THE GEOLOGIC TIME SCALE The geologic time scale is a system used by scientists to describe and organize Earth’s 4.6 billion year history. It divides time into structured units and subunits, helping researchers trace major geological, biological, and climatic events through deep time. At the broadest level are the eons, the largest divisions of time. There are four formally defined eons: Hadean from Earth’s formation 4.6 billion years ago to about 4.0 billion years ago. Archean from 4.0 billion t