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Asteroid impacts cause crustal crisis: study
Asteroids bombarding the Earth triggered major earthquakes that disrupted the evolution of our planet's crust, say Australian researchers.

The research by Dr Andrew Glikson and John Vickers from the Australian National University in Canberra appears in a recent issue of the journal Australian Journal of Earth Sciences.

The researchers say clusters of asteroids tens of kilometres in diameter slammed into the Earth around between 3.47 and 1.85 billion years ago.

An asteroid 20 to 50 kilometres in diameter could blast an impact basin hundreds of kilometres wide and several tens of kilometres deep, piercing into the Earth's mantle, the hot, partly-melted material that underlies the crust, they write.

The sudden pressure would cause the mantle to rebound and create massive volcanic activity, possibly rearranging mantle convection systems, which move the plates of the Earth's crust, the researchers say.

"Asteroids of this size cause more than just a crater," says Glikson. "They can result in volcanic activity, seismic activity triggering earthquakes and faulting, and major tsunamis."

A series of at least three massive asteroids hit the Pilbara region in Western Australia and the Barberton greenstone belt in South Africa between 3.26 and 3.24 billion years ago, says Glikson.
A very big bang

The impact shifted convection cell patterns, partially melting the crust and generating granitic magmas which rose into upper levels of the crust. They claim seismically triggered faulting and magmatic activity are represented by unconformities - major breaks in the sequences of rocks.

According to Glikson, evidence of the tectonic upheaval coincides with deposits of millimetre-sized spherules of glass called microkrystites. These spherules formed from the vapourised material shot up by the original asteroid impacts.

"When an asteroid or comet explodes a lot of material melts and is scattered on the crater floor, but some is vapourised and dispersed as a vapour plume," he says.

These spherules can be differentiated by other volcanic glass because they contain impact features and high levels of platinum group elements, such as iridium.

Iridium-rich microkrystites deposited in a layer around the globe led to geologists to identify the K-T boundary, which marks the demise of the dinosaurs thought to be caused by an asteroid impact 65 million years ago.
'Not at all proven'

But geoscientist Dr Martin Van Kranendonk from the Geological Survey of Western Australia in Perth says the theory is "not at all proven".

For instance, he says the impact that marks the K-T boundary had "no effect on tectonics" whatsoever.

Another area of contention is a deposit of broken-up rocks in the Pilbara called breccia, which Glikson associates with impact-induced seismic activity. Van Kranendonk believes this formed in a volcanic caldera, a circular depression that forms after a volcanic eruption.

"It's nice to have ideas and maybe there is something behind this. But there is not enough evidence [to support the impact theory] at this stage."

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