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For the division of living organisms, see Archaea.
Archean Eon
4000 - 2500 million years ago
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Scale:
Millions of years

The Archean Eon (/ɑrˈkən/, also spelled Archaean; formerly Archaeozoic /ɑrkəˈzɪk/, also spelled Archeozoic or Archæozoic) is a geologic eon before the Proterozoic Eon, before 2.5 Ga (billion years), or 2,500 million years ago.

Classification issues[edit]

Instead of being based on stratigraphy as all other geological ages are, the beginning of the Archean eon is defined chronometrically. The lower boundary (starting point) of 4 billion years is officially recognized by the International Commission on Stratigraphy.[1]

The Archean customarily starts at 4 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".

Earth at the beginning of the Archean[edit]

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 as high as 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 plate tectonic activity existed in the Archean is an active area of modern research.[2]

There are two schools of thought concerning the amount of continental crust that was present in the Archean. One school maintains that no large continents existed until late in the Archean: small protocontinents were common, prevented from coalescing into larger units by the high rate of geologic activity.[citation needed] The other school follows the teaching of Richard Armstrong, who argued that the continents grew to their present volume in the first 500 million years of Earth history and have maintained a near-constant ever since: throughout most of Earth history, recycling of continental material crust back to the mantle in subduction or collision zones balances crustal growth.[citation needed]

Opinion is also divided about the mechanism of continental crustal growth. Those scientists who doubt that plate tectonics operated in the Archean argue that the felsic protocontinents formed at hotspots rather than subduction zones. Through a process called "sagduction", which refers to partial melting in downward-directed diapirs, a variety of mafic magmas produce intermediate and felsic rocks.[citation needed] Others accept that granite formation in island arcs and convergent margins was part of the plate tectonic process, which has operated since at least the start of the Archean.[citation needed]

An explanation for the general lack of Hadean rocks (older than 3800 Ma) is the efficiency of the processes that either cycled these rocks back into the mantle or effaced any isotopic record of their antiquity. All rocks in the continental crust are subject to metamorphism, partial melting and tectonic erosion during multiple orogenic events and the chance of survival at the surface decreases with increasing age. In addition, a period of intense meteorite bombardment in the period 4.0-3.8 Ga pulverized all rocks at the Earth's surface during the period. Some think that the similar age of the oldest surviving rocks and the "late heavy bombardment" is not coincidental.[citation needed]

Palaeoenvironment[edit]

The Archean atmosphere is thought to have nearly lacked free oxygen. Astronomers think that the sun had about 70–75% of the present luminosity, yet temperatures appear to have been near modern levels even within 500 Ma of Earth's formation, which is puzzling (the faint young sun paradox). The presence of liquid water is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths. The equable temperatures may reflect the presence of larger amounts of greenhouse gases than later in the Earth's history.[3][4] Alternatively, Earth's albedo may have been lower at the time, due to less land area and cloud cover.[5]

By the end of the Archaean c. 2500 Ma (million years ago), 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, and deep oceanic basins are known to have existed by the presence of banded iron formations, chert beds, chemical sediments and pillow basalts.

Geology[edit]

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.[6]

In contrast to Proterozoic rocks, 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.[7] 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.[8]

Life[edit]

Scientific evidence suggests that life began on Earth at least 3.5 billion years ago.[9][10] The earliest evidences for life on Earth are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland[11] and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.[12][13]

Fossils of cyanobacterial mats (stromatolites, which were instrumental in creating the free oxygen in the atmosphere[14] ) are found throughout the Archean, becoming especially common late in the eon, while a few probable bacterial fossils are known from chert beds.[15] 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.[16] No fossil evidence has been discovered for ultramicroscopic intracellular replicators such as viruses.

See also[edit]

References[edit]

