The oldest rock and 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 only exposed on the surface in very few places.^[1]

There is some controversy about the oldest rocks based on the oldest dated mineral zircon. Some of the oldest surface rock can be found in the Canadian Shield, Australia, Africa and in other more specific places around the world. 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 has been dated to 4.031 ± 0.003 billion years, and is part of the Acasta Gneiss of the Slave craton in northwestern Canada.^[2] Since 2008, the oldest rock on earth has been discovered by McGill University in the Nuvvuagittuq Belt on the coast of Hudson Bay, in northern Quebec, and is dated from 3.8 to 4.28 billion years old.^[3]

Contents 1 Oldest rocks by category 1.1 Oldest terrestrial material 1.2 Earth's oldest rock formation 1.3 Oldest rock on Earth 1.4 Recent research 2 See also 3 References 4 External links

[edit] Oldest rocks by category

[edit] Oldest terrestrial material

The oldest material of terrestrial origin that has been dated is a zircon mineral of 4,404 ± 8 Ma enclosed in a metamorphic gneiss in the Jack Hills of the Narryer Gneiss Terrane of Western Australia. The 4,404 ± 8 Ma zircon is a slight outlier, with the oldest consistently-dated zircon falling closer to 4.35 Ga.^[4] This zircon is part of a population of zircons within the gneiss of greater than 3,900 Ma; the gneiss is considered to be no older than 3,800 Ma, which is the age of the youngest zircon in the rock.

[edit] Earth's oldest rock formation

The oldest rock formation is, depending on the latest research, either part of the Isua Greenstone Belt, Narryer Gneiss Terrane or the Acasta Gneiss. The difficulty in assigning the title to one particular block of gneiss is that the gneisses are all extremely deformed, and the oldest rock may be represented by only one streak of minerals in a mylonite, representing a layer of sediment or an old dike. 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 other formations of early Archaean gneisses, are the oldest formations or rocks on Earth; doubtless new analyses will continue to change our conceptions of the structure and nature of these ancient continental fragments.

Nevertheless, the oldest cratons on Earth include the Kaapvaal craton, the Western Gneiss Terrane of the Yilgarn craton (~2.9 - >3.2 Ga), the Pilbara Craton (~3.4 Ga), and portions of the Canadian Shield (~2.4 - >3.6 Ga). Parts of the poorly studied Dharwar craton in India are greater than 3.0 Ga. The oldest dated rocks of Baltic Shield are 3.5 Ga old ^[5].

[edit] Oldest rock on Earth

The Acasta Gneiss in the Canadian Shield in the Northwest Territories, Canada is composed of the Archaean igneous and gneissic cores of ancient mountain chains that have been exposed in a glacial peneplain. Analyses of zircons from a felsic orthogneiss with presumed granitic protolith returned an age of 4.031 ± 0.003 Ga^[2].

On September 25, 2008, researchers from McGill University, Carnegie Institution for Science and UQAM announced that a rock formation, the Nuvvuagittuq greenstone belt, exposed on the eastern shore of Hudson Bay in northern Quebec had a Sm-Nd age for extraction from the mantle of 4.28 billion years.^[6][7][8][9] 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.^[10] But after O'Neil & al. continued their researches, they confirmed the date of the rocks by various other methods, which was to be the same as the chemical analysis.^{[citation needed]}

[edit] Recent research

The zircons from the Western Australian Jack Hills returned an age of 4.404 billion years, interpreted to be the age of crystallization. These zircons also show another interesting 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 the 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 (the rare earth elements), record more recent hydrothermal alteration of the zircons rather than the composition of the magma at the time of their original crystallization.^{[citation needed]}. In a paper published in the journal Earth 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 a ion 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 the 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.^[11]

[edit] See also

• Archean
• Hadean
• Zircon
• History of Earth

[edit] References

[edit] External links

• Very old Australian zircons with a story to tell
• On the Acasta Gneiss
• Yuichiro Ueno homepage
• Abstract and fullt ext of the results from O'Neil's research, published by Science