The Earth did not always have rocks on the surface. Yes, the planet is classified as a "rocky" one, but when land appeared out of the sea billions of years ago, there might not have been as many rocks as we see today.
A new study by researchers in Canada has found the oldest rock formations on the planet. The 4.02 billion-year-old silica-rich felsic rocks were discovered at the Acasta River. Researchers speculate that the rocks could have formed at extremely high temperatures at a shallow depth. As Earth's crust might have been young and really thin.
Actual temperatures needed to melt this shallow crust could have come from a meteorite bombardment, say the researchers. This event likely happened around half a billion years after the birth of Earth. The violent crashes melted the crust, which is rich in iron, and created granite as seen today.
Called felsic rocks — rich in silica and quartz — found at the Acasta River in Canada, notes the release are the oldest rocks. 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.
Scientists have known for a while now that Acasta rocks are different than most felsic rocks seen around the world — like the many granite rocks. A team of scientists from Australia and China have actually modelled exactly how the oldest felsic rocks were formed. As a result, they found that these rock formations could have only come to be if they were put through extremely high temperatures at low pressures.
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. "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.
Using phase equilibria and trace element modelling scientists showed that the Acasta rocks were formed by partially melting mafic rocks at low pressure. If this was to be done, researchers note that about 900°C temperature was needed this points to "a drastic event", they say.
The drastic event most likely came in the form of intense heat from a meteorite bombardment.
Researchers estimated that rocks buried in the top three kilometres of the mafic crust would have melted while producing the rocks seen today. Scientists say that most of these rocks have eroded over time, which explains their rarity.
"We believe that these rocks may be the only surviving remnants of a barrage of extraterrestrial impacts which characterized the first 600 million years of Earth History," said Johnson.
These results were presented for the first time at the Goldschmidt conference in Boston, followed by publication in journal Nature Geoscience.
