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Archean Eon
3800 - 2500 million years ago
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Scale:
Millions of years

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".

Contents

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 may have comprised a mix of remnant heat from planetary accretion, heat from the formation of the iron core, and radiogenic heat produced by short-lived radionuclides such as uranium-235.

Most of the surviving Archean rocks are metamorphic or igneous. Volcanic activity was considerably higher than today, with numerous hot spots, rift valleys, and lava eruptions, including unusual types such as komatiite. Nevertheless, intrusive igneous rocks predominate throughout the crystalline cratonic remnants of the surviving Archean crust. These are magmas that 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 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 subcontinental lithospheric mantle 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 geoscientific research. [1]

No large continents existed 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 hotspots rather than subduction zones, from a variety of sources: igneous differentiation of mafic rocks to produce intermediate and felsic rocks, mafic magma melting of more felsic rocks and forced granitization of intermediate rocks, partial melting of mafic rock, and 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.[2]

An explanation for the general lack of Hadean rocks (older than 3800 Ma) is the massive quantity of rocky and icy debris present in the early Solar System. After the eight planets formed, large numbers of leftover protoplanets, asteroids, and comets pursued eccentric orbits throughout our system, bombarding the early Earth and the other planets and moons until approximately 3800 Ma. This activity may have prevented any large crustal fragments from forming by literally shattering the early protocontinents. A barrage of particularly large impactors known as the Late Heavy Bombardment represents the climax of this violent era.

Archean palaeoenvironment

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 gneisses produced by metamorphism of sedimentary protoliths. 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 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 arcs, intracontinental rifts, continent-continent collisions and widespread globe-spanning orogenic events suggesting the assembly and destruction of one and perhaps several supercontinents. Liquid water was prevalent, despite the faint young sun paradox, and deep oceanic basins are known to have existed by the presence of banded iron formations, chert beds, chemical sediments and pillow basalts.

Archean geology

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, Scotland, India, Brazil, 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 continental crust equivalent to only 5-40% of the present amount formed during the Archean.[3]

In contrast to the Proterozoic, Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations. Carbonate rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic.[4] Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks. The meta-igneous rocks were derived from volcanic island arcs, while the metasediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. Greenstone belts represent sutures between protocontinents.[5]

Archean life

Fossils of cyanobacterial mats (stromatolites, which were instrumental in creating the free oxygen in the atmosphere[citation needed]) are found throughout the Archean, becoming especially common late in the eon, while a few probable bacterial fossils are known from chert beds.[6] 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 eukaryotic fossils, though they might have evolved during the Archean without leaving any fossils.[7] No fossil evidence yet exists for ultramicroscopic intracellular replicators such as viruses.

See also

References

  1. ^ Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6 p. 297-301
  2. ^ Stanley, pp. 297-301
  3. ^ Stanley, pp. 301-2
  4. ^ John D. Cooper, Richard H. Miller, and Jacqueline Patterson, A Trip Through Time: Principles of Historical Geology, (Columbus: Merrill Publishing Company, 1986), p. 180.
  5. ^ Stanley, pp. 302-3
  6. ^ Stanley, 307
  7. ^ Stanley, pp. 306, 323

External links

Precambrian Phanerozoic  
(Hadean) Archean Proterozoic
Archean Eon
Eoarchean Paleoarchean Mesoarchean Neoarchean

Simple English

The Archean eon is the geological period after the Hadean and before the Proterozoic. It lasted from 3,800 million years ago to 2,500mya. It contains the first sedimentary rocks, and the first fossil life forms, which were cyanobacteria,[1] and acritarchs.[2]

Most of the rocks which survive are volcanic (igneous) and metamorphic in origin. Volcanic activity was everywhere; the heat flow from the Earth was at least twice as much as today. The question of when plate tectonics began is a major research area.[3] Small protocontinents formed. There were oceans already, early in the Archaean.

The atmosphere almost entirely lacked free oxygen, and instead, CO2 was a major constituent. Fossil bacterial mats, called stromatolites, are found throughout the Archaean after about 3,500mya. These were formed by cyanobacteria, who used photosynthesis and gave off oxygen as a by-product.[4]

Archean Earth

The Archean is one of the four main time periods of Earth history. When the Archean began, the Earth's heat flow was nearly three times higher than it is today. It was still twice the current level at the transition from the Archean to the Proterozoic (2,500 Ma).

Most of the surviving Archean rocks are metamorphic or igneous. Volcanic activity was considerably higher than today, with numerous hot spots, rift valleys, and lava eruptions.

The Earth of the early Archean may have supported a tectonic condition different from 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 continents formating until the mantle cooled and convection slowed down. Others argue that the subcontinental lithospheric mantle 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 geoscientific research.[5]

An explanation for the general lack of Hadean rocks (older than 3800 Ma) is the large amount of rocky and icy debris present in the early Solar System. After the eight planets formed, large numbers of leftover protoplanets, asteroids, and comets pursued eccentric orbits throughout our system, bombarding the early Earth and the other planets and moons until approximately 3800 Ma. This activity may have prevented any large crustal fragments from forming by literally shattering the early protocontinents. A barrage of particularly large impactors known as the Late Heavy Bombardment represents the climax of this violent era.

References

  1. Knoll, Andrew H. 2004. Life on a young planet: the first three billion years of evolution on Earth. Princeton, N.J. ISBN 0-691-12029-3
  2. Buick R. 2010. Early life: ancient acritarchs. Nature 463: 885–886. doi:10.1038/463885a. PMID 20164911.
  3. Stanley, Steven M. 1999. Earth system history. New York: Freeman. p297-301 ISBN 0-7167-2882-6
  4. Schopf J.W. 1992. Geology and paleobiology of the Archean Earth, in Schopf J.W., and Klein C. The Proterozoic biosphere: a multidisciplinary study, Cambridge University Press. ISBN 0-521-36615-1
  5. Stanley, Steven M. 1999. Earth system history. Freeman, New York. p297-301 ISBN 0-7167-2882-6


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