Archean | ||||||
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![]() Artist's impression of an Archean landscape | ||||||
Chronology | ||||||
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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 | Ten oldest U-Pb zircon ages | |||||
Lower boundary GSSA | Along the Acasta River, Northwest Territories, Canada 65°10′26″N 115°33′14″W / 65.1738°N 115.5538°W | |||||
Lower GSSA ratified | 2023[1] | |||||
Upper boundary definition | Defined Chronometrically | |||||
Upper GSSA ratified | 1991[2] |
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 4,031 to 2,500 Mya (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 rare relict crystals, 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 from today's: the prebiotic atmosphere 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 bacteria) 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.