Archean

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The Archean (pronounced /ɑrˈkiːən/, also spelled Archaean, formerly called the Archaeozoic (/ɑrkiəˈzoʊɪk/), also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga (billion years ago, or 2,500 Ma). 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. In older literature, the Hadean is included as part of the Archean. The name comes from the ancient Greek "Αρχή" (Arkhē), meaning "beginning, origin".

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Archean Earth

The Archean is one of the four principal eons of Earth history. When the Archean began, the Earth's heat flow was nearly three times higher than it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2,500 Ma). The extra heat was the result of a mix of remnant heat from planetary accretion, heat from the formation of the Earth's core, and heat produced by radioactive elements.

Most surviving Archean rocks are metamorphic or igneous. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite. Granitic rocks predominate throughout the crystalline remnants of the surviving Archean crust. Examples include great melt sheets and voluminous plutonic masses of granite, diorite, layered intrusions, anorthosites and monzonites known as sanukitoids.

The Earth of the early Archean may have supported a tectonic regime unlike that of the present. Some scientists argue that, because the Earth was much hotter, tectonic activity was more vigorous than it is today, resulting in a much faster 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 oceanic lithosphere was too buoyant to subduct, and that the rarity 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 research.[1]

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