Acasta Gneiss

Acasta Gneiss

The Acasta Gneiss is a rock outcrop of Archaean tonalite gneiss in the Slave craton in Northwest Territories, Canada. It was found in 1989 and was named for the nearby Acasta River east of Great Slave Lake, some 350 km north of Yellowknife. cite web
url=http://www.canada.com/montrealgazette/news/story.html?id=0fd4386e-ebb1-4f32-bcf3-5cb6768cfbef
title=World's oldest rocks found in Quebec
publisher= The Gazette
date=Septembre 25th, 2008
accessdate=2008-09-25] cite journal
url=http://www.sciencemag.org/cgi/content/short/321/5897/1828
title=Neodymium-142 Evidence for Hadean Mafic Crust
journal= Science Magazine
author= Jonathan O'Neil
coauthors=Richard W. Carlson, Don Francis, Ross K. Stevenson
date=Septembre 26th, 2008
publisher=HighWire Press
issue=5897
volume=321
pages=1828 - 1831
doi=10.1126/science.1161925
accessdate=2008-09-26] The Acasta outcrop is found in a remote area of the Tlicho people land settlement. It is the second oldest exposed rock in the world.

Geology

The rock exposed in the outcrop formed just over four billion (4 x 109) years ago; an age based on radiometric dating of zircon crystals at 4.03 Ga, [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.] which were the oldest rocks in the world at that time. It was the oldest known rock outcrop in the world until a McGill University team reported a 4.28 billion year old outcrop on the eastern shores of Hudson Bay, 40 kilometres south of Inukjuak, Quebec, Canada.

The Acasata gneiss is important in establishing the early history of the continental crust. It was formed in the Basin Groups unofficial period of the Hadean eon, which came before the Archean: see Timetable of the Precambrian.

Exhibit

In 2003 a team from the Smithsonian Institution collected a four-tonne boulder of Acasta Gneiss for display outside the National Museum of the American Indian in Washington, D.C.

ee also

* Age of the Earth

References

*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

External links

* [http://homepage.mac.com/yuee/H/research3.html Yuichiro Ueno research homepage]


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