Geologic time describes the immense span of time—billions of years-revealed in the complex rock surface of the Earth. Geochronology is the science of finding out how old rocks and minerals are. Absolute time and relative time are terms used to describe the age of rocks and events used by geologists. Radiometric age determination is a method used by geologists to determine the actual age, in years, of rocks and minerals. Knowledge of stratigraphy, the branch of geology that catalogues the earth's successions of rock layers, is essential to establish the relative ages of rock units. By finding which rock unit came first, the order of the events in earth history can be sorted out.

Before scientific methods were used to find out about geologic time, ideas about time and earth history came from religious theories. The Hindu and Mayan religions believed in endlessly repeating...