4.6 billion years on one page
A readable guide to eons, eras and periods — with verified dates.
Dividing deep time
Geologists divide Earth’s history into eons, eras, periods and smaller units. The boundaries are defined by changes in the rock record — often extinctions or the first appearance of a fossil species — and are dated using radiometric methods.
Where the dates come from
The dates used on this website follow the International Chronostratigraphic Chart published by the International Commission on Stratigraphy (ICS). Boundaries are refined as dating improves; for example, the base of the Cretaceous is currently given as approximately 143.1 million years ago, revised from an earlier figure of about 145 million.
Treat dates printed on artistic illustrations with caution. Always check them against the current ICS chart.