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.
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.
If you want to see these oldest of the oldest rocks in Britain you can do so quite easily, provided you can get to the far north of Scotland of course. When the A838 road to the north of Laxford Bridge was upgraded some years ago, several outcrops of the oldest gneiss were exposed in road cuttings. One outcrop, photographed to the right, is at grid reference NC 232 485.
The grey banded rock, called Badcallian gneisses, was formed even earlier, between 2.7 billion and 3 billion years ago. The name comes from the Badcallian metamorphic event (named after nearby Badcall Bay) that took place later, around 2.5 millions years ago.
Other unusual places of interest in Britain:
Most featureless place
Furthest point from a road
Lowest point on land