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.^[1][2] The Acasta outcrop is found in a remote area of the Tlicho people land settlement. It is the oldest known exposed rock in the world.

Contents 1 Geology 2 Contention for record 3 Exhibit 4 See also 5 References 6 External links

[edit] 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, ^[3] The Acasta 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.

[edit] Contention for record

A McGill University team has reported a 4.28 billion year old outcrop on the eastern shores of Hudson Bay, 40 kilometres south of Inukjuak, Quebec, Canada.^[1] However, the dating method used did not involve similar radiometric dating of zircon crystals and it remains somewhat contentious if the reported date represents the age that the rock itself formed or a residual isotopic signature of older material that melted to form the rock.

[edit] 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.

[edit] See also

• Age of the Earth

[edit] 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

[edit] External links

• Yuichiro Ueno research homepage

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Coordinates: 65°11′N 115°41′W / 65.183°N 115.683°W / 65.183; -115.683