Geologic Time Scale: Eons, Eras and Periods

Looking at the Big Picture

Fossilized shark tooth
Sharks first evolved over 400 million years ago in the Paleozoic Era. Andrew Alden photo

This geologic time scale shows and gives dates for all of the defined eons, eras and periods of the ICS International Chronostratigraphic Chart. It does not include epochs and ages. More detailed time ranges are given for the Cenozoic Era, but beyond that there is a small amount of uncertainty on precise dates. For example, although the date listed for the beginning of the Ordovician Period is 485 million years ago, it is actually 485.4 with an uncertainty (±) of 1.9 million years.

 Where possible, I've linked to a geology or paleontology article for more information. More details beneath the table.

EonEraPeriodDates (Ma)
PhanerozoicCenozoicQuaternary2.58-0
Neogene23.03-2.58
Paleogene66-23.03
MesozoicCretaceous145-66
Jurassic201-145
Triassic252-201
PaleozoicPermian299-252
Carboniferous359-299
Devonian419-359
Silurian444-419
Ordovician485-444
Cambrian541-485
ProterozoicNeoproterozoicEdiacaran635-541
Cryogenian720-635
Tonian1000-720
MesoproterozoicStenian1200-1000
Ectasian1400-1200
Calymmian1600-1400
PaleoproterozoicStatherian1800-1600
Orosirian2050-1800
Rhyacian2300-2050
Siderian2500-2300
ArcheanNeoarchean 2800-2500
Mesoarchean 3200-2800
Paleoarchean 3600-3200
Eoarchean 4000-3600
Hadean  4600-4000
EonEraPeriodDates (Ma)
(c) 2013 Andrew Alden, licensed to About.com, Inc. (fair use policy). Data from Geologic Time Scale of 2015.

Back to the top-level geologic time scale

The periods of the Phanerozoic Eon are further subdivided into epochs; see those in the Phanerozoic Eon geologic time scale. Epochs are further subdivided into ages; see those in the Paleozoic Era, Mesozoic Era and Cenozoic Era geologic time scales. 

The Proterozoic and Archean Eons, along with the once "informal" Hadean Eon, are together called Precambrian time.

Of course, these units are not equal in length. Eons, eras and periods are usually separated by a significant geologic event and are unique in their climate, landscape and biodiversity. The Cenozoic Era, for example, is known as the "Age of Mammals." The Carboniferous Period, on the other hand, is named for the large coal beds that were formed during this time ("Carboniferous" means coal-bearing). As you may have guessed from its name, the Cryogenian Period was a time of great glaciations. 

The dates shown on this geologic time scale were specified by the International Commission on Stratigraphy in 2015. The colors were specified by the Committee for the Geologic Map of the World in 2009.

PS - All in all, there are 4 eons, 10 eras and 22 periods. The eons can be memorized pretty easily by mnemonics - we were taught "Please Pass A Ham" for Phanerozoic, Proterozoic, Archean and Hadean. If you exclude the Precambrian, the eras and periods can be memorized easily too. Check here for a few helpful hints. 

Edited by Brooks Mitchell

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Alden, Andrew. "Geologic Time Scale: Eons, Eras and Periods." ThoughtCo, Feb. 28, 2017, thoughtco.com/geologic-time-scale-eons-eras-periods-1440796. Alden, Andrew. (2017, February 28). Geologic Time Scale: Eons, Eras and Periods. Retrieved from https://www.thoughtco.com/geologic-time-scale-eons-eras-periods-1440796 Alden, Andrew. "Geologic Time Scale: Eons, Eras and Periods." ThoughtCo. https://www.thoughtco.com/geologic-time-scale-eons-eras-periods-1440796 (accessed November 18, 2017).