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 3960-3800 million years ago) and extends to the Proterozoic, 2500 million years ago (mya). 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 3800 mya.

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 mya. The Precambrian is commonly divided, from earliest to most recent, into the Hadean, Archean, and Proterozic 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 Archean was formerly called the Archaeozoic (also spelled Archeozoic).

reviseDuring the Precambrian, processes were set in motion that prepared the foundation for the subsequent time period with multicellular animals. These processes included the origin of life, the development of an oxygen atmosphere, and the development of eukaryotes.

Archean Earth

At the beginning of the Archean, the Earth's heat flow was nearly three times higher than it is today, and it was still twice the current level by the beginning of the Proterozoic. The extra heat may have been remnant heat from the planetary accretion, partly heat of 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 metmorphic igneous rocks. Volcanic activity was considerably more active than today, with numerous hot spots, and 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.

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 from 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 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 bombardment may have prevented any large crustal fragments from forming by literally shattering the early protocontinents.

Archean palaeoenvironment

The Archean atmosphere apparently lacked free oxygen. Temperatures appear to have been near modern levels even within 500 million years of Earth 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.

By the end of the Archaean, 2500 to 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 (mountain building) 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.

Archean Geology

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 7% of the world's current cratons (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 5-40% of the present continental crust formed during the Archean (Stanley 1999).

In contrast to the 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).

Archean Life

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.

Life was probably present throughout the Archean, but may have been limited to simple non-nucleated single-celled organisms, called prokaryotes (and formerly known as Monera); there are no known eukaryotic fossils, though they might have originated during the Archean and simply not left any fossils (Stanley 1999). However, no fossil evidence yet exists for ultramicroscopic intracellular organisms such as viruses.

References

ISBN links support NWE through referral fees

• Stanley, S. M. 1999. Earth System History. New York: W.H. Freeman and Company. ISBN 0-7167-2882-6.

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