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Geologic Time Scale: Eons and Eras


This table shows the highest-level units of the geologic time scale: eons and eras. More details beneath the table.

EonEraDates (m.y.)
(c) 2013 Andrew Alden, licensed to About.com, Inc. (fair use policy). Data from Geologic Time Scale of 2013)

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 2013. 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.)

It used to be that the geologic time scale was, dare I say, carved in stone. The Cambrian, Ordovician, Silurian and so on marched in their rigorous order, and that's all we needed to know. The exact dates involved were hardly important, since the assignment of an age relied only on fossils. That was a lifetime ago. Today the time scale is in a furious ferment, in the process of being nailed down by isotopic dating methods, and it seems like the International Commission on Stratigraphy updates the official scale every year now. So do I.

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