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Acasta Gneiss
The Acasta Gneiss is a tonalite gneiss in the Slave craton in Northwest Territories, Canada. The rock body is exposed on an island about 300 kilometres north of Yellowknife. The rock of the outcrop was metamorphosed 3.58 to 4.031 billion years ago and is the oldest known intact crustal fragment on Earth.〔(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 )〕
First described in 1989, it was named for the nearby Acasta River east of Great Bear Lake. The Acasta outcrop is found in a remote area of the Tłı̨chǫ people land settlement. It is the oldest known exposed rock in the world.
== Formation ==
The metamorphic rock exposed in the outcrop was previously a granitoid that formed 4.2 billion years ago, an age based on radiometric dating of zircon crystals at 4.2 Ga.〔Tsuyoshi Iizuka, at al., ''Geology and Zircon Geochronolgy of the Acasta Gneiss Complex,'' Precambrian Research, 153 (2007) pp. 179 - 208〕 The Acasta Gneiss is important in establishing the early history of the continental crust. Acasta Gneiss was formed in the Basin Groups unofficial period of the Hadean eon, which came before the Archean: see Timetable of the Precambrian.
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