Posts Tagged ‘pre-cambrian’

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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What living things on Earth have been in existence the longest? I don’t mean which individual animal or plant has lived the longest, such as species of coral or sponge that are known to have been living upwards of 2,000 years, or terrestrial animals such as tortoise that have lived for over 150 years. Nor plants such as the bristlecone pine from North America, one of which is 5,062 years old (measured by ring count), nor the Llangernyw Yew in the churchyard of the village of Llangernyw in North Wales, one of the oldest individuals tree in the world, and believed to be aged between 4,000 years and 5,000 years old. I mean what life forms have been in existence the longest and are still living today?

stromatolites, shark bay, western australia

Stromatolites in Shark Bay, Western Australia

The answer is Stromatolites (or stromatoliths). These are rare rock-like structures found in just a few hypersaline lakes and marine environments around the world, and which, from the fossil record, are known to have been in existence for some 3.5 billion years. They existed in abundance after the earth had been formed when there were no animals or plants.  Because they were prodigious photosynthesizers, their waste product, oxygen, entered the atmosphere in great quantities, making the earth suitable for other forms of life. Over time, organisms developed that grazed on stromatolites, and by the end of the Pre-Cambrian Period (about 570 million years ago), they numbered only 20% of their peak.

Stromatolites are created by the accumulation of photosynthesizing cyanobacteria (often incorrectly called blue-green algae). These bacteria are prokaryotic bacteria, and are neither algae nor plant. When they colonize to form a stromatolite, they number some 3 billion organisms per square metre. The bacteria secrets a mucus coating that traps sediment, and calcium carbonate precipitates from the water providing a hard, cement-like material to fuse the sediment together. New cyanobacteria grow over the sediment and over time a rock-like structure is formed.

fossil stromatolites, cross section, 1.8 billion year old, great slave lake, canada

Cross section of 1.8 billion year old fossil stromatolites from rock formations at Great Slave Lake, Canada

Scientists had long known about stromatolites from the fossil record, but were surprised to find them still in existence, when they were discovered in 1956 at Shark Bay, Western Australia (now the Hamelin Pool Marine Nature Reserve). Other places where stromatolites are found are Lagoa  Salgada, Rio Grande do Norte in Brazil and two inland sites in Mexico at Cuatro Cienegas and Lake Alchichica. Unexpectedly, there is one marine site that is not hypersaline, Exuma Cays in the Bahamas.

And according to the BBC, a very young colony of stromatolites, just a single layer thick, was found at the Giant’s Causeway in Northern Ireland in 2011.

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