The oldest rocks ever found on Earth may have been born in an asteroid bombardment that happened over four billion years ago.
Found at the Acasta River in Canada about three decades ago, these ancient granite, or felsic, rocks formed approximately 600 million years after the creation of the Earth, before any life arose. They contain a distinctive mix of elements compared to rocks that formed later, suggesting they may have been created by a different geological process.
Tim Johnson at Curtin University, Australia and his colleagues simulated the potential conditions in which these rocks could have formed. They concluded that partial melting of the Earth’s surface at a temperature of 800 to 900°C under very low pressure may have contributed to their creation.
It would have been impossible for the young Earth reach such high temperatures unaided, Johnson says. Instead, he says the late heavy bombardment, a period of intense asteroids impacts on Earth that also widely cratered the moon, may be responsible.
“We know that the Earth was bombarded for 600–700 million years after its birth,” Johnson says. “The fact that they are the only felsic rocks older than four billion years that we know of instantly got me thinking about impacts as a possible cause.”
With such large-scale meteorite showers, rocks like the ones found at Acasta River could have been prevalent at the time, Johnson says. However, later plate tectonics would have swallowed most of them, meaning these rocks could be the only survivors of the extra-terrestrial impacts that happened early in Earth’s history.
In future, Johnson is hoping to explore places like Siberia, Russia, to discover more geological evidence to support his idea.