Geologists at Trinity College Dublin have discovered that the very oldest pieces of rock on Earth are to be found at the bottom of craters left by asteroid impacts.
The finding may overturn previous research suggesting these rock components - zircon crystals - form where pieces of the Earth’s crust smash together to build mountains.
Zircon crystals are found in all kinds of rocks across the planet, said Dr Gavin Kenny, based in Trinity’s school of natural sciences.
The Earth formed about 4.5 billion years ago and the crystals represent the oldest constituent of rock found anywhere in the world.
The oldest crystals are found in Australia and may be 4.3 or 4.4 billion years old, but the big question is where they came from.
The Trinity team and colleagues from the Swedish Museum of Natural History in Stockholm believe they have the answer following their research, which was published on Thursday evening by the journal Geology.
“People want to study these crystals to see what they can tell us from that time,” said Dr Kenny, who is first author on the paper.
The crystals are tiny, about the width of a human hair. But scientists have learned to get a great deal of information from them, including their age and the conditions in which they formed.
Tremendous heat must first melt rock and then cool to allow the formation of zircon crystals, leading US researchers to suggest the oldest crystals must have come from plate tectonics.
This is when pieces of the earth’s crust crash together, forcing one plate under the other and melting large amounts of rock.
Impact craters
The Trinity group took a different approach, looking at the bottom of impact craters.
The Earth would have been pummelled by asteroids and orbiting space debris when young and the Earth’s surface was solidifying.
In 2014 the scientists travelled to the Sudbury impact crater in Ontario with funding from the Irish Research Council and Science Foundation Ireland.
There they collected thousands of zircons that matched in age the two-billion-year-old impact crater.
“When an impactor hits it melts a huge batch of the crust. In the crater we studied a pool of molten rock that would have been 3-4km thick. As that cooled the zircons formed.” Dr Kenny said.
The crystals were sent to Stockholm for chemical analysis and the researchers found the Sudbury zircons were an exact match for the much older Australian zircons.
This greatly bolsters their view that asteroid impact and not tectonics is the source of the zircons seen in rocks all across the world.
And it shows that impacts were happening very early in Earth’s formation, as reflected in the age of the crystals from Australia.
