Earth:Hadean

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Short description: First eon of geological time, beginning with the formation of the Earth about 4.6 billion years ago
Hadean
~4600 – 4000 Ma
Hadean.png
Artist's impression of a Hadean landscape.
Chronology
Proposed subdivisionsSee text
Etymology
Synonym(s)Priscoan Period
Harland et al., 1989
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Definition
Chronological unitEon
Stratigraphic unitEonothem
First proposed byPreston Cloud, 1972
Time span formalityInformal
Lower boundary definitionFormation of the Earth
Lower boundary GSSPN/A
GSSP ratifiedN/A
Upper boundary definitionDefined Chronometrically
Upper boundary GSSPN/A
GSSP ratifiedN/A

The Hadean ( /ˈheɪdiən, heɪˈdiːən/ HAY-dee-ən, hay-DEE-ən) is a geologic eon of Earth history preceding the Archean. It began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the International Commission on Stratigraphy (ICS), 4 billion years ago.[1] (As of 2016), the ICS describes its status as "informal".[2] The term was coined after the Greek mythical underworld Hades, by American geologist Preston Cloud, originally to label the period before the earliest-known rocks on Earth.[3][4] W. Brian Harland later coined an almost synonymous term, the Priscoan Period, from priscus, the Latin word for 'ancient'.[5] Other, older texts refer to the eon as the Pre-Archean.[6][7]

Etymology

Backscatter electron micrograph of detrital zircons from the Hadean (4.404 ± 0.008 Ga) metasediments of the Jack Hills, Narryer Gneiss Terrane, Western Australia
Artist's impression of Earth and Moon towards the end of the Hadean, when the first water vapor clouds and oceans appeared on Earth

"Hadean" (from Hades, the Greek god of the underworld, and the underworld itself) describes the hellish conditions then prevailing on Earth: the planet had just formed and was still very hot owing to its recent accretion, the abundance of short-lived radioactive elements, and frequent collisions with other Solar System bodies.

Subdivisions

Since few geological traces of this eon remain on Earth, there is no official subdivision. However, the lunar geologic timescale embraces several major divisions relating to the Hadean, so these are sometimes used in an informal sense to refer to the same periods of time on Earth.

The lunar divisions are:

In 2010, an alternative scale was proposed that includes the addition of the Chaotian and Prenephelean Eons preceding the Hadean, and divides the Hadean into three eras with two periods each. The Paleohadean Era consists of the Hephaestean (Template:Gigaannum) and the Jacobian periods (Template:Gigaannum). The Mesohadean is divided into the Canadian (Template:Gigaannum) and the Procrustean periods (Template:Gigaannum). The Neohadean is divided into the Acastan (Template:Gigaannum) and the Promethean periods (Template:Gigaannum).[8] (As of May 2021), this has not been adopted by the IUGS.[9]

Hadean rocks

Further information: Physics:Oldest dated rocks

In the last decades of the 20th-century geologists identified a few Hadean rocks from western Greenland , northwestern Canada , and Western Australia. In 2015, traces of carbon minerals interpreted as "remains of biotic life" were found in 4.1-billion-year-old rocks in Western Australia.[10][11]

The oldest dated zircon crystals, enclosed in a metamorphosed sandstone conglomerate in the Jack Hills of the Narryer Gneiss Terrane of Western Australia, date to 4.404 ± 0.008 Ga.[12] This zircon is a slight outlier, with the oldest consistently-dated zircon falling closer to 4.35 Ga[12]—around 200 million years after the hypothesized time of the Earth's formation.

In many other areas, xenocryst (or relict) Hadean zircons enclosed in older rocks indicate that younger rocks have formed on older terranes and have incorporated some of the older material. One example occurs in the Guiana shield from the Iwokrama Formation of southern Guyana where zircon cores have been dated at 4.22 Ga.[13]

Atmosphere and oceans

A sizable quantity of water would have been in the material that formed the Earth.[14] Water molecules would have escaped Earth's gravity more easily when it was less massive during its formation. Hydrogen and helium are expected to continually escape (even to the present day) due to atmospheric escape.

Part of the ancient planet is theorized to have been disrupted by the impact that created the Moon, which should have caused melting of one or two large regions of the Earth. Earth's present composition suggests that there was not complete remelting as it is difficult to completely melt and mix huge rock masses.[15] 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 CO2 atmosphere with hydrogen and water vapor. Liquid water oceans existed despite the surface temperature of 230 °C (446 °F) because at an atmospheric pressure of above 27 atmospheres, caused by the heavy CO2 atmosphere, water is still liquid. 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.[16]

Studies of zircons have found that liquid water must have existed as long ago as 4.4 billion years ago, very soon after the formation of the Earth.[17] This requires the presence of an atmosphere. The cool early Earth theory covers a range from about 4.4 to about 4.1 billion years.

