Earth`s oldest known evolved rocks result of collision of asteroids

Artist’s concept of the period in Earth’s early history about 4 billion years ago when meteorites bombarded our world. Image via Paleoblog

Artist’s concept of the period in Earth’s early history about 4 billion years ago when meteorites bombarded our world. Image via Paleoblog

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.

The team of researchers studied a type of rock called Idiwhaa gneiss, which is more than 4 billion years old, found in northwest Canada and the oldest large swath of rock on Earth. The planet doesn't really have an atmosphere to shield it from meteorite impacts, and the planetary surface is still in its nascent days. This is one of the oldest and most stable tectonic structures on our planet, which makes it an ideal place to search for rocks from that period.

Scientists meeting in Boston this week at the Goldschmidt conference reported on 4.02-billion-year-old rocks from Acasta River, Canada, site of the world's oldest exposed rocks.

The felsic rocks (rocks rich in silica/quartz) found at the Acasta River in Canada, are the Earth's oldest rocks, although there are older mineral crystals**. The term "felsic" combines the words "feldspar" and "silica", while the word "mafic" combines "magnesium" and "ferric". Researchers speculate that the rocks could have formed at extremely high temperatures at a shallow depth.

Scientists think rocks from space may be responsible for the very oldest rocks on Earth.

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"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 Tim Johnson, co-author of the paper and a senior lecturer in applied geology at Curtin University, Australia.

Using phase equilibria and trace element modelling scientists showed that the Acasta rocks were formed by partially melting mafic rocks at low pressure. This process happened at very low pressures, indicating that they were close to the surface.

"Given the predicted high flux of meteorites in the late Hadean, impact melting may have been the predominant mechanism that generated Hadean felsic rocks". If this was to be done, researchers note that about 900°C temperature was needed this points to "a drastic event", they say.

Researchers estimated that rocks buried in the top three kilometres of the mafic crust would have melted while producing the rocks seen today. "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", Johnson continues.

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.

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