Acasta Gneiss
Encyclopedia
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.
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.
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 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.
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.
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
outcrop
Outcrop
An outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. -Features:Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial deposits are covered by a mantle of soil and vegetation and cannot be...
of Hadean
Hadean
The Hadean is the geologic eon before the Archean. It started with the formation of the Earth about 4.7 Ga and ended roughly 3.8 Ga, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld", referring to the "hellish"...
tonalite
Tonalite
Tonalite is an igneous, plutonic rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.In older references tonalite is...
gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
in the Slave craton
Slave craton
The Slave craton is a Canadian geological formation located in the Northwest Territories and Nunavut. This craton is approximately in size and forms part of the Canadian Shield. It is dominated by ca. 2.73-2.63 Ga greenstones and turbidite sequences and ca. 2.72-2.58 Ga plutonic rock, with large...
in Northwest Territories
Northwest Territories
The Northwest Territories is a federal territory of Canada.Located in northern Canada, the territory borders Canada's two other territories, Yukon to the west and Nunavut to the east, and three provinces: British Columbia to the southwest, and Alberta and Saskatchewan to the south...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. 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
Great Bear Lake
Great Bear Lake is the largest lake entirely within Canada , the third or fourth largest in North America, and the seventh or eighth largest in the world...
. The Acasta outcrop is found in a remote area of the Tlicho people land settlement. It is the oldest known exposed rock
Oldest rock
The oldest dated rocks on Earth, as an aggregate of minerals that have not been subsequently melted or disaggregated by erosion, are from the Archean Eon. Such rocks are exposed on the surface in very few places....
in the world.
Geology
The rock exposed in the outcrop is derived from a 4.2 billion year old granitiod; an age based on radiometric datingRadiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
of zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
crystals at 4.2 Ga . The Acasta Gneiss is important in establishing the early history of the continental crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
. It was formed in the Basin Groups
Basin Groups
Basin Groups refers to 9 informal subdivisions of the lunar Pre-Nectarian geologic period.-Definition:The motivation for creating the Basin Groups subdivisions was to place 30 pre-Nectarian impact basins into 9 relative age groups...
unofficial period of the Hadean
Hadean
The Hadean is the geologic eon before the Archean. It started with the formation of the Earth about 4.7 Ga and ended roughly 3.8 Ga, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld", referring to the "hellish"...
eon, which came before the Archean: see Timetable of the Precambrian.
Contention for record
A McGill UniversityMcGill University
Mohammed Fathy is a public research university located in Montreal, Quebec, Canada. The university bears the name of James McGill, a prominent Montreal merchant from Glasgow, Scotland, whose bequest formed the beginning of the university...
team has reported a 4.28 billion year old outcrop in the Nuvvuagittuq greenstone belt
Nuvvuagittuq greenstone belt
Nuvvuagittuq greenstone belt, also called the Porpoise Cove greenstone belt, is a greenstone belt on the eastern shore of Hudson Bay in northern Quebec, Canada. With an age of about 4.28 billion years, it is the only portion of the Earth's crust known to have formed during the Hadean...
on the eastern shores of Hudson Bay
Hudson Bay
Hudson Bay , sometimes called Hudson's Bay, is a large body of saltwater in northeastern Canada. It drains a very large area, about , that includes parts of Ontario, Quebec, Saskatchewan, Alberta, most of Manitoba, southeastern Nunavut, as well as parts of North Dakota, South Dakota, Minnesota,...
, 40 kilometres south of Inukjuak
Inukjuak, Quebec
Inukjuak , alternatively spelled Inoucdjouac, former name and current postal name Port Harrison, is an Inuit settlement located on Hudson Bay at the mouth of the Innuksuak River in the Nunavik region of northern Quebec, Canada. Its population is 1,294...
, Quebec
Quebec
Quebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. However, the dating method used did not involve similar radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
of zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
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 InstitutionSmithsonian Institution
The Smithsonian Institution is an educational and research institute and associated museum complex, administered and funded by the government of the United States and by funds from its endowment, contributions, and profits from its retail operations, concessions, licensing activities, and magazines...
collected a four-tonne boulder of Acasta Gneiss for display outside the National Museum of the American Indian
National Museum of the American Indian
The National Museum of the American Indian is a museum operated under the auspices of the Smithsonian Institution that is dedicated to the life, languages, literature, history, and arts of the native Americans of the Western Hemisphere...
in Washington, D.C.
There is also a sample of it exposed in the Museu de Geociências da Universidade de Brasília, Brazil.