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49-681: There has been some debate as to when and even if Vaalbara existed. An Archaean – Palaeoproterozoic (2.8–2.1 Ga) link between South Africa and Western Australia was first proposed by A. Button in 1976. He found a wide range of similarities between the Transvaal Basin in South Africa and the Hamersley Basin in Australia. Button, however, placed Madagascar between Africa and Australia and concluded that Gondwana must have had

98-468: A feature in later, more oxic oceans. Despite the lack of free oxygen, the rate of organic carbon burial appears to have been roughly the same as in the present. Due to extremely low oxygen levels, sulphate was rare in the Archean ocean, and sulphides were produced primarily through reduction of organically sourced sulphite or through mineralisation of compounds containing reduced sulphur. The Archean ocean

147-526: A greenstone belt and the granites they abut, and the Proterozoic where greenstone belts sit upon granite-gneiss basements and / or other greenstone belts, and the Phanerozoic where clear examples of island arc volcanism, arc sedimentation and ophiolite sequences become more dominant. This change in nature is interpreted as a response to the maturity of the plate tectonics processes throughout

196-586: A hotter mantle, rheologically weaker plates, and increased tensile stresses on subducting plates due to their crustal material metamorphosing from basalt into eclogite as they sank. 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 . Evidence from banded iron formations, chert beds, chemical sediments and pillow basalts demonstrates that liquid water

245-492: A long stable tectonic history. Similarly, in the reconstruction of Rogers 1993 , 1996 the oldest continent is Ur . In Rogers' reconstructions, however, Kaapvaal and Pilbara are placed far apart already in their Gondwana configuration, a reconstruction contradicted by later orogenic events and incompatible with the Vaalbara hypothesis. Cheney 1996 , nevertheless, found a three-fold stratigraphic similarity and proposed that

294-536: Is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths . The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history. Extensive abiotic denitrification took place on the Archean Earth, pumping the greenhouse gas nitrous oxide into the atmosphere. Alternatively, Earth's albedo may have been lower at

343-412: Is possible, although the existence of this 3.6–3.2 Ga continent cannot be proven. South Africa's Kaapvaal craton and Western Australia's Pilbara craton have similar early Precambrian cover sequences. Kaapvaal's Barberton granite-greenstone terrane and Pilbara's eastern block show evidence of four large meteorite impacts between 3.2 and 3.5 billion years ago. Similar greenstone belts are found at

392-515: Is that rocks from western Australia and southern Africa were assembled in a continent called Vaalbara as far back as 3,600 Ma. Archean rock makes up only about 8% of Earth's present-day continental crust; the rest of the Archean continents have been recycled. By the Neoarchean , plate tectonic activity may have been similar to that of the modern Earth, although there was a significantly greater occurrence of slab detachment resulting from

441-404: Is the age of the oldest known intact rock formations on Earth. Evidence of rocks from the preceding Hadean Eon are therefore restricted by definition to non-rock and non-terrestrial sources such as individual mineral grains and lunar samples. When the Archean began, the Earth's heat flow was nearly three times as high as it is today, and it was still twice the current level at the transition from

490-470: Is thought to have almost completely lacked free oxygen ; oxygen levels were less than 0.001% of their present atmospheric level, with some analyses suggesting they were as low as 0.00001% of modern levels. However, transient episodes of heightened oxygen concentrations are known from this eon around 2,980–2,960 Ma, 2,700 Ma, and 2,501 Ma. The pulses of increased oxygenation at 2,700 and 2,501 Ma have both been considered by some as potential start points of

539-531: The Archaeozoic , is the second of the four geologic eons of Earth 's history , preceded by the Hadean Eon and followed by the Proterozoic . The Archean represents the time period from 4,031 to 2,500 Mya (million years ago). The Late Heavy Bombardment is hypothesized to overlap with the beginning of the Archean. The Huronian glaciation occurred at the end of the eon. The Earth during

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588-496: The Archean greenstone belts. These similarities include a decrease in the amount of ultramafic and mafic rocks as you move up the stratigraphic column , in addition to an increase in pyroclastics , felsic and/or andesite rocks. Also, the rock successions tend to have clastics in the upper portion and tholeiitic suites in the lower. Calc-alkaline dikes are common in these suites. Archaean greenstones are found in

637-631: The Great Oxygenation Event , which most scholars consider to have begun in the Palaeoproterozoic ( c.  2.4 Ga ). Furthermore, oases of relatively high oxygen levels existed in some nearshore shallow marine settings by the Mesoarchean. The ocean was broadly reducing and lacked any persistent redoxcline , a water layer between oxygenated and anoxic layers with a strong redox gradient, which would become

