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	Archean - Wikipedia, the free encyclopedia
		The Archean (Archaean) is a geologic eon that refers to the time before the Proterozoic, 2500 Ma (million years ago).
		Archean rocks are known from Greenland, the Canadian Shield, northwest Australia, and southern Africa.
		Probable remains of bacterial mats (stromatolites) are found throughout the Archean, and a few probable bacterial fossils are known from chert beds.
	en.wikipedia.org /wiki/Archean (202 words)

	ARCHEAN SYSTEM - LoveToKnow Article on ARCHEAN SYSTEM (Site not responding. Last check: 2007-10-21)
		Archean Rocks fo evertheless there are large tracts, both in the Old World and oi the New, in which a subdivision of this assemblage of ancient p cks is not only possible hut desirable.
		It is quite clear in ai rtain regions that there is a lowermost group with a prevailing ul anitoid, gneissic and schistose facies, mainly of igneous origin, C rove which there are one or several groups bearing a distinctly dimentary aspect.
		There are several features which are peculiarly characteristic the Archean rocks :(1) the extraordinary complexity of the ;emblage of igneous materials; (2) the extreme metamorphism d deformation which nearly all the rocks have suffered; and the inextricable intermixture of igneous rocks with those which a sedimentary origin is postulated.
	www.1911encyclopedia.org /A/AR/ARCHEAN_SYSTEM.htm (756 words)

	Preliminary Precambrian Basement Map Showing Geologic-Geolphysical Domains-Wyoming.html
		Archean rocks on both sides of the structure are grossly similar, but they are higher grade (amphibolite facies) in the hanging-wall block on the east than in the footwall (greenschist facies).
		Thus, the structural evolution of the Archean infrastructure in the Wyoming craton is complex; detailed structural analysis such as done by R.L. Bauer and colleagues in the Laramie Mountains is required for a thorough understanding of the tectonics.
		As a result of the collision, Archean rocks of the Wyoming province were intensely redeformed and metamorphosed for a distance of at least 75 km inboard from the suture in the Laramie Mountains.
	pubs.usgs.gov /of/2001/ofr-01-0199 (4542 words)

	Heterogeneity of Archean continental roots: Evidence from Chemical Tomography (Site not responding. Last check: 2007-10-21)
		Archean SCLM is distinctively different from younger mantle; it is highly depleted, commonly is strongly stratified, and contains specific rock types (especially subcalcic harzburgites) that are extremely rare or absent in younger SCLM.
		Attempts to explain the formation of Archean SCLM by reference to Uniformitarian processes, such as the subduction of oceanic or island-arc mantle ("lithospheric stacking"), founder on the marked differences in geochemical trends between Archean xenolith suites and modern examples of highly depleted mantle, such as island-arc xenolith suites and ophiolites.
		The distinctly lower Fe content of the Archean xenoliths, and the marked positive correlation between Cr and Al at low Al, imply that no Cr-Al phase was present on the liquidus during the melting event that produced the Archean peridotites.
	www.es.mq.edu.au /GEMOC/Abs2001/Griffin2.htm (892 words)

	ARCHEAN (Site not responding. Last check: 2007-10-21)
		The Archean Eon represents a fascinating segment of Earth' s history.
		The first signs of stable continental crust and life emerged between 3.5-4.0 Ga. The Archean Eon is notable for the development of continental blocks and permobile belts.
		Archean tectonics were marked by a lack of stable cratons--essentially no large masses of continental crust had accumulated 3.9 Ga. Instead, thin lithospheric material of microcontinents and volcanic arcs collided, small "marginal" ocean basins opened and were later subducted.
	www.uwsp.edu /geo/faculty/hefferan/Geol106/CLASS3/Archean.htm (115 words)

	THE GLACIAL LAKE AGASSIZ--Chapter III
		The most Southwestern outcrops of Archean formations in Minnesota are 10 to 20 miles southwest of the Minnesota River in Redwood and Yellow Medicine counties, where small isolated exposures of granite, gneiss, and schists occur.
		Though the western boundary of the Archean area in Minnesota is mainly covered by drift and by remnants of Cretaceous beds beneath the drift, it is somewhat definitely known for a distance of 160 miles from New Ulm west, northwest, and north, to the south end of Lake Agassiz.
		The surface of the feldspathic Archean rocks was doubtless in many places decomposed and kaolinized as it is now seen where they are uncovered in the Minnesota Valley, and as such rocks are frequently changed to a considerable depth in regions that have not been glaciated.
	www.lib.ndsu.nodak.edu /govdocs/text/lakeagassiz/chapter3.html (10755 words)

