Geology is a general term for the group of sciences that study the solid Earth, its materials, and processes. The numerous subdisciplines of geology may be divided into two major categories: physical geology and historical geology. Physical geology, which includes such sciences as geophysics, geochemistry, mineralogy, and hydrogeology studies the matter of which the Earth is made and the interaction of that material with water, air, and organisms. Historical geology studies fossils and rock strata to understand the history of life forms and of the Earth itself.

The word geology comes from the Greek words ge , "Earth," and logos, "reason". The Greeks were probably not the first people to develop theories about the composition and changing form of the Earth, but they did provide us with some of the oldest written records of these endeavors. Such philosophers as Anaximander (c. 610-547 B.C.), Empedocles (c. 490-430 B.C...