Taking the pulse of the ancient world
Geologists find a region that pre-dates plate tectonics. Andrew Masterson reports.
A region of extreme geological stability in a remote part of Australia has been found to contain rocks so ancient they formed before plate tectonics developed.
Scientists led by geologist Daniel Wiemer of Australia’s Queensland University of Technology used a combination of thermodynamic modelling and uranium-lead dating to conclude that the rocks comprising the Pilbara craton – in the vast interior of the continent’s northwest – formed between 3.6 and 3.4 billion years ago.
Plate tectonics – which describes the movement of huge rocky plates that move slowly across an inner mantle – is thought to have begun 3.2 billion years ago.
“There is still a school of thought that the early Earth crust was formed by something similar to the plate tectonics that we observe today, where the plates are moving relative to each other and in places like the Pacific Ring of Fire you have one plate descending under another,” says co-author David Murphy.
“Our data suggest that this wasn’t the case in early Earth history and there was something very different happening to the way fragments of crust stabilised to form continents.”
And that different process, say Murphy and his colleagues, comprised several “gravitational overturn events”.
At the time the Pilbara rocks were forming, the young Earth’s surface was estimated to be at its hottest and its crust very unstable. This resulted in heavier elements migrating slowly down from the surface and being absorbed into a 43-kilometre magma layer beneath. At the same time, lighter elements were pushed upwards.
A single gravitational overturn event, the scientists estimate, took about 100 million years to complete. The Pilbara craton – the term denotes simply an area not subject to seismic activity – underwent three successive overturn cycles before coming to rest at least 200 million years before tectonics began. The result was the distinctive granite rocks that typify the region.
In a paper published in the journal Nature Geoscience, the researchers point to similar findings arising from research into cratons in India and Africa. This, they suggest, might indicate that the early Earth was subject to regular “pulses”.