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{{Atención}}{{Nongalego}}
O eón '''Arcaico''' previamente chamado como Arqueozoico é un eón xeolóxico na historia da Terra situado entre o [[Hadeico]] e o [[Proterozoico]], que comprende entre 3.800 e 2.500 millóns de anos antes do presente. Estas datas a pesar de estar baseadas na estratigrafía non están plenamente recoñecidas pola Comisión Internacional de Estratigrafía.
O nome provén do grego clásico "Αρχή" (Arkhē) que significa "orixe".
 
==A Terra Arcaica==
No principio do Arcaico, a calor interna da [[Terra]] era preto de 3 veces maior que na actualidade, e aínda dúas veces maior que o seu nívelnivel a principio do [[Proterozoico]] (2,500 {MA}). A calor calor era a remanente do proceso de *acrección planetaria, parciamenteparcialmente procedente da calor de formación do núcleo interno formado por ferro, e parcialementeparcialmente procedente da calor radioactvaradioactiva pola corta vida de átomos radioactivos como o [[uranio]]-235.
 
A maioría das rochas arcaicas que podemos observar na actualidade son [[rochas metamórficas|metamórficametamórficas]s] e [[rocha ígnea]]s. A actividade [[Volcán|Volcánica]] era considerablemente maior que a actual, con numerosos [[Hotspot (xeoloxía)|hot spot]]sspots, [[rift valley]]svalleys, e erupción de lavas incluindoincluíndo tipos inusuais como [[komatitas]]. Sen embargo as rochas ígneas intrusivas predominan dentro dos [[cratón]]scratóns procedentes da codia do Arcaico sobreviven hoxe. These are magmas which infiltrated into host rocks, but solidified before they could erupt at the Earth's surface. Examples include great melt sheets and voluminous plutonic masses of [[granite]], [[diorite]], [[layered intrusion]]s, [[anorthosite]]s and [[monzonite]]s known as [[sanukitoid]]s.
 
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 [[craton]]isation and continent formation until the mantle cooled and convection slowed down. Others argue that the subcontinental lithospheric mantle was 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, [[partial melting]] of mafic rock, and from the [[metamorphism|metamorphic]] alteration of felsic sedimentary rocks. Such continental fragments may not have been preserved unless they were buoyant enough or fortunate enough to avoid energetic subduction zones.<ref>Stanley, pp. 297-301</ref>
 
An explanation for the general lack of Hadean rocks (older 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.
 
===A Atmosfera Arcaica===
The Archean atmosphere is thought to have lacked free [[oxygen]]. Temperatures appear to have been near modern levels even within 500 Ma of Earth's formation, with liquid water present, as evidenced by certain highly deformed [[gneiss]]es produced by metamorphism of [[sediment]]ary [[protolith]]s. Astronomers think that the sun was about one-third dimmer than at present, 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.
 
==A Xeoloxía Arcaica==
Although a few mineral grains are known that are Hadean, 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 continental [[crust]] equivalent to only 5-40% of the present amount 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 units of metamorphosed [[mafic]] igneous and sedimentary rocks. The meta-igneous rocks were derived from volcanic [[island arc]]s, while the metasediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a [[forearc|forearc basin]]. Greenstone belts represent sutures between protocontinents.<ref>Stanley, pp. 302-3</ref>
 
==A Vida Arcaica==
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 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]] (formerly known as Monera). There are no known [[eukaryote|eukaryotic]] fossils, though they might have evolved during the Archean without leaving any fossils.<ref>Stanley, pp. 306, 323</ref> No fossil evidence yet exists for ultramicroscopic intracellular replicators such as [[viruses]].
 
==Referencias==
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