Theoldest dated rocks formed onEarth, as anaggregate ofminerals that have not been subsequently broken down byerosion or melted, are more than 4 billion years old, formed during theHadeanEon ofEarth's geological history, and mark the start of theArchean Eon, which is defined to start with the formation of the oldest intact rocks on Earth.
Archeanrocks are exposed on Earth's surface in very few places, such as in thegeologic shields ofCanada,Australia, andAfrica. The ages of thesefelsic 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 theAcasta Gneiss of theSlave Craton in northwestern Canada.[1] Researchers atMcGill University found a rock with a very oldmodel age for extraction from themantle (3.8 to 4.28 billion years ago) in theNuvvuagittuq greenstone belt on the coast ofHudson Bay, in northernQuebec;[2] the true age of these samples is still under debate, and they may actually be closer to 3.8 billion years old.[3] Older than these rocks are crystals of the mineralzircon, which can survive the disaggregation of their parent rock and be found and dated in younger rock formations.
In January 2019,NASA scientists reported the discovery of the oldest known Earth rock, found on theMoon.Apollo 14 astronauts returned several rocks from the Moon and, later, scientists determined that a fragment from a rock nicknamedBig Bertha, which had been chosen by astronautAlan Shepard, contained "a bit of Earth from about 4 billion years ago". The rock fragment containedquartz,feldspar, and zircon, all common on Earth, but highly uncommon on the Moon.[4]Pre-solar grains inmeteorites are older than theSolar System, with some grains extracted from theMurchison meteorite claimed to be 7 billion years old.[5][6][7]
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The oldest material of terrestrial origin that has beendated is azircon mineral of 4.404 ±0.008Ga enclosed in ametamorphosedsandstoneconglomerate in theJack Hills of theNarryer Gneiss terrane ofWestern Australia.[8] The 4.404 ±0.008 Ga zircon is a slight outlier, with the oldest consistently dated zircon falling closer to 4.35 Ga.[8] This zircon is part of a population of zircons within the metamorphosed conglomerate, which is believed to have been deposited about 3.060 Ga, which is the age of the youngest detrital zircon in the rock. Recent developments inatom-probe tomography have led to a further constraint on the age of the oldest continental zircon, with the most recent age quoted as 4.374 ±0.006 Ga.[9]
The discovery of the oldest known Earth rock, found on the Moon, was reported in January 2019 byNASA scientists.Apollo 14 astronauts returned several rocks from the Moon and, later, scientists determined that a fragment from one of the rocks, nicknamedBig Bertha, contained "a bit of Earth from about 4 billion years ago". The rock fragment containedquartz,feldspar, andzircon, all common on Earth, but highly uncommon on the Moon.[4]
The oldest outcropping rock formation is, depending on the latest research, either part of theIsua Greenstone Belt,Narryer Gneiss terrane,Nuvvuagittuq Greenstone Belt,Napier Complex, or theAcasta Gneiss (on theSlave Craton). The difficulty in assigning the title to one particular block of gneiss is that thegneisses are all extremely deformed, and the oldest rock may be represented by only one streak of minerals in amylonite, representing a layer of sediment or an olddike. This may be difficult to find or map; hence, the oldest dates yet resolved are as much generated by luck in sampling as by understanding the rocks themselves.
It is thus premature to claim that any of these rocks, or indeed that of other formations of Hadean gneisses, is the oldest formations or rocks on Earth; doubtless, new analyses will continue to change conceptions of the structure and nature of these ancient continental fragments.
Nevertheless, the oldestcratons on Earth include theKaapvaal craton, the Western Gneiss terrane of theYilgarn craton (~2.9 – >3.2Ga), thePilbara Craton (~3.4 Ga), and portions of the Canadian Shield (~2.4 – >3.6 Ga). Parts ofDharwar Craton in India are greater than 3.0 Ga. The oldest dated rocks of theBaltic Shield are 3.5 Ga old.[10]
Other old formations include the Saglek Gneiss Complex, dated at 3.8-3.9 Ga; the Anshan Area, dated at 3.8 Ga; theItsaq (Isua) Gneiss Complex, dated at 3.7-3.8 Ga; and the Ancient Gneiss Complex, dated at 3.6 Ga.
TheAcasta Gneiss in the Canadian Shield in theNorthwest Territories, Canada is composed of theArchaeanigneous andgneissic cores of ancient mountain chains that have been exposed in aglacialpeneplain. Analyses of zircons from a felsic orthogneiss with presumed granitic protolith returned an age of 4.031 ±0.003 Ga.[1]
On September 25, 2008, researchers fromMcGill University,Carnegie Institution for Science andUQAM announced that a rock formation, theNuvvuagittuq greenstone belt, exposed on the eastern shore ofHudson Bay in northernQuebec had a Sm–Nd model age for extraction from the mantle of 4.28 billion years.[11][12][13][14] However, it is argued that the actual age of formation of this rock, as opposed to the extraction of its magma from the mantle, is likely closer to 3.8 billion years, according to Simon Wilde of the Institute for Geoscience Research in Australia.[3]
The zircons from the Western AustralianJack Hills returned an age of 4.404 billion years, interpreted to be the age of crystallization. These zircons also show another feature; their oxygen isotopic composition has been interpreted to indicate that more than 4.4 billion years ago there was already water on the surface of Earth. The importance and accuracy of these interpretations is currently the subject of scientific debate. It may be that the oxygen isotopes and other compositional features (therare-earth elements) record more recenthydrothermal alteration of the zircons rather than the composition of the magma at the time of their original crystallization.[15] In a paper published in the journalEarth and Planetary Science Letters, a team of scientists suggest that rocky continents and liquid water existed at least 4.3 billion years ago and were subjected to heavy weathering by an acrid climate. Using anion microprobe to analyze isotope ratios of the element lithium in zircons from the Jack Hills in Western Australia, and comparing these chemical fingerprints to lithium compositions in zircons from continental crust and primitive rocks similar to Earth's mantle, they found evidence that the young planet already had the beginnings of continents, relatively cool temperatures and liquid water by the time the Australian zircons formed.[16]
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One of the oldestMartian meteorites found on Earth,Allan Hills 84001 has been measured to have crystallized from molten rock 4.091 billion years ago.[17]
TheGenesis Rock (Lunar sample 15415), obtained from theMoon by astronauts duringApollo 15 mission, has been dated at 4.08 billion years.[18] DuringApollo 16, older rocks, including Lunar sample 67215, dated at 4.46 billion years, were brought back.[19]
Some types of meteorite are older than the Earth, having formed in theearly Solar System, before theplanet formation process was completed. The meteorite Northwest Africa 11119 (NWA 11119) has been dated to 4.5648 ± 0.0003 billion years.[20]
Some solid inclusions within meteorites are older than the surrounding rock.Calcium-aluminium rich inclusions (CAIs) in meteorites are the oldest solids that formed in theSolar System, so are conventionally used to set its formation date as 4567.30 ± 0.16 Myr.[21][22]Pre-solar grains are even older; they formed in theinterstellar medium and pre-date the formation of the Solar System. Some pre-solar grains extracted from theMurchison meteorite have been claimed to be 7 billion years old.[6][7]
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