Archean - Definition and Overview

 id:eon    value:rgb(1,0.7,1)  legend:Archaean_eon  # light purple
 id:era    value:rgb(1,1,0.7)  legend:Era   # light yellow
 id:black  value:black

 bar:eon
 bar:era

 width:100 align:center textcolor:black mark:(line,black)

 bar:eon color:eon

 from:2500  till:3800 # text:Archaean~(2500-3800)

 bar:era color:era

 from:2500   till:2800  text:Neoarchean~(2500-2800)
 from:2800   till:3200   text:Mesoarchean~(2800-3200)
 from:3200   till:3600  text:Paleoarchean~(3200-3600)
 from:3600   till:3800   text:Eoarchean~(3600-3800)

 tabs:(25-left)

 pos:(15,350)
 text:"Preceded by:"
 text:^Hadean eon

 pos:(15,90)
 text:"Followed by:"
 text:^Proterozoic eon and
 text:^Paleoproterozoic era

The Archean is a geologic eon; it is a somewhat antiquated term for the time span between 2500 million years before the present and 3800 million years before the present. The Archean period follows the Hadean eon and is followed by the Paleoproterozoic eon. Although a few pebbles 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.