Geologic Time Scale: Eons and Eras

This table shows the highest-level units of the geologic time scale: eons and eras. Where available, the names link to more detailed descriptions or significant events that occurred during that specific eon or era. More details beneath the table.

(c) 2013 Andrew Alden, licensed to About.com, Inc. (fair use policy). Data from Geologic Time Scale of 2015)

All of geologic time, from the Earth's origin about 4.54 billion years ago (Ga) to today, is divided into four eons. The oldest, the Hadean, wasn't recognized officially until 2012, when the ICS removed its informal classification. Its name is derived from Hades, in reference to the hellish conditions - rampant volcanism and violent cosmic collisions - that existed from the formation of Earth to 4 billion years ago.

The Archean remains somewhat of a mystery to geologists, as most fossil or mineral evidence from that time has been metamorphosed. The Proterozoic is more understood. Oxygen levels in the atmosphere began increasing around 2.2 Ga (thanks to cyanobacteria), allowing eukaryotes and multicellular life to flourish. The two eons and their seven eras are together informally referred to as Precambrian time.

The Phanerozoic encompasses everything within the past 541 million years. It's lower boundary is marked by the Cambrian Explosion, a rapid (~20 million year) evolutionary event in which complex organisms first evolved.

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 2015. 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. More precise dating methods and other scientific advancements have changed that. Today, the time scale is updated yearly, and the boundaries between time spans have become more clearly defined.

Edited by Brooks Mitchell