According to a new study, the oldest rock formation known on Earth, dated at 4.02 billion-years-old, may have been created after our planet was bombarded with meteorites. These rocks reside in Canada’s Acasta River and scientists believe that they were first formed at an extremely shallow depth and also at very high temperatures.
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 almost certainly would have been meteorites involved. The meteorite impacts are believed to have occurred around half a billion years after the creation of Earth, as Phys.org report.
These silica abundant felsic rocks in the Acasta River are markedly different from other felsic rocks on Earth, including common granite. In their new study, scientists from China and Australia created a unique model of these rocks and learned through careful research that the only possible way they could have been created is if they were forged at very low pressures, along with extremely high temperatures.
Scientists believe that at the very early stages of the Earth, its crust was probably made out of mafic rocks that did not contain much silica in them, which has led scientists to speculate for a long time how these felsic rocks were created.
— Phys.org (@physorg_com) August 13, 2018
Tim Johnson, who headed up the recent study and is from Curtin University in Perth, explained that meteorites are one of the only things that could have been capable of producing the oldest rocks on Earth, especially given the fact that these particular ones would have needed temperatures of 900 degrees Celsius to form, a heat so intense that meteorites almost certainly were involved.
“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. We used phase equilibria and trace element modelling to show that the Acasta River rocks were produced by partial melting of the original mafic rocks at very low pressures. It would have needed something special to produce the 900°C temperatures needed to generate these early felsic rocks at such low pressures, and that probably means a drastic event, most likely the intense heating caused by meteorite bombardment.”
Due to the intense heat of the meteorite strikes, Johnson estimates that the upper three kilometers of the Earth’s crust would have been completely obliterated and melted into the oldest rock formation in the world. 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.
“We believe that these rocks may be the only surviving remnants of a barrage of extraterrestial impacts which characterized the first 600 million years of Earth History.”
The new study that has determined the oldest known rocks on Earth were most likely formed by meteorites has been published in Nature Geoscience.