  1. ^ "International Chronostratigraphic Chart v.2013/01". International Commission on Stratigraphy. January 2013. Retrieved April 6, 2013. 
  2. ^ Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman and Company. pp. 297–301. ISBN 0-7167-2882-6. 
  3. ^ Walker, James C. G. (June 1985). "Carbon dioxide on the early earth". Origins of Life and Evolution of the Biosphere 16 (2): 117−127. Bibcode:1985OLEB...16..117W. doi:10.1007/BF01809466. Retrieved 2010-01-30. 
  4. ^ Pavlov, Alexander A.; Kasting, James F.; Brown, Lisa L.; Rages, Kathy A.; Freedman, Richard (May 2000). "Greenhouse warming by CH4 in the atmosphere of early Earth". Journal of Geophysical Research 105 (E5): 11981−11990. Bibcode:2000JGR...10511981P. doi:10.1029/1999JE001134. 
  5. ^ Rosing, Minik T.; Bird, Dennis K.; Sleep, Norman H.; Bjerrum, Christian J. (April 1, 2010). "No climate paradox under the faint early Sun". Nature 464 (7289): 744–747. Bibcode:2010Natur.464..744R. doi:10.1038/nature08955. PMID 20360739. 
  6. ^ Stanley, pp. 301-2
  7. ^ Cooper, John D.; Miller, Richard H.; Patterson, Jacqueline (1986). A Trip Through Time: Principles of Historical Geology. Columbus: Merrill Publishing Company. p. 180. ISBN 0675201403. 
  8. ^ Stanley, pp. 302-3
  9. ^ de Duve, Christian (October 1995). "The Beginnings of Life on Earth". American Scientist. Retrieved 15 January 2014. 
  10. ^ Timmer, John (4 September 2012). "3.5 billion year old organic deposits show signs of life". Ars Technica. Retrieved 15 January 2014. 
  11. ^ Yoko Ohtomo, Takeshi Kakegawa, Akizumi Ishida, Toshiro Nagase, Minik T. Rosing (8 December 2013). "Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks". Nature Geoscience. doi:10.1038/ngeo2025. Retrieved 9 Dec 2013. 
  12. ^ Borenstein, Seth (13 November 2013). "Oldest fossil found: Meet your microbial mom". AP News. Retrieved 15 November 2013. 
  13. ^ Noffke, Nora; Christian, Daniel; Wacey, David; Hazen, Robert M. (8 November 2013). "Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia". Astrobiology (journal). doi:10.1089/ast.2013.1030. Retrieved 15 November 2013. 
  14. ^ "Early life: Oxygen enters the atmosphere". BBC. Retrieved September 20, 2012. 
  15. ^ Stanley, 307
  16. ^ Stanley, pp. 306, 323

External links[edit]


Archean Eon
Eoarchean Paleoarchean Mesoarchean Neoarchean

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Archean — Please support Wikipedia.
This page uses Creative Commons Licensed content from Wikipedia. A portion of the proceeds from advertising on Digplanet goes to supporting Wikipedia.
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Tue, 02 Sep 2014 05:00:00 -0700

ANCIENT KING (IRE) Peter G Moody. ANGEL GABRIAL (IRE) Richard Fahey. ARALDO (GB) Michael Moroney. ARCHEAN (USA) David Vandyke. ASTRONOMOS (GB) David Vandyke. AU REVOIR (IRE) Andre Fabre. AVERAU Peter Morgan & Craig Widdison.

Temple University News

Temple University News
Wed, 27 Aug 2014 13:15:00 -0700

Krull-Davatzes said that the rare rock she unearthed was created by a large meteor—at least three times bigger than the meteor that killed the dinosaurs—that impacted the Earth at a spot that was then the Archean ocean; plunging deep into the ocean's ...
 
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Thu, 28 Aug 2014 18:27:28 -0700

Illustration public domain via Wikimedia Commons. Early Precambrian (Archean) Minnesota (3,800 - 2,500 million years ago). Minnesota's oldest rocks - Lower Precambrian or Archean rocks - lie in alternating belts within the Canadian Shield, which ...
 
SYS-CON Media (press release)
Wed, 27 Aug 2014 05:56:15 -0700

The Clearwater Project is underlain by a mix of pelitic to psammitic metasediments that overlie older Archean gneisses. Graphitic horizons are typically present at the contact between the two and are a target for drilling, especially in areas where ...
 
Yahoo Finance UK
Tue, 26 Aug 2014 10:15:00 -0700

The archean basement rocks encountered under the cover rocks consisted of various volcanic, sub-volcanic and volcano-sedimentary rock types, possibly indicative of an extensive rift environment. However, these volcanic rocks do not explain the massive ...
 
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Wed, 27 Aug 2014 05:22:30 -0700

The Decouverte claims of 5,600 ha, cover an synclinal fold nose of Archean units, including iron formation, and metavolcanics, mainly mafic, transected by a major NE trending shear zone which can provide a pathway for hydrothermal fluids and good ...
 
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Tue, 26 Aug 2014 00:50:02 -0700

It has invested in holding company of Avantha Group, Chennai-based industrial conglomerate Archean Group as well as a privately held holding arm of Apollo Hospitals Enterprise's promoter, Magma Fincorp, Bharti Infratel, Coffee Day Resorts, Dalmia ...
 
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Wed, 27 Aug 2014 06:07:31 -0700

The Nantais Property covers an under-explored 20-km-long Archean greenstone belt entirely controlled by Azimut. The project comprises 383 claims with a surface area of 160.5 km2. It is located about 80 km south of Glencore's world-class Raglan ...
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