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41-414: Archaean may refer to: Archaean or Archean , a geologic eon between Hadean and Proterozoic Something related to Archaea , a domain of single-celled microorganisms Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title Archaean . If an internal link led you here, you may wish to change

82-463: 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

123-586: 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

164-436: A section. The samples are analyzed to determine their detrital remanent magnetism (DRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. For sedimentary rocks this is possible because, as they fall through the water column, very fine-grained magnetic minerals (< 17  μm ) behave like tiny compasses , orienting themselves with Earth's magnetic field . Upon burial, that orientation

205-431: Is also commonly used to delineate the nature and extent of hydrocarbon -bearing reservoir rocks, seals, and traps of petroleum geology . Chronostratigraphy is the branch of stratigraphy that places an absolute age, rather than a relative age on rock strata . The branch is concerned with deriving geochronological data for rock units, both directly and inferentially, so that a sequence of time-relative events that created

246-534: Is due to physical contrasts in rock type ( lithology ). This variation can occur vertically as layering (bedding), or laterally, and reflects changes in environments of deposition (known as facies change). These variations provide a lithostratigraphy or lithologic stratigraphy of the rock unit. Key concepts in stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries imply about their original depositional environment. The basic concept in stratigraphy, called

287-525: 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

328-411: Is preserved. For volcanic rocks, magnetic minerals, which form in the melt, orient themselves with the ambient magnetic field, and are fixed in place upon crystallization of the lava. Oriented paleomagnetic core samples are collected in the field; mudstones , siltstones , and very fine-grained sandstones are the preferred lithologies because the magnetic grains are finer and more likely to orient with

369-509: 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

410-404: 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

451-470: 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

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492-572: 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

533-607: 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

574-431: The law of superposition , states: in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence. Chemostratigraphy studies the changes in the relative proportions of trace elements and isotopes within and between lithologic units. Carbon and oxygen isotope ratios vary with time, and researchers can use those to map subtle changes that occurred in the paleoenvironment. This has led to

615-547: The natural remanent magnetization (NRM) to reveal the DRM. Following statistical analysis, the results are used to generate a local magnetostratigraphic column that can then be compared against the Global Magnetic Polarity Time Scale. This technique is used to date sequences that generally lack fossils or interbedded igneous rocks. The continuous nature of the sampling means that it is also a powerful technique for

656-405: 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

697-511: 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

738-528: 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

779-509: 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

820-404: 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:

861-502: 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. Stratigraphy Catholic priest Nicholas Steno established

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902-515: 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

943-1028: 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

984-464: 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

1025-588: The ambient field during deposition. If the ancient magnetic field were oriented similar to today's field ( North Magnetic Pole near the North Rotational Pole ), the strata would retain a normal polarity. If the data indicate that the North Magnetic Pole were near the South Rotational Pole , the strata would exhibit reversed polarity. Results of the individual samples are analyzed by removing

1066-512: The gap may be due to removal by erosion, in which case it may be called a stratigraphic vacuity. It is called a hiatus because deposition was on hold for a period of time. A physical gap may represent both a period of non-deposition and a period of erosion. A geologic fault may cause the appearance of a hiatus. Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout

1107-506: The link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=Archaean&oldid=932693426 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages Archean The Archean Eon ( IPA : / ɑːr ˈ k iː ə n / ar- KEE -ən , also spelled Archaean or Archæan ), in older sources sometimes called

1148-412: 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

1189-555: 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,

1230-457: 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,

1271-437: The rock layers. Strata from widespread locations containing the same fossil fauna and flora are said to be correlatable in time. Biologic stratigraphy was based on William Smith's principle of faunal succession , which predated, and was one of the first and most powerful lines of evidence for, biological evolution . It provides strong evidence for the formation ( speciation ) and extinction of species . The geologic time scale

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1312-432: The rocks formation can be derived. The ultimate aim of chronostratigraphy is to place dates on the sequence of deposition of all rocks within a geological region, and then to every region, and by extension to provide an entire geologic record of the Earth. A gap or missing strata in the geological record of an area is called a stratigraphic hiatus. This may be the result of a halt in the deposition of sediment. Alternatively,

1353-405: The significance of strata or rock layering and the importance of fossil markers for correlating strata; he created the first geologic map of England. Other influential applications of stratigraphy in the early 19th century were by Georges Cuvier and Alexandre Brongniart , who studied the geology of the region around Paris. Variation in rock units, most obviously displayed as visible layering,

1394-406: The specialized field of isotopic stratigraphy. Cyclostratigraphy documents the often cyclic changes in the relative proportions of minerals (particularly carbonates ), grain size, thickness of sediment layers ( varves ) and fossil diversity with time, related to seasonal or longer term changes in palaeoclimates . Biostratigraphy or paleontologic stratigraphy is based on fossil evidence in

1435-429: The theoretical basis for stratigraphy when he introduced the law of superposition , the principle of original horizontality and the principle of lateral continuity in a 1669 work on the fossilization of organic remains in layers of sediment. The first practical large-scale application of stratigraphy was by William Smith in the 1790s and early 19th century. Known as the "Father of English geology", Smith recognized

1476-837: 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

1517-455: 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

1558-511: 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

1599-525: Was developed during the 19th century, based on the evidence of biologic stratigraphy and faunal succession. This timescale remained a relative scale until the development of radiometric dating , which was based on an absolute time framework, leading to the development of chronostratigraphy. One important development is the Vail curve , which attempts to define a global historical sea-level curve according to inferences from worldwide stratigraphic patterns. Stratigraphy

1640-557: 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

1681-575: 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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