The Archean (pronounced /ɑrˈkiːən/, also spelled Archaean, formerly called the Archaeozoic (/ɑrkiəˈzoʊɪk/), also spelled Archeozoic or Archæozoic) is a geologic eon before the Proterozoic and Paleoproterozoic, ending 2. The geologic time scale is a chronologic schema (or idealized Model) relating Stratigraphy to time that is used by Geologists and other The Precambrian ( Pre-Cambrian) is an informal name for the supereon comprising the eons of the Geologic timescale that came before the current Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit The Proterozoic (ˌproʊtərəˈzoʊɪk is a geological eon representing a period before the first abundant complex life on Earth. The Paleoproterozoic (ˌpeɪlɪoʊˌproʊtərəˈzoʊɪk also spelled Palaeoproterozoic) is the first of the three sub-divisions ( eras) of the 5 Ga (billion years ago). Annum is one form of the Latin noun meaning Year, not a form normally used for derivatives in modern languages the accusative singular Instead of being based on stratigraphy, this date is defined chronometrically. Stratigraphy, a branch of Geology, studies rock layers and layering ( stratification) The lower boundary (starting point) has not been officially recognized by the International Commission on Stratigraphy, but it is usually set to 3. The International Commission on Stratigraphy (ICS, sometimes referred to by the unofficial " International Stratigraphic Commission " is a daughter or major Subcommittee 8 Ga, at the end of the Hadean eon. The Hadean (ˈheɪdiən is the geologic eon before the Archean.

Archean Earth

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. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The Proterozoic (ˌproʊtərəˈzoʊɪk is a geological eon representing a period before the first abundant complex life on Earth. 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. Uranium (jʊˈreɪniəm is a silvery-gray Metallic Chemical element in the

The majority of Archean rocks which exist are metamorphic and igneous rocks, the bulk of the latter being intrusive. Metamorphic rock is the result of the transformation of an existing rock type the protolith, in a process called Metamorphism, which means "change Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock Volcanic activity was considerably higher than today, with numerous hot spots, and rift valleys, and eruption of unusual lavas such as komatiite. Plate tectonics and hotspots Divergent plate boundaries At the In Geology, a hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic Rift or fault. Komatiites are Ultramafic mantle-derived Volcanic rocks They have low SiO2 low K2O low Al2O3 and high to extremely high 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 which exist today. Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. Diorite (ˈdaɪəraɪt is a grey to dark grey intermediate intrusive Igneous rock composed principally of Plagioclase Feldspar (typically Ultramafic to mafic layered intrusions are found in typically ancient Cratons and are rare but worldwide in distribution Anorthosite (æˈnɔrθəsaɪt/ /ə- is a Phaneritic, Intrusive Igneous rock characterized by a predominance of Plagioclase Feldspar Monzonite is an intermediate Igneous Intrusive rock composed of approximately equal amounts of sodic to intermediate Plagioclase and Orthoclase Sanukitoids are a variety of high-Mg Granitoid found in convergent margin settings A craton ( Greek kratos / κρἀτος ( neut. "strength" is an old and stable part of the Continental crust that has survived

The Earth of the early Archean may have had a different tectonic style. Some scientists think that, because the Earth was hotter, plate tectonic activity was more vigorous than it is today, resulting in a much greater rate of recycling of crustal material. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere 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. In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other The question of whether or not plate tectonic activity existed in the Archean is an active area of modern geoscientific research. ^[1]

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 felsic 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. Felsic is a term used in Geology to refer to Silicate minerals, Magma, and rocks which are enriched in the lighter elements such as Silicon In Geology, a hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other Igneous differentiation is an umbrella term for the various processes by which Magmas undergo bulk chemical change during the Partial melting process cooling Mafic is an adjective describing a Silicate mineral or rock that is rich in magnesium and iron the term was derived by contracting "magnesium" and "ferric" Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet Metamorphism can be defined as the solid state recrystallisation of pre-existing rocks due to changes in heat and/or pressure and/or introduction of fluids i Such continental fragments may not have been preserved if they were not buoyant enough or fortunate enough to avoid energetic subduction zones. ^[2]

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. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but A meteorite is a natural object originating in Outer space that survives an impact with the Earth 's surface 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. The Late Heavy Bombardment (commonly referred to as the lunar cataclysm, or LHB) is a period of time approximately 3800 to 4100 million years ago ( mya

