Archean

"Archaean" redirects here. For the division of living organisms, see Archaea.

Template:Archaean Infobox The Archean, also spelled Archaean, is a geologic eon that begins with the formation of the earth's crust and the oldest earth rocks 3.8-3.96 billion years ago (3,960-3,800 million years ago) and extends to the Proterozoic, 2.5 billion years ago (2,500 million years ago). Instead of being based on stratigraphy, the upper (more recent) boundary is defined chronometrically. The lower boundary has not been officially recognized by the International Commission on stratigraphy, but it is usually set at the end of the Hadean eon, about 3,800 million years ago.

The Archean is considered part of the Precambrian, an informal name for the billions of years of the geologic timescale that came before the current Phanerozoic eon (which marked the appearance of abundant macroscopic hard-shelled fossils some 542 million years ago). The Precambrian is commonly divided, from earliest to most recent, into the Hadean, Archean, and Proterozoic eons, but some authorities only recognize two subdivisions, the Archaean and the Proterozoic, beginning the Precambrian from the time of the oldest preserved rocks rather than from the formation of the earth. In this later view, the Archean is the first (oldest) division of the Precambrian.

The origin of life traces to the Archean, with prokaryote fossils known from 3.5 billion years ago. During the Archean, processes were also set in motion that prepared the foundation for multicellular life, with the development of an oxygen atmosphere and, it is speculated, possibly the appearance of the first eukaryotes around 2.7 billion years ago, near the end of the Archean (Mayr 2001). Mayr considers the origin of eukaryotes to be "the most important and dramatic event in the history of life."

The Archean was formerly called the Archaeozoic (also spelled Archeozoic).

Archean Earth

Although a few mineral grains are known that are older, 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, western Australia, and southern Africa. Although the first continents formed during this eon, rock of this age makes up only seven percent of the world's current cratons (the old and stable part of the continental crust that has survived the merging and splitting of continents and supercontinents). Even allowing for erosion and destruction of past formations, evidence suggests that only five to 40 percent of the present continental crust formed during the Archean (Stanley 1999).

The Archean atmosphere apparently lacked free oxygen during most of the eon, but free oxygen increased near the end of the Archean, coinciding with and stimulating the rise of eukaryotes (Mayr 2001).

Surface temperatures appear to approach modern levels even within 500 million years of Earth’s formation, with liquid water present, due to the presence of sedimentary rocks within certain highly deformed gneisses. Astronomers think that the sun was about one-third dimmer, 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.

At the beginning of the Archean, the earth's heat flow is considered by some to have been nearly three times higher than it is today, and remains twice the current level by the beginning of the Proterozoic. The extra heat may have been partially remnant heat from the planetary accretion, partially heat from the formation of the iron core, and most likely partially caused by greater radiogenic heat production from short-lived radionuclides, such as uranium-235.

The majority of Archean rocks that exist are metamorphic igneous rocks. Volcanic activity was considerably more active than today, with numerous hot spots, rift valleys, and eruption of unusual lavas, such as komatiite. Intrusive igneous rocks, such as great melt sheets and voluminous plutonic masses of granite, diorite, ultramafic to mafic layered intrusions, anorthosites, and monzonites known as sanukitoids, predominate throughout the crystalline cratonic remnants of the Archean crust that exists today. In contrast to the subsequent Proterozoic, Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations.

Greenstone belts are typical Archean formations, consisting of alternating high- and low-grade metamorphic rocks. The high-grade rocks were derived from volcanic island arcs, while the low-grade metamorphic rocks represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. In short, greenstone belts represent sutured protocontinents (Stanley 1999).

By the end of the Archaean, 2,500 to 2,600 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 (mountain building) suggesting the assembly and destruction of one and perhaps several supercontinents.

The earth of the early Archean may have had a different tectonic style. Some scientists think because the earth was hotter, that 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 cratonisation and continent formation until the mantle cooled and convection slowed down. Others argue that the sub continental lithospheric mantle is 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 1999).

There were no large continents until late in the Archean; it is considered that small "protocontinents" were the norm, prevented from coalescing into larger units by the high rate of geologic activity. These 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 more felsic rocks and forcing granitization of intermediate rocks, partial melting of mafic rock, and the metamorphic alteration of felsic sedimentary rocks. Such continental fragments may not have been preserved if they were not buoyant enough or fortunate enough to avoid energetic subduction zones (Stanley 1999).

Another explanation for a general lack of early Archean rocks greater than 3,800 million years ago 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 3,800 million years ago. A barrage of particularly large impactors known as the late heavy bombardment may have prevented any large crustal fragments from forming by shattering the early protocontinents.

Archean Life

Life apparently originated during the Archean, with prokaryote fossils known from 3.5 billion years ago (Mayr, 2001). These earliest fossils are considered to be cyanobacteria. 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 chert beds (Stanley, 1999). In addition to the domain Bacteria (once known as Eubacteria), microfossils of the extremophilic domain Archaea have also been identified. (Some, such as Cavalier-Smith, 1998, consider the Archaea to be a subdivision of the Bacteria domain rather than a separate domain.)

Mayr notes that cyanobacteria did not change much from the time of the Archean until today, with about one-third of the early fossil species of prokaryotes "morphologically indistinguishable from still living species."

Life during the Archean may have been limited to simple non-nucleated single-celled organisms (prokaryotes); there are no known eukaryotic fossils. However, eukaryotes may have originated during the Archean and simply not left any fossils (Stanley, 1999). Mayr notes that lipids, by-products of eukaryotic metabolism, have been found in rocks that are 2,700 million years old, tracing to the Archean. There is a possibility, however, that these molecules percolated down from recent strata into these older strata, although most geologists deny this possibility (Mayr 2001).

No fossil evidence yet exists for ultramicroscopic intracellular organisms such as viruses.

References

ISBN links support NWE through referral fees

• Cavalier-Smith, T. “A Revised Six-Kingdom System of Life.” Biol. Rev. 73:203-266. 1998.
• Mayr, E. What Evolution Is. New York, NY: Basic Books. 2001. ISBN 0465044263
• Stanley, S. M. Earth System History. New York: W.H. Freeman and Company. 1999. ISBN 0-7167-2882-6

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