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{{Mergefrom|Azoic|date=May 2008}}
{{Hadean Infobox}}

The '''Hadean''' ({{pronEng|ˈheɪdiən}}) is the [[Eon (geology)|geologic eon]] before the [[Archean]].  It started at [[Earth|Earth's]] formation about 4.6 billion years ago (4600&nbsp;[[Ma (unit)|Ma]]), and ended roughly 3.8 billion years ago, though the latter date varies according to different sources. The name "Hadean" derives from [[Hades]], Greek for "unseen" or "[[Hell]]" and suggesting the [[underworld]] or referring to the conditions on Earth at the time. The [[geologist]] [[Preston Cloud]] coined the term in [[1972]], originally to label the period before the earliest-known [[Rock (geology)|rock]]s.  W. B. Harland later coined an almost synonymous term: the "'''Priscoan period'''". Other older texts simply refer to the eon as the '''Pre-Archean''', while during much of the 19th and 20th centuries, the term '''[[Azoic]]''' (meaning without life) was commonly used.

==Subdivisions==

Since few geological traces of this period remain on Earth there are no official subdivisions. However, several major divisions of the [[lunar geologic timescale]] occurred during the Hadean, and so these are sometimes used unofficially to refer to the same periods of time on Earth.

==Hadean rocks==
{{further|[[Oldest rock]]}}
In the last decades of the [[20th century]] geologists identified a few Hadean rocks from Western [[Greenland]], Northwestern [[Canada]] and [[Western Australia]].  The oldest known rock formations (the [[Isua greenstone belt]]) comprise sediments from Greenland dated around 3.8 billion years ago somewhat altered by a [[volcano|volcanic]] [[dike (geology)|dike]] that penetrated the rocks after they were deposited.  Individual [[zircon]] [[crystal]]s redeposited in [[sediment]]s in [[Western Canada]] and the [[Jack Hills]] region of [[Western Australia]] are much older.  The oldest dated zircons date from about 4400 Ma<ref>[http://www.nature.com/nature/journal/v409/n6817/abs/409175A0.html Wilde, S. A.; Valley, J.W.; Peck, W.H. and Graham, C.M. (2001) "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago" ''Nature'' 409: pp. 175-178] Abstract</ref> - very close to the hypothesized time of the [[History of Earth#Origin|Earth's formation]].  

The Greenland sediments include [[Banded iron formation|banded iron]] beds.  They contain possibly [[organic compound|organic]] [[carbon]] and imply some possibility that [[photosynthesis|photosynthetic]] [[life]] had already emerged at that time.  The oldest known [[fossil]]s (from Australia) date from a few hundred million years later.

The [[late heavy bombardment]] happened during Hadean times and affected the [[Earth]] and the [[Moon]].

==Atmosphere and oceans==

A sizeable quantity of [[water]] would have been in the material which formed the Earth.<ref>[http://www.ingentaconnect.com/content/arizona/maps/2005/00000040/00000004/art00003;jsessionid=7ibpocfkopqql.alice IngentaConnect Origin of water in the terrestrial planets<!-- Bot generated title -->]</ref> Water molecules would have escaped Earth's gravity until the planet attained a radius of about 40% of its current size; after that point, water (and other volatile substances) would have been retained.<ref>[http://history.nasa.gov/SP-345/ch26.htm chapter 26<!-- Bot generated title -->]</ref>  Hydrogen and helium are expected to continually leak from the atmosphere, but the lack of denser [[noble gas]]es in the modern atmosphere suggests that something disastrous happened to the early [[atmosphere]].

