Posts Tagged ‘jack hills’

sandstone, jack hills, western australia, zircon, bruce watson, mark harrison, oldest

These are the sandstone rocks in Jack Hills in Western Australia, in which zircon crystals found by geochemists Bruce Watson and Mark Harrison in 2005, were later dated as being 4.4 billion years old, the oldest material so far found on Earth.

There was a news item on the BBC website a few days ago that tiny crystals of zircon, a blue semi-precious stone, that had been found in a much younger sandstone in the Jack Hills in the Murchison river basin in Western Australia, were the oldest fragments of the earth’s crust. The age of the crystal, which was dated using the decay of trace uranium atoms within it, is 4.4 billion years, only 200 million years after the formation of the earth itself. This is in the very earliest part of the Pre-Cambrian era that makes up seven-eighths of geologic time. The significance of this discovery is not so much how old the rock was, but that it is evidence of the earth having had a solid crust much earlier than had been thought and consequently of having been able to host life very early in its history.

As for the oldest rocks in the world, that is rocks consisting of minerals that have not been subsequently melted or broken down by erosion – unlike the Jack Hills zircon – there are four contenders, depending on the latest research. The rocks are all gneisses (gneiss is pronounced ‘nice’), rocks formed, or metamorphosed, by the action of heat and pressure on earlier rocks. Gneisses are hard, folded, and characterised by darker and lighter coloured bands, and they are widely distributed around the world.

The four locations are in south-western Greenland; the Jack Hills area of Western Australia as above; and in two locations in Canada, the eastern shore of the Hudson Bay and the Canadian shield in the Northwest territory. These rocks have all been dated as being between 3.8 and 4.4 billion years old. The difficulty in assigning the title of oldest to one particular formation of gneiss is that they are all extremely deformed, hence claiming one site to have the oldest rocks may be as much a matter of luck in sampling as by an understanding of the rocks themselves.

geologic time scale, pre-cambrian, age of the earth, jack hills, zircon, chalk, dover, cretaceous

In this geologic time chart, the Pre-Cambrian period, because it covers seven-eighths of the age of the earth occupies most of the lower scale, with the remaining one-eighth expanded into the upper scale. For comparison purposes the Jack Hills zircon were formed in the dark-brown Hadean period at the very beginning of the Pre-cambrian era, and the chalk cliffs of Dover were formed in the light green Cretaceous peiod above, only about 100 million years ago.
Incidentally this is an American chart, and in the UK the two blue periods between the Permian period and the Devonian, are called the Carboniferous period, which is when coal was formed.

But where are the oldest rocks in Britain?

The oldest rocks, the Lewisian gneiss of the Pre-Cambrian era, date from at least 2.7 billion years old – close to two-thirds of the age of the planet – and can be found at the surface in the far north-west of mainland Scotland and on the Hebridean islands. This rock is thought to underlie much of the Britain Isles although boreholes have only penetrated the first few kilometres. The main outcrops of Lewisian gneiss are on the islands of the Outer Hebrides, including Lewis, from which the formation takes its name, but the oldest of these rocks, are on the mainland around Scourie and Laxford Bridge, small villages halfway between Ullapool and Durness.

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