A September 2008 study of zircons found that Australian Hadean rock holds minerals pointing to the existence of plate tectonics as early as 4 billion years ago (approximately 600 million years after Earth's formation).[18][19] If this is true, the time when Earth finished its transition from having a hot, molten surface and atmosphere full of carbon dioxide, to being very much like it is today, can be roughly dated to about 4.0 billion years ago. The actions of plate tectonics and the oceans trapped vast amounts of carbon dioxide, thereby reducing the greenhouse effect and leading to a much cooler surface temperature and the formation of solid rock, and possibly even life.[18][19]

See also

References

  1. ↑ "International Chronostratigraphic Chart". International Commission on Stratigraphy. https://stratigraphy.org/chart. 
  2. ↑ Ogg, J. G.; Ogg, G.; Gradstein, F. M. (2016). A Concise Geologic Time Scale: 2016. Elsevier. p. 20. ISBN 978-0-444-63771-0. 
  3. ↑ Cloud, Preston (1972). "A working model of the primitive Earth". American Journal of Science 272 (6): 537–548. doi:10.2475/ajs.272.6.537. Bibcode: 1972AmJS..272..537C. 
  4. ↑ Bleeker, W. (2004). "10. Toward a "natural" Precambrian time scale". in Gradstein, Felix M.; Ogg, James G.; Smith, Alan G.. A Geologic Time Scale 2004. Cambridge, England, UK: Cambridge University Press. p. 145. ISBN 9780521786737. https://books.google.com/books?id=rse4v1P-f9kC&pg=PA145. 
  5. ↑ Oxford Dictionary, "Priscoan"
  6. ↑ Shaw, D.M. (1975). "Early History of the Earth". Proceedings of the NATO Advanced Study Institute (Leicester: John Wiley (London)): 33–53. 
  7. ↑ Jarvis, Gary T.; Campbell, Ian H. (December 1983). "Archean komatiites and geotherms: Solution to an apparent contradiction". Geophysical Research Letters 10 (12): 1133–1136. doi:10.1029/GL010i012p01133. Bibcode: 1983GeoRL..10.1133J. 
  8. ↑ "The eons of Chaos and Hades". Solid Earth. 26 January 2010. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100036717_2010036774.pdf. 
  9. ↑ "Chart". International Commission on Stratigraphy. May 2021. https://stratigraphy.org/chart. Retrieved 14 June 2021. 
  10. ↑ Borenstein, Seth (19 October 2015). "Hints of life on what was thought to be desolate early Earth". Excite. Associated Press (Yonkers, NY: Mindspark Interactive Network). http://apnews.excite.com/article/20151019/us-sci--earliest_life-a400435d0d.html. 
  11. ↑ Bell, Elizabeth A.; Boehnike, Patrick; Harrison, T. Mark et al. (19 October 2015). "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon". Proc. Natl. Acad. Sci. U.S.A. (Washington, D.C.: National Academy of Sciences) 112 (47): 14518–21. doi:10.1073/pnas.1517557112. ISSN 1091-6490. PMID 26483481. PMC 4664351. Bibcode: 2015PNAS..11214518B. http://www.pnas.org/content/early/2015/10/14/1517557112.full.pdf. Retrieved 2015-10-20. 
  12. ↑ 12.0 12.1 Wilde, Simon A.; Valley, John W.; Peck, William H.; Graham, Colin M. (2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago". Nature 409 (6817): 175–178. doi:10.1038/35051550. PMID 11196637. Bibcode: 2001Natur.409..175W. 
  13. ↑ Nadeau, Serge; Chen, Wei; Reece, Jimmy; Lachhman, Deokumar; Ault, Randy; Faraco, Maria; Fraga, Leda; Reis, Nelson et al. (2013-12-01). "Guyana: the Lost Hadean crust of South America?". Brazilian Journal of Geology 43 (4): 601–606. doi:10.5327/Z2317-48892013000400002. 
  14. ↑ Drake, Michael J. (2005), "Origin of water in the terrestrial planets", Meteoritics & Planetary Science 40 (4): 515–656, doi:10.1111/j.1945-5100.2005.tb00958.x, Bibcode: 2005M&PS...40..515J .
  15. ↑ Taylor, G. Jeffrey. "Origin of the Earth and Moon". NASA. http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=446. 
  16. ↑ Sleep, N. H.; Zahnle, K.; Neuhoff, P. S. (2001), "Initiation of clement surface conditions on the earliest Earth", PNAS 98 (7): 3666–3672, doi:10.1073/pnas.071045698, PMID 11259665, Bibcode: 2001PNAS...98.3666S .
  17. ↑ "Hell's milder side". Australian National University. http://wwwrses.anu.edu.au/admin/index.php?p=harrison. 
    "There was no such thing as hell on Earth". Australian National University. 18 November 2005. http://info.anu.edu.au/mac/Media/Media_Releases/_2005/_November/_181105harrisoncontinents.asp. 
    Valley, John W.; Peck, William H.; King, Elizabeth M.; Wilde, Simon A. (April 2002). "A Cool Early Earth". Geology 30 (4): 351–354. doi:10.1130/0091-7613(2002)030<0351:ACEE>2.0.CO;2. PMID 16196254. Bibcode: 2002Geo....30..351V. http://www.geology.wisc.edu/%7Evalley/zircons/cool_early/cool_early_home.html. 
  18. ↑ 18.0 18.1 Chang, Kenneth (December 2, 2008). "A New Picture of the Early Earth". The New York Times. https://www.nytimes.com/2008/12/02/science/02eart.html?_r=1. 
  19. ↑ 19.0 19.1 Abramov, Oleg; Mojzsis, Stephen J. (December 2008). "Thermal State of the Lithosphere During Late Heavy Bombardment: Implications for Early Life". AGU Fall Meeting Abstracts (Fall Meeting 2008: American Geophysical Union) 1 (2008 Fall Meeting): V11E–08. Bibcode: 2008AGUFM.V11E..08A. 

Further reading

External links



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