686-760: The Kalahari and West Australian (Yilgarn and Pilbara) cratons around 1.95–1.8 Ga. The Archaean–Palaeoproterozoic Grunehogna Craton in Queen Maud Land , East Antarctica , formed the eastern part of the Kalahari Craton for at least a billion years. Grunehogna collided with the rest of East Antarctica during the Mesoproterozoic assembly of the supercontinent Rodinia and the Grenville orogeny . The Neoproterozoic Pan-African orogeny and

735-856: The Slave craton , northern Canada , Pilbara craton and Yilgarn Craton , Western Australia , Gawler Craton in South Australia , and in the Wyoming Craton in the US. Examples are found in South and Eastern Africa, namely the Kaapvaal craton and also in the cratonic core of Madagascar , as well as West Africa and Brazil , northern Scandinavia and the Kola Peninsula (see Baltic Shield ). Proterozoic greenstones occur sandwiched between

784-405: The prebiotic atmosphere was a reducing atmosphere rich in methane and lacking free oxygen . The earliest known life , mostly represented by shallow-water microbial mats called stromatolites , started in the Archean and remained simple prokaryotes ( archaea and bacteria ) throughout the eon. The earliest photosynthetic processes, especially those by early cyanobacteria , appeared in

833-601: The Archaean in both Africa and Australia. The oldest widely accepted evidence of photosynthesis by early life forms is molecular fossils found in 2.7 Ga-old shales in the Pilbara Craton . These fossils have been interpreted as traces of eukaryotes and cyanobacteria , though some scientists argue that these biomarkers must have entered these rocks later and date the fossils to 2.15–1.68 Ga. This later time span agrees with estimates based on molecular clocks which dates

882-519: The Archean and become common late in the Archean. Cyanobacteria were instrumental in creating free oxygen in the atmosphere. Further evidence for early life is found in 3.47 billion-year-old baryte , in the Warrawoona Group of Western Australia. This mineral shows sulfur fractionation of as much as 21.1%, which is evidence of sulfate-reducing bacteria that metabolize sulfur-32 more readily than sulfur-34. Evidence of life in

931-544: The Archean spanned Earth's early history from its formation about 4,540 million years ago until 2,500 million years ago. Instead of being based on stratigraphy , the beginning and end of the Archean Eon are defined chronometrically . The eon's lower boundary or starting point of 4,031±3 million years ago is officially recognized by the International Commission on Stratigraphy , which

980-519: The Archean to the Proterozoic (2,500  Ma ). The extra heat was partly remnant heat from planetary accretion , from the formation of the metallic core , and partly arose from the decay of radioactive elements. As a result, the Earth's mantle was significantly hotter than today. Although a few mineral grains have survived from the Hadean , the oldest rock formations exposed on the surface of

1029-404: The Archean was mostly a water world : there was continental crust , but much of it was under an ocean deeper than today's oceans. Except for some rare relict crystals , today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent activity. The Earth's atmosphere was also vastly different in composition from today's:

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1078-774: The Archean without leaving any. Fossil steranes , indicative of eukaryotes, have been reported from Archean strata but were shown to derive from contamination with younger organic matter. No fossil evidence has been discovered for ultramicroscopic intracellular replicators such as viruses . Fossilized microbes from terrestrial microbial mats show that life was already established on land 3.22 billion years ago. Greenstone belt Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies. The name comes from

1127-525: The Archean, the conditions necessary to sustain life could not have occurred until the Archean Eon. Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called prokaryotes . In addition to the domain Bacteria , microfossils of the domain Archaea have also been identified. There are no known eukaryotic fossils from the earliest Archean, though they might have evolved during

1176-998: The Earth are Archean. Archean rocks are found in Greenland , Siberia , the Canadian Shield , Montana , Wyoming (exposed parts of the Wyoming Craton ), Minnesota (Minnesota River Valley), the Baltic Shield , the Rhodope Massif , Scotland , India , Brazil , western Australia , and southern Africa . Granitic rocks predominate throughout the crystalline remnants of the surviving Archean crust. These include great melt sheets and voluminous plutonic masses of granite , diorite , layered intrusions , anorthosites and monzonites known as sanukitoids . Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes , mudstones , volcanic sediments, and banded iron formations . Volcanic activity

1225-492: The Earth's geological history. Archaean plate tectonics did not take place on mature crust and as such the presence of thrust-in allochthonous greenstone belts is expected. By the Proterozoic, magmatism was occurring around cratons and with established sedimentary sources, with little recycling of the crust , allowing preservation of more sediments. By the Phanerozoic, extensive continental cover and lower heat flow from

