geological time

Time scale embracing the history of the Earth from its physical origin to the present day. Geological time is traditionally divided into eons (Archaean or Archaeozoic, Proterozoic, and Phanerozoic in ascending chronological order), which in turn are subdivided into eras, periods, epochs, ages, and finally chrons.

The terms eon, era, period, epoch, age, and chron are geochronological units representing intervals of geological time. Rocks representing an interval of geological time comprise a chronostratigraphic (or time-stratigraphic) unit. Each of the hierarchical geochronological terms has a chronostratigraphic equivalent. Thus, rocks formed during an eon (a geochronological unit) are members of an eonothem (the chronostratigraphic unit equivalent of eon). Rocks of an era belong to an erathem. The chronostratigraphic equivalents of period, epoch, age, and chron are system, series, stage, and chronozone, respectively.

In 2004 a new period was added to geological time, the Ediacaran. Dating from 600 to 542 million years ago, the Ediacaran represents the last geological period of the Neoproterozoic age. The period had been called the Vendian period since 1952, but this term was not internationally agreed upon. The Ediacaran is the first time period to be officially added to geological time since 1884.