Acasta Gneiss

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The Acasta Gneiss is a rock outcrop of Hadean tonalite gneiss in the Slave craton in Northwest Territories, Canada. Located on an island about 300 kilometres north of Yellowknife, the Acasta River rock deposit, believed to be 4.031 to 3.58 billion years old, is the oldest known intact crustal fragment on Earth.

Found 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 Tlicho people land settlement. It is the oldest known exposed rock in the world.


The rock exposed in the outcrop is derived from a 4.2 billion year old granitiod; an age based on radiometric dating of zircon crystals at 4.2 Ga . 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.

Contention for record

A McGill University team has reported a 4.28 billion year old outcrop in the Nuvvuagittuq greenstone belt on the eastern shores of Hudson Bay, 40 kilometres south of Inukjuak, Quebec, Canada. However, the dating method used did not involve similar......
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