1274-470: The Late Hadean is more controversial. In 2015, biogenic carbon was detected in zircons dated to 4.1 billion years ago, but this evidence is preliminary and needs validation. Earth was very hostile to life before 4,300 to 4,200 Ma, and the conclusion is that before the Archean Eon, life as we know it would have been challenged by these environmental conditions. While life could have arisen before

1323-652: The Pilbara and Yilgarn cratons in Australia, and adjoining the Gawler Craton and within the extensive Proterozoic mobile belts of Australia, within West Africa, throughout the metamorphic complexes surrounding the Archaean core of Madagascar; the eastern United States , northern Canada and northern Scandinavia. The Abitibi greenstone belt in Ontario and Quebec is one of the largest Archean greenstone belts in

1372-600: The assembly of Gondwana/ Pannotia produced large shear zones between Grunehogna and Kalahari. During the Jurassic break-up of Gondwana, these shear zones finally separated Grunehogna and the rest of Antarctica from Africa. In the Annandags Peaks in Antarctica, the only exposed parts of Grunehogna, detrital zircons from several crustal sources have been dated to 3.9–3.0 Ga suggesting intracrustal recycling

1421-714: The eukaryote last common ancestor at 1.8–1.7 Ga. If the Pilbara fossils are traces of early eukaryotes, they could represent groups that went extinct before modern groups emerged. [REDACTED] Africa [REDACTED] Antarctica [REDACTED] Asia [REDACTED] Australia [REDACTED] Europe [REDACTED] North America [REDACTED] South America [REDACTED] Afro-Eurasia [REDACTED] Americas [REDACTED] Eurasia [REDACTED] Oceania Archean The Archean Eon ( IPA : / ɑːr ˈ k iː ə n / ar- KEE -ən , also spelled Archaean or Archæan ), in older sources sometimes called

1470-495: The existence of Vaalbara and explain similarities between the two cratons as the product of global processes. They point, for example, to thick volcanic deposits on other cratons such as Amazonia , São Francisco , and Karnataka . Zimgarn, another proposed supercraton composed of the Zimbabwe and Yilgarn cratons at 2.41 Ga, is distinct from Vaalbara. Zimgarn should have disintegrated around 2.1–2.0 Ga to reassemble as

1519-535: The granite and gneiss events, because the vast majority of greenstones are interpreted as altered basalts and other volcanic or sedimentary rocks . As such, understanding the nature and origin of greenstone belts is the most fruitful way of studying Archaean geological history. Greenstone belts have been interpreted as having formed at ancient oceanic spreading centers and island arc terranes . Greenstone belts are primarily formed of volcanic rocks, dominated by basalt , with minor sedimentary rocks inter-leaving

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1568-406: The greater volume of otherwise homogeneous granite - gneiss within a craton contains a significantly larger degree of heterogeneity and complications and forms a tectonic marker far more distinct than the much more voluminous and homogeneous granites. Additionally, a greenstone belt contains far more information on tectonic and metamorphic events, deformations, and paleogeologic conditions than

1617-400: The green hue imparted by the colour of the metamorphic minerals within the mafic rocks: The typical green minerals are chlorite , actinolite , and other green amphiboles . Greenstone belts also often contain ore deposits of gold , silver , copper , zinc , and lead . A greenstone belt is typically several dozens to several thousand kilometres long. Typically, a greenstone belt within

1666-431: The mantle has seen greater preservation of sediments and greater influence of continental masses. Greenstones, aside from containing basalts, also give rise to several types of metamorphic rocks which are used synonymously with ' metabasalt ' et cetera; greenschist , whiteschist and blueschist are all terms spawned from the study of greenstone belts. The West African early Proterozoic greenstone belts are similar to

1715-747: The margins of the Superior Craton of Canada. The high temperatures created by the impacts' forces fused sediments into small glassy spherules. Spherules of 3.5 billion years old exist in South Africa, and spherules of a similar age have been found in Western Australia; they are the oldest-known terrestrial impact products. The spherules resemble the glassy chondrules (rounded granules) in carbonaceous chondrites , which are found in carbon-rich meteorites and lunar soils. Remarkably similar lithostratigraphic and chronostratigraphic structural sequences between these two cratons have been noted for

1764-412: The metamorphosed sediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. Greenstone belts, which include both types of metamorphosed rock, represent sutures between the protocontinents. Plate tectonics likely started vigorously in the Hadean , but slowed down in the Archean. The slowing of plate tectonics was probably due to an increase in

1813-569: The mid/late Archean and led to a permanent chemical change in the ocean and the atmosphere after the Archean. The word Archean is derived from the Greek word arkhē ( αρχή ), meaning 'beginning, origin'. The Pre-Cambrian had been believed to be without life (azoic); however, fossils were found in deposits that were judged to belong to the Azoic age . Before the Hadean Eon was recognized,

