|
Fragment exhibited at the Natural History Museum in ViennaThe 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.[1] Found in 1989, it was named for the nearby Acasta River east of Great Bear Lake.[2][3] 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.
Geology The rock exposed in the outcrop is derived from a 4.2 billion year old granitoid; an age based on radiometric dating of zircon crystals at 4.2 Ga.[4] 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.[2] 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. This dating has now been refuted and is considered much younger. 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. There is also a sample of it exposed in the Museu de Geoci ncias da Universidade de Bras lia, Brazil. See also 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 ca:Gneis acasta cs:Acasta Gneiss da:Acasta-gnejskomplekset de:Acasta-Gneis nl:Acasta Gneiss
|
Encyclopedia
top