Part of the young planet is theorized to have been disrupted by the [[giant impact hypothesis|impact which created the Moon]], which should have caused melting of one or two large areas.  Present composition does not match complete melting and it is hard to completely melt and mix huge rock masses.<ref>[http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=446 Solar System Exploration: Science & Technology: Science Features: View Feature<!-- Bot generated title -->]</ref>  However, a fair fraction of material should have been vaporized by this impact, creating a rock vapor atmosphere around the young planet.  The rock vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a heavy [[carbon dioxide]] atmosphere with [[hydrogen]] and [[water vapor]].  Liquid water oceans existed despite the surface temperature of 230°C because of the atmospheric pressure of the heavy CO<sub>2</sub> atmosphere.  As cooling continued, subduction and dissolving in ocean water removed most CO<sub>2</sub> from the atmosphere but levels  oscillated wildly as new surface and mantle cycles appeared.<ref>[http://www.pnas.org/cgi/content/full/98/7/3666 Inaugural Article: Initiation of clement surface conditions on the earliest Earth - Sleep et al. 98 (7): 3666 - Proceedings of the National Academy of Sciences<!-- Bot generated title -->]</ref>

Study of [[zircon]]s has found that liquid water must have existed as long ago as 4400 Ma, very soon after the formation of the Earth.<ref>[http://wwwrses.anu.edu.au/admin/index.php?p=harrison ANU - Research School of Earth Sciences - ANU College of Science - Harrison<!-- Bot generated title -->]</ref><ref>[http://info.anu.edu.au/mac/Media/Media_Releases/_2005/_November/_181105harrisoncontinents.asp ANU - OVC - MEDIA - MEDIA RELEASES - 2005 - NOVEMBER - 181105HARRISONCONTINENTS<!-- Bot generated title -->]</ref><ref>[http://www.geology.wisc.edu/%7Evalley/zircons/cool_early/cool_early_home.html A Cool Early Earth<!-- Bot generated title -->]</ref> This requires the presence of an atmosphere.

==See also==
{{Commonscat|Hadean}}
* [[oldest rock]]
* [[Timetable of the Precambrian]]
* [[History of Earth]] - The first sections describe the formation of the earth
* [[Solar nebula]] - For details of the pre-Hadean development of the solar system
* [[Formation and evolution of the Solar System]]

==References==
{{reflist|2}}
* Valley, John W., William H. Peck, Elizabeth M. King (1999) ''Zircons Are Forever'', The Outcrop for 1999, University of Wisconsin-Madison [http://www.geology.wisc.edu/~valley/zircons/zircon_home.html Wgeology.wisc.edu] &ndash; ''Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago'' Accessed Jan. 10, 2006
* Wilde S.A., Valley J.W., Peck W.H. and Graham C.M. (2001) ''Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.'' Nature, v. 409, pp. 175-178.
* Wyche, S., D. R. Nelson and A. Riganti (2004) ''4350–3130 Ma detrital zircons in the Southern Cross Granite–Greenstone Terrane, Western Australia: implications for the early evolution of the Yilgarn Craton'', Australian Journal of Earth Sciences Volume 51 [http://www.blackwell-synergy.com/doi/abs/10.1046/j.1400-0952.2003.01042.x?cookieSet=1 Zircon ages from W. Australia - Absract] Accessed Jan. 10, 2006

==External links==
[http://www.peripatus.gen.nz/paleontology/Hadean.html Description of the Hadean Era]


{{Eon Footer}}
{{Hadean Footer}}


[[Category:Geologic time scale]]
[[Category:Precambrian]]
[[Category:Geological history of Earth]]

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[[cs:Hadaikum]]
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[[el:Καταρχαιοζωικός αιώνας]]
[[es:Eón Hadeico]]
[[fr:Hadéen]]
[[hr:Hadij]]
[[id:Hadean]]
[[it:Adeano]]
[[he:האדן]]
[[kk:Хадей]]
[[la:Aeon Hadaicus]]
[[lt:Hadėjus]]
[[hu:Hadaikum]]
[[ms:Hadean]]
[[nl:Hadeïcum]]
[[ja:冥王代]]
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[[pt:Hadeano]]
[[ru:Катархей]]
[[sk:Hadaikum]]
[[sl:Had (geologija)]]
[[sh:Had (period)]]
[[fi:Hadeinen eoni]]
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[[vi:Liên đại Hỏa Thành]]
[[uk:Гадейський еон]]
[[zh:冥古宙]]