Origin of Earth's oldest crystals found

The oldest pieces of rock on Earth - zircon crystals - are likely to have formed in the craters left by violent asteroid impacts that peppered our planet, rather than via plate tectonics as was previously believed, scientists say.
Rocks that formed over the course of Earth's history allow geologists to infer things such as when water first appeared on the planet, how our climate has varied, and even where life came from, researchers said.
However, we can only go back in time so far, as the only material we have from the very early Earth comes in the form of tiny, naturally occurring zircon crystals, they said.

Ten years ago, a team of researchers in the US argued that the ancient zircon crystals probably formed when tectonic plates moving around on the Earth's surface collided with each other in a similar fashion to the disruption taking place in the Andes Mountains today, where the ocean floor under the Pacific Ocean is plunging under South America.
However, current evidence suggests that plate tectonics - as we know it today - was not occurring on the early Earth. So the question remained that where did the crystals come from.
Recently, geologists suggested these grains may have formed in huge impact craters produced as chunks of rock from space, up to several kilometres in diameter, slammed into a young Earth.

To test this idea, researchers from Trinity College Dublin in Ireland decided to study a much younger impact crater to see if zircon crystals similar to the very old ones could possibly have formed in these violent settings.

In 2014, the researchers and colleagues from Irish Research Council and Science Foundation Ireland collected thousands of zircons from the Sudbury impact crater in Canada - the best preserved large impact crater on Earth and the planet's second oldest confirmed crater at almost two billion years old.
After analysing the crystals, they discovered that the crystal compositions were indistinguishable from the ancient set.

"What we found was quite surprising. Many people thought the very ancient zircon crystals could not have formed in impact craters, but we now know they could have," said Gavin Kenny from Trinity College.
"There is a lot we still do not fully understand about these little guys but it looks like we may now be able to form a more coherent story of Earth's early years - one which fits with the idea that our planet suffered far more frequent bombardment from asteroids early on than it has in relatively recent times," said Kenny.
The findings were published in the journal Geology.