The geological timescale provides a common framework for describing the Earth's long and complex geological history.

Geologists, palaeontologists, and other geoscientists study the Earth's history by combining and interpreting information found in rocks and minerals. This requires the geological timescale. The timescale, which is regulated by the International Commission on Stratigraphy, relates stratigraphy to geological time periods, and uses standard nomenclature, ages, and colour codes. The time scale is hierarchical, consisting of (from shortest to longest units) ages, epochs, periods, eras and eons; periods can cover several tens of millions of years.

Geoscientists use several methods to subdivide the timescale:

  • Relative chronology (stratigraphy) is used to determine the relative order of geological events. The method is based on the principle of younger rocks lying on top of older materials.
  • Where fossil evidence defines major changes in the dominance of particular life forms, it can be used to correlate between geographically distinct areas
  • Geochronology includes the use of absolute (radiometric) dating, which is based on the radioactive decay of elements found in rock-forming minerals, along with palaeomagnetism, which gives information about the direction and intensity of the Earth's magnetic field.

The different divisions correspond to important events in Earth's history. For example the Phanerozoic (541 ± 1  million years ago to the present day) is the geological eon in which animal life has been abundant, while both the start and end of the Triassic (252.2 ± 0.5 to 201.3 ± 0.2 million years ago) are characterised by mass-extinction episodes in the biosphere.

The times of boundaries in the timescale should not be regarded as definitive or static - they are updated as new information comes to light and as new and improved dating techniques are developed.

Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. (2013; updated). The ICS International Chronostratigraphic Chart. Episodes 36: 199-204.