47-462: Azoic Age , Azoic Era , Azoic Period and Azoic Eon were terms used before 1950 to describe the age of rocks formed before the appearance of life in the geologic sequence. The word "Azoic" is derived from the Greek a- meaning without and zoön meaning animal (or living being), it was first used to mean without death . Azoic was used as early as 1846 by a geologist named Adams, and gradually replaced
94-563: A gravity well , and the kinetic energy of accreted matter. Controversy over the exact nature of mantle convection makes the linked evolution of Earth's heat budget and the dynamics and structure of the mantle difficult to unravel. There is evidence that the processes of plate tectonics were not active in the Earth before 3.2 billion years ago, and that early Earth's internal heat loss could have been dominated by advection via heat-pipe volcanism . Terrestrial bodies with lower heat flows, such as
141-463: 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
188-586: 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
235-418: A lack of rock samples from below 200 km depth, it is difficult to determine precisely the radiogenic heat throughout the whole mantle, although some estimates are available. For the Earth's core, geochemical studies indicate that it is unlikely to be a significant source of radiogenic heat due to an expected low concentration of radioactive elements partitioning into iron. Radiogenic heat production in
282-417: A variable conductivity in the Earth's interior could expand the computed age of the Earth to billions of years, as later confirmed by radiometric dating. Contrary to the usual representation of Thomson's argument, the observed thermal gradient of the Earth's crust would not be explained by the addition of radioactivity as a heat source. More significantly, mantle convection alters how heat is transported within
329-601: Is a rigid outer crust that is composed of thicker continental crust and thinner oceanic crust , solid but plastically flowing mantle , a liquid outer core , and a solid inner core . The fluidity of a material is proportional to temperature; thus, the solid mantle can still flow on long time scales, as a function of its temperature and therefore as a function of the flow of Earth's internal heat. The mantle convects in response to heat escaping from Earth's interior, with hotter and more buoyant mantle rising and cooler, and therefore denser, mantle sinking. This convective flow of
376-477: 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
423-612: Is fundamental to the thermal history of the Earth . The flow of heat from Earth's interior to the surface is estimated at 47±2 terawatts (TW) and comes from two main sources in roughly equal amounts: the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust, and the primordial heat left over from the formation of Earth . Earth's internal heat travels along geothermal gradients and powers most geological processes. It drives mantle convection , plate tectonics , mountain building , rock metamorphism , and volcanism . Convective heat transfer within
470-509: 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
517-404: 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
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#1732844653526564-470: 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
611-572: 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
658-607: 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
705-536: The global map of Earth heat flow . The radioactive decay of elements in the Earth's mantle and crust results in production of daughter isotopes and release of geoneutrinos and heat energy, or radiogenic heat . About 50% of the Earth's internal heat originates from radioactive decay. Four radioactive isotopes are responsible for the majority of radiogenic heat because of their enrichment relative to other radioactive isotopes: uranium-238 ( U), uranium-235 ( U), thorium-232 ( Th), and potassium-40 ( K). Due to
752-405: The ocean floor , the sensible heat absorbed from non-reflected insolation flows inward only by means of thermal conduction , and thus penetrates only a few dozen centimeters on the daily cycle and only a few dozen meters on the annual cycle. This renders solar radiation minimally relevant for processes internal to Earth's crust . Global data on heat-flow density are collected and compiled by
799-405: 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
846-511: 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
893-528: 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
940-509: 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
987-404: 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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#17328446535261034-505: 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. Earth%27s internal heat budget Earth's internal heat budget
1081-515: 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
1128-552: The Azoic "stands as the first [age] in geologic history, whether science can point out unquestionably the rocks of that age or not." He went on to say that when fossils had been found in strata which had previously been classified as Azoic, the boundary was simply moved lower. "Such changes are part of the progress of the science." Archean The Archean Eon ( IPA : / ɑːr ˈ k iː ə n / ar- KEE -ən , also spelled Archaean or Archæan ), in older sources sometimes called
1175-1028: 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
1222-419: The Earth, a proxy for radiogenic heat, yielded a new estimate of half of the total Earth internal heat source being radiogenic, and this is consistent with previous estimates. Primordial heat is the heat lost by the Earth as it continues to cool from its original formation, and this is in contrast to its still actively-produced radiogenic heat. The Earth core's heat flow—heat leaving the core and flowing into
