The Archean (pronEng|ɑrˈkiːən, also spelled Archaean, formerly called the Archaeozoic (IPA|/ɑrkiəˈzoʊ
ɪk/), also spelled Archeozoic or Archæozoic) is a geologic eon before the Proterozoicand Paleoproterozoic, before 2.5 Ga (billion years ago, or 2,500 MA|1). 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 Hadeaneon. In older literature, the Hadean is included as part of the Archean.The name comes from the ancient Greek"Αρχή" (Arkhē), meaning "beginning, origin".
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(2,500 MA). 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 still survive are metamorphic and
igneous rocks. Volcanic activity was considerably higher than today, with numerous hot spots, rift valleys, and eruption of lavas including unusual types such as komatiite. Nevertheless, intrusive igneous rocks predominate throughout the crystalline cratonic remnants of the Archean crust which survive today. 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, ultramafic to mafic layered intrusions, anorthosites and monzonites known as sanukitoids.
The Earth of the early Archean may have had a different tectonic style. Some scientists think that because the Earth was hotter,
plate tectonicactivity 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 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. [Stanley, Steven M. "Earth System History." New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6 p. 297-301]
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
felsicprotocontinents probably formed at hotspots rather than subduction zones, from a variety of sources: igneous differentiationof mafic rocks to produce intermediate and felsic rocks, maficmagma melting more felsic rocks and forcing granitizationof intermediate rocks, partial melting of mafic rock, and from the 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. [Stanley, pp. 297-301]
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
asteroids and meteorites still existed, and bombarded the early Earth until approximately 3800 Ma. A barrage of particularly large impactors known as the late heavy bombardmentmay have prevented any large crustal fragments from forming by literally shattering the early protocontinents.
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, 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, and deep oceanic basins are known to have existed by the presence of banded iron formations, chertbeds, chemical sediments and pillow basalts.
Although a few mineral grains are known that are Hadean, the oldest rock formations exposed on the surface of the
Earthare 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 continentsformed 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 crustequivalent to only 5-40% of the present amount formed during the Archean. [Stanley, pp. 301-2]
In contrast to the Proterozoic, Archean rocks are often heavily metamorphized deep-water sediments, such as
graywackes, mudstones, volcanic sediments, and banded iron formations. Carbonaterocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxidethan during the Proterozoic. [John D. Cooper, Richard H. Miller, and Jacqueline Patterson, "A Trip Through Time: Principles of Historical Geology", (Columbus: Merrill Publishing Company, 1986), p. 180.] Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed maficigneous 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. [Stanley, pp. 302-3]
Fossils of cyanobacterial mats (
stromatolites) are found throughout the Archean, becoming especially common late in the eon, while a few probable bacterial fossils are known from chertbeds. [Stanley, 307] In addition to the domain Bacteria(once known as Eubacteria), microfossils of the domain Archaeahave 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. [Stanley, pp. 306, 323] However, no fossil evidence yet exists for ultramicroscopic intracellular replicators such as viruses.
* [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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См. также в других словарях:
Archean — [är kē′ən] adj. [< Gr archaios, ARCHAIC] [sometimes a ] designating or of the geologic eon characterized by the development of the first igneous and metamorphic rocks and the first marine microorganisms; Precambrian, esp. early Precambrian… … English World dictionary
Archean — or Archaean adjective Etymology: Greek archaios Date: 1872 1. of, relating to, or being the earliest eon of geological history or the corresponding system of rocks see geologic time table 2. Precambrian • Archean noun … New Collegiate Dictionary
Archean — … Википедия
Archean — /ahr kee euhn/, adj. Geol. noting or pertaining to rocks of the Archeozoic portion of the Precambrian Era. Also, Archaean. [1870 75; < Gk archaî(os) ancient (see ARCHAEO ) + AN] * * * … Universalium
archean — adj. (Geology) of or pertaining to the earliest known forms of rocks; of or belonging to the earlier of the two divisions of Precambrian time … English contemporary dictionary
Archean — adjective US spelling of Archaean … English new terms dictionary
archean — ar·che·an … English syllables
Archean — Ar•che•an [[t]ɑrˈki ən[/t]] adj. gel of or pertaining to rocks of the Archeozoic portion of the Precambrian Era • Etymology: 1870–75 … From formal English to slang
Archean — /aˈkiən/ (say ah keeuhn) noun, adjective → Archaean … Australian-English dictionary
archean — US var. of ARCHAEAN … Useful english dictionary