Archean

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"Archaean" redirects here. For the division of living organisms, see Archaea.

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

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 from:2500   till:2800  text:Neoarchean~(2500–2800)
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 from:3200   till:3600  text:Paleoarchean~(3200–3600)
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The Archean (IPA: /ɑːˈkiːən/, also spelled Archaean, formerly called the Archaeozoic (IPA: /ɑːkiə(ʊ)ˈzəʊɪk/), also spelled Archeozoic) 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 mya at the end of the Hadean eon.

[edit] Archean Earth

At the beginning of the Archean, the Earth's heat flow was nearly three times higher than it is today, and was still twice the current level by the beginning of the Proterozoic. Volcanic activity was considerably more active than today, with numerous hot spots, and rift valleys, but Earth may have had a different tectonic style. Some scientists think because the Earth was hotter, that plate tectonic activity was also greater than it is today, but others argue that the lithosphere was too buoyant to subduct. The question of whether or not plate tectonic activity existed in the Archean is an active area of modern geoscientific research. <ref>Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6 p. 297-301</ref>

There were no large continents until late in the Archean; small protocontinents were the norm, prevented from coalescing into larger units by the high rate of geologic activity. These felsic protocontinents probably formed at hot spots rather than subduction zones, from a variety of sources: mafic magma melting more felsic rocks, partial melting of mafic rock, and from the metamorphic alteration of felsic sedimentary rocks.<ref>Stanley, pp. 297-301</ref>

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.

[edit] Archean Geology

Although a few mineral grains 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, western Australia, and southern Africa. Although the first continents formed during this eon, rock of this age makes up only 7% of the world's current cratons; even allowing for erosion and destruction of past formations, evidence suggests that only 5-40% of the present continental crust formed during the Archean.<ref>Stanley, pp. 301-2</ref>

In contrast to the Proterozoic, Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations. Greenstone belts are typical Archean formations, consisting of alternating high and low-grade metamorphic rocks. The high-grade rocks were derived from volcanic island arcs, while the low-grade metamorphic rocks represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. In short, greenstone belts represent sutured protocontinents.<ref>Stanley, pp. 302-3</ref>

[edit] Archean Life

Fossils of cyanobacterial mats (stromatolites) are found throughout the Archean—becoming especially common late in the eon—while a few probable bacterial fossils are known from chert beds.<ref>Stanley, 307</ref> In addition to the domain Bacteria (once known as Eubacteria), microfossils of the extremophilic domain Archaea have also been identified.

Life was probably present throughout the Archean, but may have been limited to simple non-nucleated single-celled organisms, called Prokaryota (and formerly known as Monera); there are no known eukaryotic fossils, though they might have evolved during the Archean and simply not left any fossils.<ref>Stanley, pp. 306, 323</ref> However, no fossil evidence yet exists for ultramicroscopic intracellular organisms such as viruses.

[edit] References:

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[edit] External links

• GeoWhen Database

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