Editing Archean (section)

==Geology==
When the Archean began, the Earth's [[Earth's internal heat budget|heat flow]] was nearly three times as high as it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2,500&nbsp;{{tooltip|2=Megaannum, or 1 million years|Ma}}). The extra heat was partly remnant heat from [[Accretion (astrophysics)|planetary accretion]], from the formation of the [[structure of the Earth|metallic core]], and partly arose from the decay of [[radioactive]] elements. As a result, the Earth's mantle was significantly hotter than today.<ref>{{cite journal |last1=Galer |first1=Stephen J. G. |last2=Mezger |first2=Klaus |date=1 December 1998 |title=Metamorphism, denudation and sea level in the Archean and cooling of the Earth |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926898000837 |journal=[[Precambrian Research]] |volume=92 |issue=4 |pages=389–412 |doi=10.1016/S0301-9268(98)00083-7 |bibcode=1998PreR...92..389G |access-date=24 November 2022}}</ref>
[[File:Evolution of Earth's radiogenic heat.svg|thumb|upright=1.5|center|The evolution of Earth's [[Radiogenic nuclide|radiogenic heat]] flow over time]]

Although a few mineral grains have survived from the [[Hadean]], the oldest rock formations exposed on the surface of the Earth are Archean. Archean rocks are found in [[Geology of Greenland|Greenland]], [[Geology of Siberia|Siberia]], the [[Canadian Shield]], [[Geology of Montana|Montana]], [[Geology of Wyoming|Wyoming]] (exposed parts of the [[Wyoming craton|Wyoming Craton]]), [[ Geology of Minnesota|Minnesota]] (Minnesota River Valley), the [[Baltic Shield]], the [[Geology of Bulgaria|Rhodope Massif]], [[Geology of Scotland|Scotland]], [[Geological history of India|India]], [[Geology of Brazil|Brazil]], western [[Geology of Australia|Australia]], and southern [[Geology of Africa|Africa]].{{Citation needed|date=May 2020}} [[Granitic]] rocks predominate throughout the crystalline remnants of the surviving Archean crust. These include great melt sheets and voluminous [[intrusive rock|plutonic]] masses of [[granite]], [[diorite]], [[layered intrusion]]s, [[anorthosite]]s and [[monzonite]]s known as [[sanukitoid]]s. Archean rocks are often heavily metamorphosed deep-water sediments, such as [[graywacke]]s, [[mudstone]]s, volcanic sediments, and [[banded iron formation]]s. [[Volcanic]] activity was considerably higher than today, with numerous lava eruptions, including unusual types such as [[komatiite]].<ref name=Dostal2008>{{cite journal |vauthors=Dostal J |year=2008 |title=Igneous Rock Associations 10. Komatiites |journal=Geoscience Canada |volume=35 |issue=1 |url=https://journals.lib.unb.ca/index.php/GC/article/view/11074/11722}}</ref> [[Carbonate]] rocks are rare, indicating that the oceans were more acidic, due to dissolved [[carbon dioxide]], than during the Proterozoic.<ref>{{cite book |last1=Cooper |first1=John D. |last2=Miller |first2=Richard H. |last3=Patterson |first3=Jacqueline |date=1986 |title=A Trip Through Time: Principles of historical geology |publisher=Merrill Publishing Company |location=Columbus |isbn=978-0675201407 |page=[https://archive.org/details/tripthroughtimep0000coop/page/180 180] |url=https://archive.org/details/tripthroughtimep0000coop |url-access=registration}}</ref> [[Greenstone belt]]s 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 arc]]s, while 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 [[suture (geology)|sutures]] between the protocontinents.<ref name=Stanley1999>{{cite book |last=Stanley |first=Steven M. |year=1999 |title=Earth System History |publisher=W.H. Freeman and Company |location=New York |isbn=978-0716728825}}</ref>{{rp|pages=302–303}}

[[Plate tectonics]] likely started vigorously in the [[Hadean]], but slowed down in the Archean.<ref name=Korenaga2021>{{cite journal |last=Korenaga |first=J |year=2021 |title=Was There Land on the Early Earth? |journal=Life |doi=10.3390/life11111142 |doi-access=free |pmid=34833018 |pmc=8623345 |volume=11 |issue=11 |page=1142|bibcode=2021Life...11.1142K }}</ref><ref>{{cite journal |last=Korenaga |first=J |year=2021 |title=Hadean geodynamics and the nature of early continental crust |journal=[[Precambrian Research]] |doi=10.1016/j.precamres.2021.106178 |bibcode=2021PreR..35906178K |s2cid=233441822 |volume=359 |page=106178}}</ref> The slowing of plate tectonics was probably due to an increase in the viscosity of the [[mantle (geology)|mantle]] due to outgassing of its water.<ref name=Korenaga2021/> Plate tectonics likely produced large amounts of continental crust, but the deep oceans of the Archean probably covered the continents entirely.<ref>{{cite journal |last1=Bada |first1=J. L. |last2=Korenaga |first2=J. |year=2018 |title=Exposed areas above sea level on Earth >3.5 Gyr ago: Implications for prebiotic and primitive biotic chemistry |journal=Life |doi=10.3390/life8040055 |doi-access=free |pmid=30400350 |pmc=6316429 |volume=8 |issue=4 |page=55|bibcode=2018Life....8...55B }}</ref> Only at the end of the Archean did the continents likely emerge from the ocean.<ref name="BindemanEtAl2018">{{cite journal |last1=Bindeman |first1=I. N. |last2=Zakharov |first2=D. O. |last3=Palandri |first3=J. |last4=Greber |first4=N. D. |last5=Dauphas |first5=N. |last6=Retallack |first6=Gregory J. |last7=Hofmann |first7=A. |last8=Lackey |first8=J. S. |last9=Bekker |first9=A. |date=23 May 2018 |url=https://www.nature.com/articles/s41586-018-0131-1 |title=Rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2.5 billion years ago |journal=[[Nature (journal)|Nature]] |doi=10.1038/s41586-018-0131-1 |pmid=29795252 |bibcode=2018Natur.557..545B |s2cid=43921922 |volume=557 |issue=7706 |pages=545–548 |access-date=25 December 2023}}</ref> The emergence of continents towards the end of the Archaean initiated continental weathering that left its mark on the oxygen isotope record by enriching seawater with isotopically light oxygen.<ref>{{Cite journal |last1=Johnson |first1=Benjamin W. |last2=Wing |first2=Boswell A. |date=2 March 2020 |title=Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean |url=https://www.nature.com/articles/s41561-020-0538-9 |journal=[[Nature Geoscience]] |language=en |volume=13 |issue=3 |pages=243–248 |doi=10.1038/s41561-020-0538-9 |bibcode=2020NatGe..13..243J |s2cid=211730235 |issn=1752-0908 |access-date=25 December 2023}}</ref>

