Oldest rock

The oldest rock or rocks on Earth, as an aggregate of minerals that have not undergone subsequent metamorphosis, are from the Archean Eon. Such rocks are only exposed on the surface in very few places. [Some meteorites, such as the ALH84001 Mars meteorite found in Allan Hills Antarctica, are older but are not of terrestrial origin. ]

There is some controversy about the oldest rocks based on the oldest dated mineral zircon. Some of the oldest surface rock can be found in the Canadian Shield, Australia, Africa and in other more specific places around the world. The ages of these felsic rocks are generally between 2.5 and 3.8 billion years. The approximate ages have a margin of error of millions of years. On September 25, 2008, researchers announced that a rock formation, the Nuvvuagittuq greenstone belt, exposed on the eastern shore of Hudson Bay in northern Quebec, has the oldest known rocks on Earth, with an estimated age of 4.28 billion years. [http://www.mcgill.ca/newsroom/news/item/?item_id=102000 McGill University press release] ] [http://www.msnbc.msn.com/id/26890176/ Oldest rocks on Earth found] ]

Oldest rocks by category

Oldest terrestrial material

The oldest material of terrestrial origin that has been dated is a zircon mineral of 4,404 +/- 8 Ma enclosed in a metamorphic gneiss in the Jack Hills of the Narryer Gneiss Terrane of Western Australia. [Wilde SA, Valley JW, Peck WH and Graham CM (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. http://www.geology.wisc.edu/%7Evalley/zircons/Wilde2001Nature.pdf ] This zircon is part of a population of zircons within the gneiss of greater than 3,900 Ma; the gneiss is considered to be no older than 3,800 Ma, which is the age of the youngest zircon in the rock.

Earth's oldest rock formation

The oldest rock formation is, depending on the latest research, either part of the Isua Greenstone Belt, Narryer Gneiss Terrane or the Acasta Gneiss. The difficulty in assigning the title to one particular block of gneiss is that the gneisses are all extremely deformed, and the oldest rock may be represented by only one streak of minerals in a mylonite, representing a layer of sediment or an old dyke. This may be difficult to find or map; hence the oldest dates yet resolved are as much generated by luck in sampling as by understanding the rocks themselves.

It is thus premature to claim that any of these rocks, or indeed that other formations of early Archaean gneisses, are the oldest formations or rocks on Earth; doubtless new analyses will continue to change our conceptions of the structure and nature of these ancient continental fragments.

Nevertheless, the oldest cratons on Earth include the Kaapvaal craton, the Western Gneiss Terrane of the Yilgarn craton (~2.9 - >3.2 Ga), the Pilbara Craton (~3.4 Ga), and portions of the Canadian Shield (~2.4 - >3.6 Ga). Parts of the poorly studied Dharwar craton in India are greater than 3.0 Ga.

Oldest rock on Earth

The Acasta Gneiss in the Canadian Shield in the Northwest Territories, Canada is composed of the Archaean igneous and gneissic cores of ancient mountain chains that have been exposed in a glacial peneplain. Analyses of zircons from a felsic orthogneiss with presumed granitic protolith returned an age of 4.03 Ga,Fact|date=September 2008 which is the current oldest known terrestrial rock.

A potentially older rock was found in the Jack Hills metaconglomerate of Western Australia, but there is ongoing debate surrounding its actual age (see below). A zircon crystal dating to 4.4 Ga was found in the Jack Hills that may be the current oldest known terrestrial material. [ [http://www.news.wisc.edu/10849.html World's oldest rock, newest technology to be on display] ] [ [http://www.geology.wisc.edu/zircon/Earliest%20Piece/Earliest.html The Earliest Piece of the Earth] ]

On September 25, 2008, researchers from McGill University, Carnegie Institution for Science and UQAM announced that a rock formation, the Nuvvuagittuq greenstone belt, exposed on the eastern shore of Hudson Bay in northern Quebec has the oldest known rocks on Earth, with an estimated age of 4.28 billion years. [http://www.sciencemag.org/cgi/reprint/321/5897/1828.pdf] This claim has been challenged. [ [http://environment.newscientist.com/article/dn14818-discovery-of-worlds-oldest-rocks-challenged-.html Discovery of world's oldest rocks challenged] ]

Recent research

The zircons from Jack Hills returned an age of 4.404 billion years, interpreted to be the age of crystallization. These zircons also show another interesting feature; their oxygen isotopic composition has been interpreted to indicate that more than 4.4 billion years ago there was already water on the surface of the Earth. The importance and accuracy of these interpretations is currently the subject of scientific debate. It may be that the oxygen isotopes, and other compositional features (the rare earth elements), record more recent hydrothermal alteration of the zircons rather than the composition of the magma at the time of their original crystallization.Fact|date=November 2007. In a paper published in the journal "Earth and Planetary Science Letters", a team of scientists suggest that rocky continents and liquid water existed at least 4.3 billion years ago and were subjected to heavy weathering by an acrid climate. Using a ion microprobe to analyze isotope ratios of the element lithium in zircons from the Jack Hills in western Australia, and comparing these chemical fingerprints to lithium compositions in zircons from continental crust and primitive rocks similar to the Earth's mantle, they found evidence that the young planet already had the beginnings of continents, relatively cool temperatures and liquid water by the time the Australian zircons formed. [ [http://newswise.com/articles/view/541751/ Newswise: Ancient Mineral Shows Early Earth Climate Tough on Continents] Retrieved on June 15, 2008.]

See also

* Archean
* Hadean
* Zircon
* History of Earth

References

* [http://www.geology.wisc.edu/zircon/zircon_home.html Zircons are Forever]
*
* Bowring, S.A., and Williams, I.S., 1999. Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada. Contributions to Mineralogy and Petrology, v. 134, 3-16.
* Stern, R.A., Bleeker, W., 1998. Age of the world's oldest rocks refined using Canada's SHRIMP. the Acasta gneiss complex, Northwest Territories, Canada. Geoscience Canada, v. 25, p. 27-31
* Yu A., Lee C-D and Halliday, A.N.."Lutetium-Hafnium and Uranium-Lead Systematics of Early-Middle Archean Single Zircon Grains", Ninth Annual Goldschmidt Conference. [http://www.lpi.usra.edu/meetings/gold99/pdf/7202.pdf 2]

External links

* [http://www.mala.bc.ca/~earles/zircon-jan01.htm Very old Australian zircons with a story to tell]
* [http://www.nmnh.si.edu/paleo/geotime/main/hadean4.html On the Acasta Gneiss]
* [http://homepage.mac.com/yuee/H/research3.html Yuichiro Ueno homepage]