|Geologic Time (ca. 4500 million years ago - present)|
|Precambrian (ca. 4500 - 542 million years ago)|
The Archean (or Archaean) eon is an interval of geologic time of about 1.4 billion years, beginning with the formation of Earth's crust and the oldest Earth rocks 3,960-3,800 million years ago (mya) and continuing until its boundary at 2,500 mya, with the Proterozoic eon. The Archean-Proterozoic boundary is defined chronometrically, unlike the boundaries separating many other geologic time periods, which are defined based on noticeable changes in the geologic record.
The Archean eon comprises four subdivisions called eras. From the earliest to the most recent, they are: the Eoarchean, the Paleoarchean, Mesoarchean, and the Neoarchean. The Archean eon, itself, is considered to be one part of the even longer Precambrian super-eon, which encompasses roughly four billion years of Earth history prior to the appearance of abundant macroscopic hard-shelled fossils some 542 mya, and is commonly divided, from earliest to most recent, into the Hadean, Archean, and Proterozoic eons.
The origin of life has been traced to the Archean eon, with fossils of prokaryotes (non-nucleated single-celled organisms) known from 3,500 mya. During the Archean eon, prokaryotes spread over much of Earth's surface, often in mats comprising myriads of collaborating bacteria differentiated by the type of biochemistry each performed. During the Archean eon some prokaryotes developed the molecular structures for achieving photosynthesis, which permitted them initially to use sunlight for capturing hydrogen from the atmosphere and later to use it for capturing carbon from atmospheric carbon dioxide with the release of oxygen, thus helping to prepare an environment that could support eukaryotic cells and multicellular forms of life. It is speculated that the eukaryotes may first have appeared around 2,700 mya, near the end of the Archean eon (Mayr 2001). Mayr considers the origin of eukaryotes to be "the most important and dramatic event in the history of life."
|Archean eon (3900–2500 million years ago)|
The Archean was formerly called the Archaeozoic (or Archeozoic).
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 eon (Stanley 1999).
Free oxygen was absent from Earth's atmosphere through most of the Archean eon, but atmospheric free oxygen increased near the end of the eon, coinciding with and stimulating the rise of eukaryotes (Mayr 2001).
Earth's surface temperatures appear to have approached modern levels even within 500 million years of the planet’s formation, as has been inferred from the presence of sedimentary rocks within certain highly deformed early Archean gneisses. Astronomers think that the sun was about one-third dimmer, which may have contributed to lower global temperatures than otherwise expected. Further, the lesser energy supplied by the sun may have been counterbalanced by larger amounts of atmospheric greenhouse gases than later in Earth's history.
Earth's heat flow is considered by some to have been nearly three times higher at the beginning of the Archean eon than it is today, and to have fallen to twice the current level by the beginning of the Proterozoic eon. The greater heat flow than today's may have derived in part from remnant heat from the earlier planetary accretion, in part from heat from the formation of the iron core, and most likely in greater part from radiogenic heat production from short-lived radionuclides, such as uranium-235.
Most extant Archean rocks are of either the metamorphic or igneous type. Volcanic activity was considerably greater then than today, with numerous hot spots, rift valleys, and eruptions of unusual lavas, such as komatiite with its high melting temperature. In addition to its extensive volcanic eruptions, the Archean Earth's subterranean regions were also extremely active with flows of magma producing intrusive igneous rocks that predominate throughout the crystalline cratonic remnants of the Archean crust that survive today. After the magmas infiltrated into host rocks, they solidified before they could erupt at the Earth's surface, forming instead great melt sheets and voluminous rock masses comprising various combinations of the most common elements, silicon and oxygen, plus such other elements as aluminum, sodium, calcium, and potassium. The Archean intrusive rocks include granite, diorite, intrusions layered from ultramafic to mafic (high melting temperature to medium melting temperature), anorthosites and monzonites known as sanukitoids. In contrast to the subsequent Proterozoic rocks, 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, cmprising 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 mya, plate tectonic activity may have been similar to that of the modern earth, as 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 early Archean Earth may have had a different tectonic style. Some scientists think that because the earth was hotter, the 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 at 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 mya 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 mya. 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.
Life apparently originated during the Archean, with prokaryote fossils known from 3,500 mya (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 eon 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 mya, 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 in the Archean eon.
The upper or later boundary of the Archean eon with the Proterozoic eon is set at roughly the time when oxygen began to accumulate in the atmosphere—while much of the oxygen being produced by photosynthetic bacteria was still reacting with dissolved iron to form iron oxides that settled to the bottom. The boundary is not keyed to particular geological strata but rather is defined in the rock strata by chronometric dating of the strata.
Although the lower boundary of the Archean eon is typically set at the formation of Earth's crust and the oldest Earth rocks 3,960-3,800 million years ago (mya), that boundary has not been officially recognized by the International Commission on Stratigraphy.
- Cavalier-Smith, T. “A Revised Six-Kingdom System of Life.” Biol. Rev. 73 (1998): 203-266.
- Mayr, E. What Evolution Is. New York: Basic Books. 2001. ISBN 0465044263.
- Stanley, S. M. Earth System History. New York: W.H. Freeman and Company. 1999. ISBN 0716728826.
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