Rocks found in a Canadian river could be all that remain of a massive meteor barrage hitting the planet 4 billion years ago.
The pale stones of the Acasta River have long been considered unusual by geologists, and are thought to be the oldest rocks on Earth.
Understanding how these so-called Idiwhaa gneisses formed therefore provides an unmatched insight into the planet’s violent history.
A new study presented at the Geochemical Society’s Goldschmidt Conference has laid out a new theory that explains the formation of these ancient rocks.
“We believe that these rocks may be the only surviving remnants of a barrage of extraterrestial impacts which characterized the first 600 million years of Earth history,” said geologist Dr Tim Johnson from Curtin University, who led the research.
Dr Johnson and his colleagues examined the chemical composition of the Idiwhaa gneisses, and used computer models to show that it was possible to make these rocks by melting the ancient crust.
The gneisses are felsic, meaning they are rich in silica and quartz, whereas the primitive Earth crust likely consisted of dark, silica-poor “mafic” rocks.
Getting from mafic to felsic would have required at tremendous burst of heat sustained at very low pressures – making meteors the most likely explanation.
“It would have needed something special to produce the 900C temperatures needed to generate these early felsic rocks at such low pressures,” explained Dr Johnson.
“That probably means a drastic event, most likely the intense heating caused by meteorite bombardment.”
In the paper outlining their findings, which was published in Nature Geoscience, the researchers suggested such events were common during the planet’s youth, and played a key role in shaping its geology.
“We estimate that rocks within the uppermost 3km of mafic crust would have been melted in producing the rocks we see today,” explained Dr Johnson.
“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 almost nothing remains.”
Their results were welcomed by other scientists as a feasible explanation for the mysterious Idiwhaa gneisses, especially considering our prior knowledge of meteorite impacts.
“The idea of making felsic melts by large or giant impacts seems plausible considering the high-energy nature of these events and the pockmarked ancient surfaces of other inner Solar System planets and moons,” said Dr Balz Kamber from Trinity College Dublin, who was not involved in the study.
“However, the implied pressure-temperature regime might also permit melting of shallow crust below a super-heated impact melt sea. In other words, an indirect consequence of the impact itself.”
