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.
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