Britannica Sources used in this answer

How is the geologic time scale divided into eons, eras, periods, epochs, and ages?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Grand Canyon rock layering — geochronology The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.

The geologic time scale serves as a standardized calendar for Earth's history, subdividing time into hierarchical units based on stratigraphy—the classification and correlation of rock strata. In descending order of duration, these units are eons, eras, periods, epochs, and ages.1

The boundaries between these intervals are primarily delineated by the fossil record, specifically the emergence and disappearance of widespread species within rock layers. Because life has undergone continuous evolutionary changes, these biological markers allow scientists to correlate strata globally.12

While the relative order of these units is established through biostratigraphic boundaries, the scale is also numerically quantified using radiometric dating methods, such as the analysis of isotope pairs like potassium-argon or uranium-lead. This dual approach—combining relative chronostratigraphic sequences with absolute chronometric dates—provides the framework used by the International Commission on Stratigraphy to maintain the standard geologic time scale.123