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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 age of these felsic rocks are generally between 2.5 and 3.8 billion years old.
Additional recommended knowledge
Oldest Terrestrial Material
The oldest material of terrestrial origin that has been dated is a zircon mineral of 4,404 +/- 8 Ma from a sedimentary gneiss in the Jack Hills of the Narryer Gneiss Terrane of Australia.  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.
Oldest Formation on Earth
The oldest rock formation on Earth 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 dyke. This may be difficult to find or map, and 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 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.
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 4.03 Ga, which is the current oldest known terrestrial rock.
A potentially older rock was found in the Jack Hills metaconglomerate of Western Australia, but there is some controversy surrounding its actual age (see below). A zircon crystal dating to 4.4 Ga was found in the Jack Hills that may be the current oldest known terrestrial material.  
The zircons from 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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Oldest_rock". A list of authors is available in Wikipedia.|