1862-461: The oxygen isotope record by enriching seawater with isotopically light oxygen. Due to recycling and metamorphosis of the Archean crust, there is a lack of extensive geological evidence for specific continents. One hypothesis is that rocks that are now in India, western Australia, and southern Africa formed a continent called Ur as of 3,100 Ma. Another hypothesis, which conflicts with the first,

1911-495: The period between 3.5 and 2.7 Ga. Paleomagnetic data from two ultramafic complexes in the cratons showed that at 3.87 Ga the two cratons could have been part of the same supercontinent. Both the Pilbara and Kaapvaal cratons show extensional faults which were active about 3.47 Ga during felsic volcanism and coeval with the impact layers. The Pilbara and Kaapvaal cratons contain well-preserved Archaean microfossils. Drilling has revealed traces of microbial life and photosynthesis from

1960-853: The time, due to less land area and cloud cover. The processes that gave rise to life on Earth are not completely understood, but there is substantial evidence that life came into existence either near the end of the Hadean Eon or early in the Archean Eon. The earliest evidence for life on Earth is graphite of biogenic origin found in 3.7 billion–year-old metasedimentary rocks discovered in Western Greenland . The earliest identifiable fossils consist of stromatolites , which are microbial mats formed in shallow water by cyanobacteria . The earliest stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia . Stromatolites are found throughout

2009-404: The two cratons had a rotational 30° latitudinal separation in the time period of 2.78–2.77 Ga, which indicates they were no longer joined after c. 2.8 billion years ago. Vaalbara thus remained stable for 1–0.4 Ga and hence had a life span similar to that of later supercontinents such as Gondwana and Rodinia . Some palaeomagnetic reconstructions suggest a Palaeoarchaean proto-Vaalbara

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2058-423: The two cratons once formed a continent which he named Vaalbara. This model is supported by the palaeomagnetic data of Zegers, de Wit & White 1998 . Reconstructions of the palaeolatitudes of the two cratons at 2.78–2.77 Ga are ambiguous however. In the reconstruction of Wingate 1998 they fail to overlap, but they do in more recent reconstructions, for example Strik et al. 2003 . Other scientists dispute

2107-404: The viscosity of the mantle due to outgassing of its water. Plate tectonics likely produced large amounts of continental crust, but the deep oceans of the Archean probably covered the continents entirely. Only at the end of the Archean did the continents likely emerge from the ocean. The emergence of continents towards the end of the Archaean initiated continental weathering that left its mark on

2156-422: The volcanic formations. Through time, the degree of sediment contained within greenstone belts has risen, and the amount of ultramafic rock (either as layered intrusions or as volcanic komatiite ) has decreased. There is also a change in the structure and relationship of greenstone belts to their basements between the Archaean where there is little clear relationship, if any, between basalt- peridotite sheets of

2205-802: The world. In Antarctica, the Proterozoic-aged Fisher Massif closely resembles the composition and structure of a greenstone belt. One of the best known greenstone belts in the world is the South African Barberton greenstone belt , where gold was first discovered in South Africa . The Barberton Greenstone belt was first uniquely identified by Prof Annhauser at the University of the Witwatersrand , Johannesburg . His work in mapping and detailing

2254-563: Was an important part in the formation of the first cratons. The Kaapvaal craton is marked by dramatic events such as the intrusion of the Bushveld Complex (2.045 Ga) and the Vredefort impact event (2.025 Ga), and no traces of these events have been found in the Pilbara craton, clearly indicating that the two cratons were separated before 2.05 Ga. Furthermore, geochronological and palaeomagnetic evidence show that

2303-511: Was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite . Carbonate rocks are rare, indicating that the oceans were more acidic, due to dissolved carbon dioxide , than during the Proterozoic. Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks, including Archean felsic volcanic rocks . The metamorphosed igneous rocks were derived from volcanic island arcs , while

2352-560: Was enriched in heavier oxygen isotopes relative to the modern ocean, though δ18O values decreased to levels comparable to those of modern oceans over the course of the later part of the eon as a result of increased continental weathering. Astronomers think that the Sun had about 75–80 percent of its present luminosity, yet temperatures on Earth appear to have been near modern levels only 500 million years after Earth's formation (the faint young Sun paradox ). The presence of liquid water

2401-580: Was prevalent and deep oceanic basins already existed. Asteroid impacts were frequent in the early Archean. Evidence from spherule layers suggests that impacts continued into the later Archean, at an average rate of about one impactor with a diameter greater than 10 kilometers (6 mi) every 15 million years. This is about the size of the Chicxulub impactor. These impacts would have been an important oxygen sink and would have caused drastic fluctuations of atmospheric oxygen levels. The Archean atmosphere

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