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 (continent)|Ur]] as of 3,100&nbsp;Ma.<ref>{{cite journal |vauthors=Rogers JJ |year=1996 |title=A history of continents in the past three billion years |journal=Journal of Geology |doi=10.1086/629803 |jstor=30068065 |bibcode=1996JG....104...91R |s2cid=128776432 |volume=104 |issue=1 |pages=91–107}}</ref> Another hypothesis, which conflicts with the first, is that rocks from western Australia and southern Africa were assembled in a continent called [[Vaalbara]] as far back as 3,600&nbsp;Ma.<ref>{{cite journal |vauthors=Cheney ES |year=1996 |title=Sequence stratigraphy and plate tectonic significance of the Transvaal succession of southern Africa and its equivalent in Western Australia |journal=Precambrian Research |doi=10.1016/0301-9268(95)00085-2 |bibcode=1996PreR...79....3C |volume=79 |issue=1–2 |pages=3–24}}</ref> Archean rock makes up only about 8% of Earth's present-day continental crust; the rest of the Archean continents have been recycled.<ref name=Korenaga2021/>

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 a hotter mantle, [[rheology|rheologically]] weaker plates, and increased tensile stresses on [[Subduction|subducting]] plates due to their crustal material metamorphosing from [[basalt]] into [[eclogite]] as they sank.<ref name=MartyDauphas2003>{{cite journal |last1=Marty |first1=Bernard |last2=Dauphas |first2=Nicolas |date=15 February 2003 |title=The nitrogen record of crust–mantle interaction and mantle convection from Archean to Present |journal=[[Earth and Planetary Science Letters]] |doi=10.1016/S0012-821X(02)01108-1 |volume=206 |issue=3–4 |pages=397–410 |bibcode=2003E&PSL.206..397M |url=https://www.sciencedirect.com/science/article/abs/pii/S0012821X02011081 |access-date=16 November 2022}}</ref><ref name=HallaEtAl2009>{{cite journal |last1=Halla |first1=Jaana |last2=Van Hunen |first2=Jeroen |last3=Heilimo |first3=Esa |last4=Hölttä |first4=Pentti |date=October 2009 |title=Geochemical and numerical constraints on Neoarchean plate tectonics |journal=[[Precambrian Research]] |doi=10.1016/j.precamres.2009.07.008 |volume=174 |issue=1–2 |pages=155–162 |bibcode=2009PreR..174..155H |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926809001661#! |access-date=12 November 2022}}</ref> There are well-preserved [[sedimentary basin]]s, and evidence of [[volcanic arc]]s, intracontinental [[rift]]s, continent-continent collisions and widespread globe-spanning [[Orogeny|orogenic events]] suggesting the assembly and destruction of one and perhaps several [[supercontinent]]s. Evidence from banded iron formations, [[chert]] beds, chemical sediments and [[Basalt|pillow basalts]] demonstrates that liquid water was prevalent and deep oceanic basins already existed.

Asteroid impacts were frequent in the early Archean.<ref>{{cite journal |last1=Borgeat |first1=Xavier |last2=Tackley |first2=Paul J. |date=12 July 2022 |title=Hadean/Eoarchean tectonics and mantle mixing induced by impacts: a three-dimensional study |journal=Progress in Earth and Planetary Science |doi=10.1186/s40645-022-00497-0 |s2cid=243973728 |volume=9 |issue=1 |page=38 |bibcode=2022PEPS....9...38B |hdl=20.500.11850/559385 |hdl-access=free |doi-access=free }}</ref> 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 {{convert|10|km|sigfig=1|sp=us}} every 15 million years. This is about the size of the [[Chicxulub crater|Chicxulub]] impactor. These impacts would have been an important oxygen sink and would have caused drastic fluctuations of atmospheric oxygen levels.<ref name=MarchiEtal2021>{{cite journal |last1=Marchi |first1=S. |last2=Drabon |first2=N. |last3=Schulz |first3=T. |last4=Schaefer |first4=L. |last5=Nesvorny |first5=D. |last6=Bottke |first6=W. F. |last7=Koeberl |first7=C. |last8=Lyons |first8=T. |date=November 2021 |title=Delayed and variable late Archaean atmospheric oxidation due to high collision rates on Earth |url=https://www.nature.com/articles/s41561-021-00835-9 |journal=[[Nature Geoscience]] |doi=10.1038/s41561-021-00835-9 |bibcode=2021NatGe..14..827M |s2cid=239055744 |volume=14 |issue=11 |pages=827–831 |access-date=25 December 2023}}</ref>