	ALAN G JONES: WC Evolution Page
		As in other Archean cratons world-wide, the WCP is thought to be underlain by a refractory, buoyant mantle root, extending down to ca.
		Because lithospheric roots are classically interpreted to result from massive basalt and komatiite extraction from the upper mantle, the assumed late Archean age for the WCP mantle root may have implications for the tectonic setting of the penecontemporaneous greenstone belts.
		Such a boundary might also be reflected in variations in crustal thickness, presence/absence of a high velocity zone at the base of the crust, distribution of electrical resistivity within the crust and upper mantle, as well as differences in seismic and electrical anisotropy, and degree of mantle metasomatism.
	www.geophysics.dias.ie /~ajones/wc/evolution.html (823 words)

	Archean Era
		Archean sulfide rocks are much less depleted than are modern sulfides in 34S, an isotope that reflects bacterial processing.
		This deep environment would have provided the microbiota with protection from the harmful UV radiation prevalent at the surface of the Earth during the Archean, when there was no protective ozone layer.
		The early Archean fossil record at best contains a handful of morphotypes, none of which is taxonomically diagnostic or physiologically informative.
	www.peripatus.gen.nz /paleontology/Archaean.html (2116 words)

	The Archean
		The Archean is the name of the age which began with the forming Earth.
		By the end of the Archean, the Earth was just beginning to come alive, but Mars was already a frozen ball with no life and no ocean.
		From 3.5 BYA to 2.5 BYA, or roughly to the end of the Archean age, iron ores began to be formed in large amounts at the bottom of the ocean.
	www.windows.ucar.edu /tour/link=/earth/past/Archean.html (550 words)

	Search Results for Archean - Encyclopædia Britannica
		Archean rocks occur in greenstone–granite belts that represent the upper crust, in granulite–gneiss belts that formed in the mid-lower crust, and in sedimentary basins, basic dikes, and layered...
		These highly deformed and metamorphosed rocks are similar to those of the Archean and occur in many Proterozoic orogenic belts, such as the Grenville in Canada, the Pan-African Mozambique belt in...
		During the Late Archean (3 to 2.5 billion years ago), relatively stable, post-orogenic conditions developed locally in the upper crust, especially in southern Africa, where the development of...
	www.britannica.com /search?query=Archean&submit=Find&source=MWTEXT (370 words)

	Archean Eon -- Britannica Concise Encyclopedia - Your gateway to all Britannica has to offer!
		The Archean eon began with the formation of the Earth's crust about 3.96 billion years ago and extended to about 2.5 billion years ago, up to the start of the Proterozoic eon; the latter is the second and younger division of the Precambrian period.
		The Onverwacht series is well known from exposures in the Komati valley in the eastern Transvaal region, South Africa, where Onverwacht rocks consist of dark, andesitic lavas, dolomitic limestones, cherts, and jaspers, as well as serpentines,...
		Composed of ancient crystalline rocks that extend far downward, these so-called Archean basement rocks seem to be the stuff of which continents are chiefly made.
	concise.britannica.com /ebc/article-9355715?tocId=9355715 (754 words)

	The Archean
		The duration of the Archean, 2.8 billion years, is more than half the expected age of the Earth.
		At the beginning of the age, the "birth" of the Earth, at 4.8 BYA (billion years ago) the Sun finished it's T-Tauri phase and blew away whatever atmosphere the Earth started with, and Earth started over with an atmosphere of hydrogen dominated components (a "reducing" atmosphere).
		From 3.5 BYA to 2.5 BYA, or roughly to the end of the Archean age, iron ores began to be deposited in large amounts at the bottom of the ocean.
	www.windows.ucar.edu /earth/past/Archean. (1568 words)

	Ore deposits in eastern Finland
		All of these lithostratigraphic units were subsequently thrust back northwards and northeastwards onto the Archean basement foreland during the Svecofennian Orogeny, with the general tectonic style having a geometry and scale reminiscent of the earliest stages of the Alpine collision zone — a comparison first made long ago, by Wegmann (1928).
		Archean basement between Kuopio and Nunnanlahti — and including the Luikonlahti area — has undergone extensive ductile reworking during the Svecofennian, usually such that feldspar porphyroclasts are recognizable in a highly strained quartz-mica matrix (Kohonen et al., 1991).
		Thus, even if an Archean age for the Nunnanlahti greenstones were vindicated by isotopic dating, the question of the timing of the hydrothermal processes that produced the soapstone alteration remains unsolved.
	www.gsf.fi /info/OG42FULL.HTML (17039 words)

