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 3.58 to 4.031 billion years old, is the oldest known intact crustal fragment on Earth (but see also Nuvvuagittuq greenstone belt, which may be older).
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 granitoid; 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
In 2008, a McGill University team 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 radiometric dating of zircon crystals and it remains somewhat contentious whether 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. Mafic rocks from the Nuvvuagittuq greenstone belt have recorded isotopic compositions that can only be produced in the Hadean (i.e. older than 4 billion years ago) and the complete isotopic study of all the lithologies included in the Nuvvuagittuq greenstone belt suggests that it was formed nearly 4.4 billion years ago 
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 Geociências da Universidade de Brasília, Brazil.
- CBC News: Ancient N.W.T. rock chunks for sale
- "World's oldest rocks found in Quebec". The Gazette. September 25, 2008. Archived from the original on 26 September 2008. Retrieved 2008-09-25.
- Jonathan O'Neil; Richard W. Carlson; Don Francis; Ross K. Stevenson (September 26, 2008). "Neodymium-142 Evidence for Hadean Mafic Crust". Science Magazine (HighWire Press) 321 (5897): 1828–1831. Bibcode:2008Sci...321.1828O. doi:10.1126/science.1161925. PMID 18818357. Archived from the original on 4 October 2008. Retrieved 2008-09-26.
- Tsuyoshi Iizuka at al . Precambrian Research 153 (2007) 179 - 208 Geology and Zircon Geochronolgy of the Acasta Gneiss Complex.
- O'Neil at al . Precambrian Research 220-221 (2012) 23 - 44 Formation age and metamorphic history of the Nuvvuagittuq Greenstone Belt.
- 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