Geologic time scale

The geologic time scale is the calendar that geologists and paleontologists use to describe Earth's history, which began about 4.6 billion years ago. Its units nest inside one another, from the longest to the shortest: eon, era, period, epoch and age. The four eons are the Hadean, Archean, Proterozoic and Phanerozoic, and the first three are together called the Precambrian. The Phanerozoic, the eon of abundant visible animal life, began roughly 540 million years ago and contains the Paleozoic, Mesozoic and Cenozoic eras.

The divisions are not equal in length. Their boundaries mark major changes recorded in rock layers, very often the appearance or disappearance of fossil groups, and several coincide with mass extinctions, such as the one that closed the Mesozoic about 66 million years ago. Radiometric dating of rocks supplies the ages in years. The International Commission on Stratigraphy sets the official divisions, fixing each boundary at a reference point in a chosen rock section, and the dates are revised as methods improve.

We live in the Holocene Epoch of the Quaternary Period, in the Cenozoic Era. A proposal to add an Anthropocene epoch, marking the age of human impact, was rejected in 2024.