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Origin of Earth's oldest crystals revealed

29 Apr 2016, 17:09 GMT+10

London, April 29 (IANS) The very oldest pieces of rock on Earth -- zircon crystals -- are likely to have formed in the craters left by violent asteroid impacts that peppered our nascent planet rather than via plate tectonics as previously believed, researchers report.

The tiny crystals probably formed in huge impact craters not long after Earth formed, some four billion years ago.

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.

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.

However, current evidence suggests that plate tectonics -- as we know it today -- was not occurring on the early Earth.

So, the question remained: 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 km in diameter, slammed into a young Earth.

To test this idea, researchers from Trinity College Dublin 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.

With the support of the Irish Reseach Council (IRC) and Science Foundation Ireland (SFI), the team collected thousands of zircons from the Sudbury impact crater in Ontario, Canada.

After analysing these crystals at the Swedish Museum of Natural History in Stockholm, 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 couldn't have formed in impact craters, but we now know they could have," said Gavin Kenny from Trinity's school of natural sciences in a paper published in the journal Geology.

"There's a lot we still don't 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," he added.

Kenny recently presented the findings at the annual Lunar and Planetary Science Conference (LPSC) in Houston, Texas.



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