	Geology 200a - Lab 6 Rocks typical of the Early Proterozoic and Archean of Canada
		4-16a; Archean; spinifex textured upper part of a komatiite flow; spinifex composed of olivine; extrusion of ultramafic lavas early in Earths history when heat flow was much higher than today; Abitib belt; Munro Township, Matheson.
		4-20; Archean; coarse-grained, equigranular, pale grey to tan weathered hornblende - biotite tonalite; emplacement of granitoid plutons represents final stage in the construction of a continent; Abitibi belt.
		4-21; Archean; silver grey to white weathered, soft, pervasively foliated tourmaline-bearing sericite schist; tourmaline looks like fl, acicular amphibole, but it is triangular in cross-section; protolith likely a potassic felsic volcanic (ash tuff); this rock is associated with gold mineralization at Canada's world class Hemlo Gold deposit.
	instruct.uwo.ca /earth-sci/200a-001/200lab6.htm (866 words)

	GEOBAROMETRY - ARCHEAN LODE-GOLD
		The depth of mineralization for Archean lode-gold deposits from the Yilgarn Craton in Australia and the Superior Province in Canada has been estimated from published data and new fluid inclusion measurements (Fig.
		In fact it appears that, at least in Western Australia, these deposits are hosted from epi- to katazonal depth levels and therefore contrast to other ore deposit classes, such as VMS and MVT deposits, that form only at shallow crustal levels near the seafloor, or within the top 5 km of the crust, respectively.
		One reason for the apparent disparity between the two Archean terrains might be the lack of detailed fluid inclusion studies, hence lack of geobarometric data, conducted in deep and shallow level deposits.
	www.geology.wisc.edu /flincs/pubs/ecrofi1.html (679 words)

	Archean geology (Site not responding. Last check: 2007-10-21)
		The Superior Province in Minnesota consists of three subprovinces defined and named in Canada (from north to south, the Wabigoon, the Quetico, and the Wawa); and a fourth, the Minnesota River Valley (MRV) subprovince, which lies south of the Wawa subprovince.
		These data, together with subhorizontal structural style, high metamorphic grade, and a sparsity of supracrustal protoliths, suggest that the MRV terrane probably was a pre-existing continental fragment that was accreted to the margin of the Superior craton by northward-verging (directed) subduction at the end of late Archean continental growth.
		Based on the history of discoveries in Canada, the Archean greenstone belts of Minnesota are intrinsically prospective for diverse mineral-deposit types including shear-zone-hosted lode gold, iron-formation-hosted stratabound gold, volcanic-hosted base-metal sulfides (VMS), Ni-Cu-PGE-Cr deposits hosted by komatiitic volcanic sequences or associated sills, and Cu-Mo-Au deposits hosted by granitoid porphyry.
	www.geo.umn.edu /mgs/mnpot/archean.html (345 words)

	Science News: Fossils show early diversity of life. (Archean-... @ HighBeam Research (Site not responding. Last check: 2007-10-21)
		The record of ancient life preserved in Earth's oldest rocks shrinks to a handful of tattered pages as paleontologists struggle back through time to the Archean era the first 2 billion years of our planet's history, Now, recently identified fossil microorganisms add a potentially important chapter to that incomplete record.
		Until about 2.2 billion years ago, according to the standard account, dissolved iron in the oceans combined with any free oxygen in the environment.
		Thus, aerobic (oxygen-using) creatures - phytoplankton, for instance - could not have existed in the Archean era described in textbooks.
	www.highbeam.com /library/doc0.asp?DOCID=1G1:13852339&refid=ip_encyclopedia_hf (583 words)

	DID LATE ARCHEAN TO PALEOPROTEROZOIC SHEARING OVERTURN THE ISUA BELT OF SW GREENLAND? CONSTRAINTS FROM STRUCTURE AND ... (Site not responding. Last check: 2007-10-21)
		Both Early- and Mid-Archean orthogneisses and igneous rocks surrounded by the Isua supracrustal belt, SW Greenland, are commonly strongly deformed at their western edge, suggesting the structure of the neighboring greenstone belt is post-Early Archean.
		We suggest that the steep SE dips in the west limb of the greenstone belt are linked to this shearing, and possibly related to motion along the Ataneq fault further west.
		The domal structure everywhere else in the central gneisses and surrounding greenstone belt was thus the original Early Archean structure.
	gsa.confex.com /gsa/2001AM/finalprogram/abstract_14241.htm (486 words)