1269-454: The Earth, invalidating Thomson's assumption of purely conductive cooling. Estimates of the total heat flow from Earth's interior to surface span a range of 43 to 49 terawatts (TW) (a terawatt is 10 watts ). One recent estimate is 47 TW, equivalent to an average heat flux of 91.6 mW/m , and is based on more than 38,000 measurements. The respective mean heat flows of continental and oceanic crust are 70.9 and 105.4 mW/m . While
1316-622: The International Heat Flow Commission of the International Association of Seismology and Physics of the Earth's Interior . Based on calculations of Earth's cooling rate, which assumed constant conductivity in the Earth's interior, in 1862 William Thomson , later Lord Kelvin , estimated the age of the Earth at 98 million years, which contrasts with the age of 4.5 billion years obtained in the 20th century by radiometric dating . As pointed out by John Perry in 1895
1363-464: 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
1410-527: The earlier term Primitive . Due to the controversy over evolution, "Azoic" was replaced, by 1900, in most usages by the term " Archaean " or "Archaeozoic The Archaean was later subdivided into the Archaean and the even earlier Hadean . Many of the rocks that had originally been thought to be of Azoic time were reclassified as Archaean, but the period itself is now essentially the Hadean. J.D. Dana in 1863, said that
1457-410: The heat loss rate would slow once the mantle solidified. Heat flow from the core is necessary for maintaining the convecting outer core and the geodynamo and Earth's magnetic field ; therefore primordial heat from the core enabled Earth's atmosphere and thus helped retain Earth's liquid water. Primordial heat energy comes from the potential energy released by collapsing a large amount of matter into
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1504-410: The mantle drives the movement of Earth's lithospheric plates ; thus, an additional reservoir of heat in the lower mantle is critical for the operation of plate tectonics and one possible source is an enrichment of radioactive elements in the lower mantle. Earth heat transport occurs by conduction , mantle convection , hydrothermal convection , and volcanic advection . Earth's internal heat flow to
1551-446: The mantle is linked to the structure of mantle convection , a topic of much debate, and it is thought that the mantle may either have a layered structure with a higher concentration of radioactive heat-producing elements in the lower mantle, or small reservoirs enriched in radioactive elements dispersed throughout the whole mantle. Geoneutrino detectors can detect the decay of U and Th and thus allow estimation of their contribution to
1598-412: 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
1645-507: 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,
1692-430: The overlying mantle—is thought to be due to primordial heat, and is estimated at 5–15 TW. Estimates of mantle primordial heat loss range between 7 and 15 TW, which is calculated as the remainder of heat after removal of core heat flow and bulk-Earth radiogenic heat production from the observed surface heat flow. The early formation of the Earth's dense core could have caused superheating and rapid heat loss, and
1739-457: 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,
1786-469: The planet's high-temperature metallic core is also theorized to sustain a geodynamo which generates Earth's magnetic field . Despite its geological significance, Earth's interior heat contributes only 0.03% of Earth's total energy budget at the surface, which is dominated by 173,000 TW of incoming solar radiation . This external energy source powers most of the planet's atmospheric, oceanic, and biologic processes. Nevertheless on land and at
1833-413: The present radiogenic heat budget, while U and K are not thus detectable. Regardless, K is estimated to contribute 4 TW of heating. However, due to the short half-lives the decay of U and K contributed a large fraction of radiogenic heat flux to the early Earth, which was also much hotter than at present. Initial results from measuring the geoneutrino products of radioactive decay from within
1880-417: The surface is thought to be 80% due to mantle convection, with the remaining heat mostly originating in the Earth's crust, with about 1% due to volcanic activity, earthquakes, and mountain building. Thus, about 99% of Earth's internal heat loss at the surface is by conduction through the crust, and mantle convection is the dominant control on heat transport from deep within the Earth. Most of the heat flow from
1927-447: The thicker continental crust is attributed to internal radiogenic sources; in contrast the thinner oceanic crust has only 2% internal radiogenic heat. The remaining heat flow at the surface would be due to basal heating of the crust from mantle convection. Heat fluxes are negatively correlated with rock age, with the highest heat fluxes from the youngest rock at mid-ocean ridge spreading centers (zones of mantle upwelling), as observed in
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1974-837: 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
2021-409: The total internal Earth heat flow to the surface is well constrained, the relative contribution of the two main sources of Earth's heat, radiogenic and primordial heat, are highly uncertain because their direct measurement is difficult. Chemical and physical models give estimated ranges of 15–41 TW and 12–30 TW for radiogenic heat and primordial heat , respectively. The structure of Earth
2068-404: 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
2115-511: 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
2162-557: 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
2209-575: 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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