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

Earth’s history includes five major mass extinctions, with up to 95% of species lost in events driven by impacts, volcanic eruptions, or potentially solar outbursts. The terminal Pleistocene extinction, though not part of the “Great Five,” saw the disappearance of North America’s megafauna, focusing on top predators and large mammals. | The Randall Carlson