	Palaeos Archean: The Archeaan Eon
		Rocks of the Lower Archean (in geology time is often referred to vertically, because younger rocks are deposited above older ones) are rare, and include the oldest known terrestrial rocks, from 3.8 to 4.2 billion years ago.
		Most of the oldest rocks are so altered through subsequent metamorphic processes it is difficult to know under what conditions they were formed.
		The situation is rather brighter with the more numerous rocks of the Younger ("Upper") Archean, from 3 to 2.5 or 2.6 billion years ago.
	www.palaeos.com /Archean/Archean.htm (895 words)

	MSN Encarta - Search Results - Archean
		major events in Earth’s history during Archean, subdivisions of Archean, Phanerozoic Eon, when complex life appeared, Precambrian, including...
		The Precambrian is divided into pre-Archean time (from the formation of the earth to 3.8 billion years ago), the Archean Eon (3.8 billion to 2.5...
		Too little is known about pre-Archean time, from the origin of the earth to 3.8 billion...
	encarta.msn.com /Archean.html (105 words)

	Penn State News
		According to thermodynamic analysis, this means that 90 to 95 percent of the hydrogen would be converted to methane.
		According to Kasting, this level of methane would compensate for the sun during the Archean which produced only 80 percent of the energy that it does today.
		In the Archean, while Cyanobacteria, bacteria capable of photosynthesis similar to algae, are producing some oxygen, the oxygen is quickly reduced.
	www.psu.edu /ur/2001/archeanatmos.html (661 words)

	Dawn's Archean Microbialites (Site not responding. Last check: 2007-10-21)
		Although Archean stromatolites have been used to support interpretations of the evolution of photosynthetic communities, the specific role of microbial communities, if any, in the formation of many of the oldest stromatolites is controversial (e.g.
		Some Archean stromatolites, however, contain compelling evidence for the presence of microbial communities.
		The 2521+/- 3 Ma Gamohaan and Frisco formations contain a wide morphological range of microbialites (or stromatolites) and associated structures.
	www.gps.caltech.edu /~sumner/Dmicrob.html (745 words)

	The North America Tapestry - Archean Rocks
		During the Archean, the cratons--the masses of rock that make up the basic, initial structure of continents--formed.
		During the Archean the earth was pummeled with meteors.
		Seawater and an atmosphere are believed to have developed during the Archean, and it was during this time that the oceans became populated with algae, bacteria, and colonies of microorganisms known as stromatolites.
	www.nationalatlas.gov /articles/geology/legend/ages/archean.html (182 words)

	Freeman-Lynde GLY126 Archean & Proterozoic Tectonics
		Canadian Shield - Wyoming, Superior, Nain, Hearne, Rae and Slave provinces (Archean cratons); Trans-Hudson and Wopmay provinces (Early Proterozoic orogens >1.8 by); Yavapai-Mazatzal and Central Plains provinces (Early Proterozoic orogens < 1.8 by); and Grenville province (Middle Proterozoic orogen)(see p.
		Archean and Earliest Proterozoic oxygen combined with previously dissolved iron (Fe) to form BIFs and did NOT accumulate in the atmosphere
		What are the general characteristics of Archean tectonics (the nature of plate tectonics in the Archean in terms of convection rates and thickness of lithosphere)?
	www.arches.uga.edu /~rfreeman/'GLY126_'8_Arch_Prot_Tect.html (927 words)

	DO ARCHEAN CHEMICAL SEDIMENTS RECORD ANCIENT SEAWATER RARE EARTH ELEMENT PATTERNS?
		Although complex and multifaceted, arguments for preservation of biogenic indicators in these Archean rocks are based on sedimentary structures (e.g., stromatolitic bedforms), microscopic features that resemble, to a degree, bacteria, and perhaps most convincing, isotopically light signatures of reduced carbon within these rocks.
		The shale-normalized La-, Ce-, Gd-, Y-anomalies, and the heavy REE enrichments of these groundwaters are statistically indistinguishable from modern seawater and Holocene microbialites, and are within the range exhibited by the Archean rocks.
		Consequently, we suggest that it may be premature, in the absence of Nd isotopic data, to unequivocally conclude that these Archean rocks record REE signatures of an Archean ocean.
	gsa.confex.com /gsa/2004AM/finalprogram/abstract_75735.htm (322 words)

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