Overberg Geoscientists Group
4200 Mya
Oldest rocks on Earth
The oldest dated rocks formed on Earth, as an aggregate of minerals that have not been subsequently broken down by erosion or melted, are more than 4 billion years old, formed during the Hadean Eon of Earth's geological history. Meteorites that were formed in other planetary systems can pre-date Earth. Particles from the Murchison meteorite were dated in January 2020 to be 7 billion years old.
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Hadean rocks are exposed on Earth's surface in very few places, such as in the geologic shields of Canada, Australia, and Africa. The ages of these felsic rocks are generally between 2.5 and 3.8 billion years. The approximate ages have a margin of error of millions of years. In 1999, the oldest known rock on Earth was dated to 4.031 ±0.003 billion years, and is part of the Acasta Gneiss of the Slave craton in northwestern Canada.
Figure 1: A sample of gneiss from the site of the Earth's oldest dated rocks (the Acasta River area of Canada). This sample has been dated at 4.03 billion years old (M. Beauregard, 2008).
Figure 2: An outcrop of the oldest known rocks on Earth in the Acasta Gneiss. Located on the eastern shore of Canada’s Hudson Bay. Dating from 4,280Ma years ago. (Geology In). The structures (boudinage) in this outcrop result from deformation of previously banded rocks.
The Acasta Gneiss is a tonalite gneiss in the Slave craton in the 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 one of the oldest known intact crustal fragments on Earth.[1]
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 formation in the world.
Older pieces of Earths crust are fragments of the mineral zircon (ZrSiO4). Zircon is common in the crust of Earth. It occurs as a common accessory mineral in igneous rocks (as primary crystallization products), in metamorphic rocks (such as the Acasta Gneiss) and as detrital grains in sedimentary rocks. The oldest zircon found to date is from the Jack Hills conglomerate in Western Australia (Geology In). These zircon crystals are over 4,400Ma years old and would have been eroded from older igneous rocks that are no longer exposed, having been eroded or covered by younger rocks.
Further reading:
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Wikipedia: Oldest Dated Rocks. https://en.wikipedia.org/wiki/Oldest_dated_rocks#:~:text=In%201999%2C%20the%20oldest%20known,Slave%20craton%20in%20northwestern%20Canada.
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Geology In: Oldest Dated Rocks Found. https://www.geologyin.com/2023/04/oldest-rocks-on-earth-found.html
John Blaine, August 2023