Earths oldest known evolved rocks result of asteroids

Christopher Davidson
August 16, 2018

Scientists have long known that the Acasta rocks are different to the majority of felsic rocks we see today, such as the granites widely used as a building or decorative material.

The oldest evolved rocks on Earth are the outcome of asteroids colliding with the the planet 4 billion years ago, an Australian research released on Tuesday revealed. As Earth's crust might have been young and really thin.

Researchers started off with the belief that the planet's primitive crust was largely made of dark, mafic rocks that were low in silica, so there was a mystery surrounding how Acasta River felsic rocks formed. This melted the iron-rich crust and formed the granites we see today.

They came to the conclusion that their formation could contribute to the partial "melting" of the earth's surface at temperatures from 800 to 900 degrees Celsius at very low pressure.

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New research has concluded that in order to achieve the kind of high temperatures that would be needed to cause the Earth's shallow crust to melt in the process, there nearly certainly would have been meteorites involved.

And while there surely would have been many other similar felsic rocks like these ones, after four billion years and the shift of the continents with plate tectonics, these rocks in the Acasta River may be the only ones from this time period that are still left on Earth. The results showed that these rocks could form at a distance of 10-50 km from the impact and it also affected the mafic rocks at 3 km depth, making them melt and create the Idiwhaa gneisses. However, there are mineral crystals that are actually much older, like the Zircon crystals found at Jack Hills in Australia, which are up to 4.4 billion years old.

"Our modelling shows that the Acasta River rocks derived from the melting of pre-existing iron-rich basaltic rock, which formed the uppermost layers of crust on the primitive Earth", said lead researcher Tim Johnson, from Curtin University, Perth. We think that these ancient felsic rocks would have been very common, but the passage of 4 billion years, and the development of plate tectonics, means that nearly nothing remains. Researchers suspect there was a drastic event, most likely a meteorite bombardment, that caused this rise in temperature.

That's according to new research published today (Aug. 13), which argues that meteorite bombardment is the most likely way to explain the temperature and pressure conditions under which 4.02-billion-year-old Canadian rocks formed. These results are presented for the first time at the Goldschmidt conference in Boston tomorrow (14 August), following publication in the peer-reviewed journal Nature Geoscience*.

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