Geologic Time Scale

The geologic time scale divides Earth's roughly 4.54 billion year history into a nested hierarchy of intervals, from largest to smallest eons, eras, periods, epochs and ages, so that any given rock layer or event can be placed in a shared framework. It is built from two complementary methods: chronostratigraphy, which orders rock sequences by relative age using principles such as superposition, original horizontality and faunal succession, and geochronology, which uses radiometric dating and paleomagnetic analysis to attach precise numerical ages to the boundaries between intervals. The International Commission on Stratigraphy maintains the scale today, publishing and periodically revising it as the International Chronostratigraphic Chart, the field's authoritative global standard.