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The '''Archean''' ({{pronEng|ɑrˈkiːən}}, also spelled '''Archaean''', formerly called the '''Archaeozoic''' ({{IPA|/ɑrkiəˈzoʊ<s>ɪ</s>k/}}), also spelled '''Archeozoic''' or '''Archæozoic''') is a [[geology|geologic]] [[eon (geology)|eon]] before the [[Proterozoic]] and [[Paleoproterozoic]], ending 2.5 [[Ga (unit)|Ga]] (billion years ago). Instead of being based on [[stratigraphy]], this date is defined chronometrically. The lower boundary (starting point) has not been officially recognized by the [[International Commission on Stratigraphy]], but it is usually set to 3.8 Ga, at the end of the [[Hadean]] eon.

==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]]. The extra heat may have been remnant heat from the planetary accretion, partly heat of formation of the iron core, and partially caused by greater radiogenic heat production from short-lived radionuclides such as [[uranium]]-235.

The majority of Archean rocks which exist are [[metamorphic rock|metamorphic]] and [[igneous rock]]s, the bulk of the latter being intrusive.{{Clarifyme|date=May 2008}}<!--Link to an explanation of what "intrusive" means in this context.--> [[Volcano|Volcanic]] activity was considerably higher than today, with numerous [[Hotspot (geology)|hot spot]]s, and [[rift valley]]s, and eruption of unusual lavas such as [[komatiite]]. Intrusive igneous rocks such as great melt sheets and voluminous plutonic masses of [[granite]], [[diorite]], [[ultramafic to mafic layered intrusions]], [[anorthosite]]s and [[monzonite]]s known as [[sanukitoid]]s predominate throughout the crystalline [[craton]]ic remnants of the Archean crust which exist today.

The Earth of the early Archean may have had a different tectonic style. Some scientists think that, because the Earth was hotter, [[plate tectonic]] activity was more vigorous than it is today, resulting in a much greater rate of recycling of crustal material. This may have prevented cratonisation and continent formation until the mantle cooled and convection slowed down. Others argue that the sub continental lithospheric mantle is too buoyant to [[subduct]] and that the lack of Archean rocks is a function of erosion by subsequent tectonic events. 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 [[hotspot (geology)|hotspots]] rather than [[subduction zone]]s, from a variety of sources: [[igneous differentiation]] of mafic rocks to produce intermediate and felsic rocks, [[mafic]] magma melting more felsic rocks and forcing [[granitization]] of intermediate rocks, [[magma|partial melting]] of mafic rock, and from the [[metamorphism|metamorphic]] alteration of felsic sedimentary rocks. Such continental fragments may not have been preserved if they were not buoyant enough or fortunate enough to avoid energetic subduction zones.<ref>Stanley, pp. 297-301</ref>

Another explanation for a general lack of early Archean rocks greater than 3800 Ma is the amount of extrasolar debris present within the early solar system. Even after planetary formation, considerable volumes of large [[asteroid]]s and [[meteorite]]s still existed, and bombarded the early Earth until approximately 3800 Ma. A barrage of particularly large impactors known as the [[late heavy bombardment]] may have prevented any large crustal fragments from forming by literally shattering the early protocontinents.

===Archean palaeoenvironment===
The Archean atmosphere apparently lacked free [[oxygen]]. Temperatures appear to have been near modern levels even within 500 Ma of Earth's formation, with liquid water present, due to the presence of sedimentary rocks within certain highly deformed [[gneiss]]es. Astronomers think that the sun was about one-third dimmer, which may have contributed to lower global temperatures than otherwise expected. This is thought to reflect larger amounts of greenhouse gases than later in the [[History of Earth|Earth's history]].

By the end of the Archaean c. 2600 Mya, plate tectonic activity may have been similar to that of the modern Earth; there are well preserved sedimentary basins and evidence of [[volcanic arc]]s, intracontinental [[rift]]s, continent-continent collisions and widespread globe-spanning [[Orogeny|orogenic events]] suggesting the assembly and destruction of one and perhaps several [[supercontinent]]s. Liquid water was prevalent, and deep oceanic basins are known to have existed by the presence of [[banded iron formation]]s, [[chert]] beds, chemical sediments and pillow basalts.

==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]], the [[Baltic shield]], [[Geology of Scotland|Scotland]], [[Geological history of India|India]], [[Brazil]], western [[Geology of Australia|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 [[craton]]s; 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 [[graywacke]]s, [[mudstone]]s, volcanic sediments, and [[banded iron formation]]s. [[Carbonate]] rocks are rare, indicating that the oceans were more acidic due to dissolved [[carbon dioxide]] than during the Proterozoic.<ref>John D. Cooper, Richard H. Miller, and Jacqueline Patterson, ''A Trip Through Time: Principles of Historical Geology'', (Columbus: Merrill Publishing Company, 1986), p. 180.</ref> [[Greenstone belt]]s are typical Archean formations, consisting of alternating high and low-grade metamorphic rocks.  The high-grade rocks were derived from volcanic [[island arc]]s, while the low-grade metamorphic rocks represent deep-sea sediments eroded from the neighboring island arcs and deposited in a [[forearc|forearc basin]]. In short, greenstone belts represent sutured protocontinents.<ref>Stanley, pp. 302-3</ref>

==Archean life==
Fossils of cyanobacterial mats ([[stromatolites]]) are found throughout the Archean—becoming especially common late in the eon—while a few probable [[bacterium|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 [[eukaryote|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 replicators such as [[viruses]].

==References==
{{reflist|2}}

{{Commonscat|Archean}}
==External links==
*[http://www.stratigraphy.org/geowhen/stages/Archean.html GeoWhen Database]
*[http://www.utdallas.edu/~rjstern/PlateTectonicsStart/ When Did Plate Tectonics Begin?]
*[http://www.palaeos.com/Archean/Archean.htm Archean Eon] - ''Paleos''


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[[Category:Geologic time scale]]
[[Category:Archaean| ]]
[[Category:Geological history of Earth]]

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