Archean
(Redirected from Archaean)
| Archaean eon |
|---|
| Geologic timescale of the Precambrian |
| (millions of years ago) |
|
<timeline> ImageSize = width:175 height:355 PlotArea = left:40 right:5 bottom:100 top:45 AlignBars = justify Period = from:2500 till:3800 TimeAxis = orientation:vertical ScaleMajor = unit:year increment:500 start:2500 ScaleMinor = unit:year increment:100 start:2500 Colors = 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 Legend = left:15 top:40 BarData = bar:eon bar:era PlotData= 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) TextData = 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 </timeline> (expand) |
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.
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| Precambrian | |||
|---|---|---|---|
| Hadean eon | Archean eon | Proterozoic eon | Phanerozoic eon |
| Archean eon | |||
|---|---|---|---|
| Eoarchean | Paleoarchean | Mesoarchean | Neoarchean |
de:Archaikum et:Arhaikum eo:Arkeozoiko fr:Archéen he:ארכאיקון ja:始生代 nl:Archaïcum pt:Arqueano sv:Arkeikum