Archean palaeoenvironment

The Archean atmosphere apparently lacked free oxygen. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Temperatures appear to have been near modern levels even within 500 Ma of Earth's formation, with liquid water present, due to the presence of sedimentary rocks within certain highly deformed gneisses. Gneiss (ˈnaɪs is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from preexisting formations that were originally 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. The history of Earth covers approximately 46 billion years (4567000000 years from Earth ’s formation out of the Solar nebula to the present

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. A volcanic arc is a chain of volcanic islands or Mountains formed by Plate tectonics as an oceanic Tectonic plate subducts under In Geology, a rift is a place where the Earth 's crust and Lithosphere are being pulled apart and is an example of Extensional tectonics Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and In Geology, a supercontinent is a Landmass comprising more than one Continental core or Craton. 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. Banded iron formations (also known as banded ironstone formations or BIF s are a distinctive type of rock often found in primordial ( Precambrian) Sedimentary Chert (ˈtʃɝt is a fine-grained Silica -rich Microcrystalline, Cryptocrystalline or Microfibrous Sedimentary rock that may contain

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. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Archean rocks are known from Greenland, the Canadian Shield, the Baltic shield, Scotland, India, Brazil, western Australia, and southern Africa. Greenland (Kalaallit Nunaat meaning "Land of the Greenlanders" Grønland is a self-governing Danish Province located between the The Canadian Shield &mdash also called the Laurentian Plateau, or Bouclier Canadien (French &mdash is a large geological shield covered by The Baltic Shield (sometimes referred to as the Fennoscandian Shield) is located in Fennoscandia ( Norway, Sweden and Finland) northwest The geology of Scotland is unusually varied for a country of its size with a large number of differing geological features The geological history of India started with the geological evolution of rest of the Earth i |utc_offset = -2 to -4 |time_zone_DST = BRST |utc_offset_DST = -2 to -5 |cctld Australia is a Continent situated on the Indo-Australian Plate. 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 only 5-40% of the present continental crust formed during the Archean. A continent is one of several large Landmasses on Earth. They are generally identified by Convention rather than any strict criteria with seven regions A craton ( Greek kratos / κρἀτος ( neut. "strength" is an old and stable part of the Continental crust that has survived ^[3]

In contrast to the Proterozoic, Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations. Greywacke ( German grauwacke, signifying a grey earthy rock is a variety of Sandstone generally characterized by its hardness dark color and poorly-sorted Mudstone (also called mudrock) is a fine grained Sedimentary rock whose original constituents were Clays or Muds Grain size is up Banded iron formations (also known as banded ironstone formations or BIF s are a distinctive type of rock often found in primordial ( Precambrian) Sedimentary Carbonate rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic. In Chemistry, a carbonate is a salt or Ester of Carbonic acid. Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single ^[4] Greenstone belts are typical Archean formations, consisting of alternating high and low-grade metamorphic rocks. Greenstone belts are zones of variably metamorphosed Mafic to Ultramafic Volcanic sequences with associated Sedimentary rocks that 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. A volcanic arc is a chain of volcanic islands or Mountains formed by Plate tectonics as an oceanic Tectonic plate subducts under A forearc is a depression in the Sea floor located between a Subduction zone and an associated Volcanic arc. In short, greenstone belts represent sutured protocontinents. ^[5]

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. Stromatolites (from Greek στρώμα strōma, mattress bed stratum and λιθος lithos, rock are layered accretionary Structures formed in The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have Chert (ˈtʃɝt is a fine-grained Silica -rich Microcrystalline, Cryptocrystalline or Microfibrous Sedimentary rock that may contain ^[6] In addition to the domain Bacteria (once known as Eubacteria), microfossils of the extremophilic domain Archaea have also been identified. The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have

Life was probably present throughout the Archean, but may have been limited to simple non-nucleated single-celled organisms, called Prokaryota (and formerly known as Monera); there are no known eukaryotic fossils, though they might have evolved during the Archean and simply not left any fossils. The prokaryotes (proʊˈkærioʊts singular prokaryote /proʊˈkæriət/ are a group of Organisms that lack a Cell nucleus (= karyon or any other Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex ^[7] However, no fossil evidence yet exists for ultramicroscopic intracellular replicators such as viruses. A virus (from the Latin virus meaning Toxin or Poison) is a sub-microscopic infectious agent that is unable

References

External links

• GeoWhen Database
• When Did Plate Tectonics Begin?
• Archean Eon - Paleos