Archean

Archean
4000 – 2500 Ma Pha. Proterozoic Archean Had.
Artist's impression of an Archean landscape.
Chronology
−4500 — – — – −4000 — – — – −3500 — – — – −3000 — – — – −2500 — – — – −2000 — – — – −1500 — – — – −1000 — – — – −500 — – — – 0 — P r e c a m b r i a n Hadean A r c h e a n P r o t e r o z o i c P h a n. Eo Paleo Meso Neo Paleo Meso Neo Paleozoic Mesozoic Cenozoic   Scale: millions of years
Etymology
Name formality	Formal
Alternate spelling(s)	Archaean, Archæan
Synonym(s)	Eozoic J.W. Dawson, 1865
Usage information
Celestial body	Earth
Regional usage	Global (ICS)
Time scale(s) used	ICS Time Scale
Definition
Chronological unit	Eon
Stratigraphic unit	Eonothem
Time span formality	Formal
Lower boundary definition	Defined Chronometrically
Lower GSSA ratified	1991[citation needed]
Upper boundary definition	Defined Chronometrically
Upper GSSA ratified	1991[1]

The Archean Eon (IPA: /ɑːrˈkiːən/ ar-KEE-ən, also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history, preceded by the Hadean eon and followed by the Proterozoic. The Archean represents the time period from 4,000 to 2,500 Ma (millions of years ago). The Late Heavy Bombardment is hypothesized to overlap with the beginning of the Archean. The Huronian glaciation occurred at the end of the eon.

The Earth during the Archean was mostly a water world: there was continental crust, but much of it was under an ocean deeper than today's oceans. Except for some trace minerals, today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent activity. The Earth's atmosphere was also vastly different in composition to today's: it was a reducing atmosphere rich in methane and lacking free oxygen.

The earliest known life, mostly represented by shallow-water microbial mats called stromatolites, started in the Archean and remained simple prokaryotes (archaea and eubacteria) throughout the eon. The earliest photosynthetic processes, especially those by early cyanobacteria, appeared in the mid/late Archean and led to a permanent chemical change in the ocean and the atmosphere after the Archean.