The Hadean (pronEng|ˈheɪdiən) is the geologic eon before the
Archean. It started at Earth's formation about 4.6 billion years ago (4600 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 underworldor referring to the conditions on Earth at the time. The geologist Preston Cloudcoined the term in 1972, originally to label the period before the earliest-known rocks. W. B. Harland later coined an almost synonymous term: the "Priscoan period". Other older texts simply refer to the eon as the Pre-Archean.
Since few geological traces of this period remain on Earth there are no official subdivisions. However, several major divisions of the
lunar geologic timescaleoccurred during the Hadean, and so these are sometimes used unofficially to refer to the same periods of time on Earth.
In the last decades of the 20th century geologists identified a few Hadean rocks from Western
Greenland, Northwestern Canadaand 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 volcanic dike that penetrated the rocks after they were deposited. Individual zircon crystals redeposited in sediments in Western Canadaand the Jack Hillsregion of Western Australiaare much older. The oldest dated zircons date from about 4400 Ma [ [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] - very close to the hypothesized time of the Earth's formation.
The Greenland sediments include banded iron beds. They contain possibly organic
carbonand imply some possibility that photosynthetic lifehad already emerged at that time. The oldest known fossils (from Australia) date from a few hundred million years later.
late heavy bombardmenthappened during Hadean times and affected the Earthand the Moon.
Atmosphere and oceans
A sizeable quantity of
waterwould have been in the material which formed the Earth. [ [http://www.ingentaconnect.com/content/arizona/maps/2005/00000040/00000004/art00003;jsessionid=7ibpocfkopqql.alice IngentaConnect Origin of water in the terrestrial planets ] ] 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. [ [http://history.nasa.gov/SP-345/ch26.htm chapter 26 ] ] Hydrogen and helium are expected to continually leak from the atmosphere, but the lack of denser noble gases 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 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. [ [http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=446 Solar System Exploration: Science & Technology: Science Features: View Feature ] ] 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 dioxideatmosphere with hydrogenand water vapor. Liquid water oceans existed despite the surface temperature of 230°C because of the atmospheric pressure of the heavy CO2 atmosphere. As cooling continued, subduction and dissolving in ocean water removed most CO2 from the atmosphere but levels oscillated wildly as new surface and mantle cycles appeared. [ [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 ] ]
zircons has found that liquid water must have existed as long ago as 4400 Ma, very soon after the formation of the Earth. [ [http://wwwrses.anu.edu.au/admin/index.php?p=harrison ANU - Research School of Earth Sciences - ANU College of Science - Harrison ] ] [ [http://info.anu.edu.au/mac/Media/Media_Releases/_2005/_November/_181105harrisoncontinents.asp ANU - OVC - MEDIA - MEDIA RELEASES - 2005 - NOVEMBER - 181105HARRISONCONTINENTS ] ] [ [http://www.geology.wisc.edu/%7Evalley/zircons/cool_early/cool_early_home.html A Cool Early Earth ] ] This requires the presence of an atmosphere.
Timetable of the Precambrian
History of Earth- The first sections describe the formation of the earth
Formation and evolution of the Solar System
* 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] – "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
* [http://www.peripatus.gen.nz/paleontology/Hadean.html Description of the Hadean Era]
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Look at other dictionaries:
Hadean — chthonian, *infernal, Tartarean, stygian, hellish … New Dictionary of Synonyms
Hadean — /heɪˈdiən/ (say hay deeuhn), /ˈheɪdiən/ (say haydeeuhn) adjective 1. of or relating to Hades. 2. relating to the geological era stretching from the time of the earliest accretion of planetary material to form the earth and the formation of the… … Australian English dictionary
Hadean — I noun the earliest eon in the history of the Earth from the first accretion of planetary material (around 4,600 million years ago) until the date of the oldest known rocks (about 3,800 million years ago); no evidence of life • Syn: ↑Hadean time … Useful english dictionary
Hadean — adjective Etymology: Hades Date: 1984 of, relating to, or being the aeon of history between the formation of the solar system and the formation of the first rocks on the earth see geologic time table • Hadean noun … New Collegiate Dictionary
Hadean — 1. adjective Of, or relating to the geologic eon from about 4,600 to 3,800 million years ago; marked by the formation of the solar system, a stable Earth Moon orbit and the first rocks. 2. noun The Hadean eon … Wiktionary
hadean — an eon of geologic time lasting from 4600m to 3800m years ago … Geography glossary
hadean — ha·de·an … English syllables
Hadean aeon — noun the earliest eon in the history of the Earth from the first accretion of planetary material (around 4,600 million years ago) until the date of the oldest known rocks (about 3,800 million years ago); no evidence of life • Syn: ↑Hadean,… … Useful english dictionary
Hadean eon — noun the earliest eon in the history of the Earth from the first accretion of planetary material (around 4,600 million years ago) until the date of the oldest known rocks (about 3,800 million years ago); no evidence of life • Syn: ↑Hadean,… … Useful english dictionary
Hadean time — noun the earliest eon in the history of the Earth from the first accretion of planetary material (around 4,600 million years ago) until the date of the oldest known rocks (about 3,800 million years ago); no evidence of life • Syn: ↑Hadean,… … Useful english dictionary