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Geologic Time

(c) 2009 Andrew Alden, licensed to About.com, Inc. (fair use policy). Data from Geologic Time Scale of 2009)

All of geologic time, from the Earth's origin about 4600 million years ago to today, is divided into three eons. The first two eons, Archean and Proterozoic, and their seven eras are together informally referred to as Precambrian time. The informal term "Hadean" refers to time before 4 billion years ago (Ga). See summaries of the Archean Eon and the Proterozoic Eon.

The eras of the Proterozoic and Phanerozoic eons are each further divided into periods, shown in this geologic time scale.

The periods of the three Phanerozoic eras are divided in turn into epochs. (See the Phanerozoic epochs listed together.) Epochs are subdivided into ages. Because there are so many ages, they are presented separately for the Paleozoic Era, the Mesozoic Era and the Cenozoic Era.

The dates shown on this table were specified by the International Commission on Stratigraphy in 2009. Colors are used to indicate the age of rocks on geologic maps. There are two major color standards, the international standard and the U.S. Geological Survey standard. (All of the geologic time scales here are made using the 2009 standard of the Committee on the Geologic Map of the World.)

Geologic Time Scale: Precambrian Time

(c) 2009 Andrew Alden, licensed to About.com, Inc. (fair use policy. Data from Geologic Time Scale of 2009.

"Precambrian" is a long-established, informal name for the 4 billion years of Earth history before hard-bodied organisms arose at the beginning of the Cambrian Period, earliest division of the Phanerozoic Eon. Aside from the names of the eons and the Ediacaran Period (also called the Vendian), few of the terms in this geologic time scale are yet in wide use. The time before 4000 million years ago (4 billion or 4 Ga) is informally known as Hadean time.

Whereas later geologic history is minutely documented by fossils showing the evolving succession of living things, Precambrian time is almost entirely divided by absolute numerical ages of rocks. Nonetheless, the end of the Archean marks a distinct change in the Earth, roughly at the time atmospheric oxygen first appeared. (Read more about the Archean Eon and the Proterozoic Eon.) Archean and Proterozoic rocks are exposed in the continents' ancient central regions, but they make up a small fraction of Earth's total surface. The dates shown on this geologic time scale were specified by the International Commission on Stratigraphy in 2009. The colors were specified by the Committe on the Geologic Map of the World in 2009.

Geologic Time Scale: The Paleozoic Era

(c) 2009 Andrew Alden, licensed to About.com, Inc. (fair use policy). Data from Geologic Time Scale of 2009.

Back to the top-level geologic time scale

This geologic time scale represents the working edge of historical geology, showing the latest names and dates of the smallest divisions of geologic time that are universally recognized. The Paleozoic Era is the first part of the Phanerozoic Eon. For anyone but specialists, the rounded-off dates in the Phanerozoic table are sufficient. Each of these dates also has a specified uncertainty, which you can look up at the source. For example, the Silurian and Devonian age boundaries have more than 2 million years uncertainty and the Cambrian units are still being refined, but the rest of the chronology is more securely known.

The dates shown on this geologic time scale were specified by the International Commission on Stratigraphy in 2009, and the colors were specified by the Committee for the Geologic Map of the World in 2009.

Geologic Time Scale: The Mesozoic Era