petrogenesis. Petrography deals with the description and classification of rocks. Petrogenesis deals with the origin and formation of rocks. A third branch of petrology, called experimental petrology, is closely related to petrogenesis. In experimental petrology, conditions of high temperature and pressure like those found deep beneath the earth's surface are produced in the laboratory to study their effect on the materials involved in the formation of rocks.