Archean

The Archean (Archaean) is a geologic eon that refers to the time before the Proterozoic, 2500 Ma (million years ago). Instead of being based on Stratigraphy, this date is defined chronometrically. The lower boundary has not been officially recognized by the International Commission on Stratigraphy, but it is usually set to 3800 Ma at the end of the Hadean eon.

Although a few stones are known that are older, the oldest rock formations exposed on the surface of the Earth are Archean or slightly older. Archean rocks are known from Greenland, the Canadian Shield, northwest Australia, and southern Africa.

The Archean atmosphere apparently lacked free Oxygen. Temperatures appear to have been near modern levels, although astronomers think that the sun was about one-third dimmer. This is thought to reflect larger amounts of greenhouse gases than later in the Earth's history.

Probable remains of bacterial mats (stromatolites) are found throughout the Archean, and a few probable bacterial fossils are known from Chert beds. It is thought that life was present throughout the Archean, but was probably confined to simple non-nucleated single-celled organisms, called prokaryotes.

External link

• GeoWhen Database

Precambrian
Hadean eon	Archean eon	Proterozoic eon	Phanerozoic eon

Archean eon
Eoarchean	Paleoarchean	Mesoarchean	Neoarchean