Acasta Gneiss

The Acasta Gneiss is a rock outcrop of Archaean tonalite gneiss in the Slave craton in Northwest Territories, Canada. It is the oldest known crustal rock outcrop in the world, and so it is important in establishing the history of the continental crust. The Acasta Gneiss is named for the nearby Acasta River east of Great Slave Lake some 350 km north of Yellowknife. 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 (4.03 Ga orthogneisses (Bowring & Williams, 1999), which are the oldest rocks in the world so far).

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

The Acasta outcrop is found in a remote area of the Tlicho land settlement.

According to some timings of the geological periods, it was formed in the Basin Groups unofficial period of the Hadean eon, which came before the Archean: see Timetable of the Precambrian.

See also

• Archean
• Hadean
• Zircon
• Oldest rock
• Slave craton
• Age of the Earth

References

• 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