Geologists from Hawaiian Volcano Observatory studying the crust of a cooled lava flow from Kilauea, Hawaii, Hawaii. © Roger Ressmeyer/Corbis. Reproduced by permission.

Geology is the study of the earth. Specifically, geologists may study mountains, valleys, plains, sea floors, minerals, rocks, fossils, and the processes that create and destroy each of these. Geology consists of two broad categories of study. Physical geology studies Earth's materials (erosion, volcanism, sediment deposition, etc.) that create and destroy the materials and landforms. Historical geology explores the development of life by studying fossils (petrified remains of ancient life) and the changes in land (for example, distribution and latitude) via rocks. The two categories overlap in their coverage: for example, to examine a fossil without also examining the rock that surrounds it tells only part of the preserved organism's history.

Physical geology further divides into more specific branches, each of which deals with its...