If the dinosaurs saw it coming, what they would have seen is a giant asteroid rocketing their way, crashing with the power of a billion Hiroshimas and eviscerating their kind from the surface the Earth (more or less). But that impact pales in comparison with the huge fist from space that smashed into the planet billions of years ago, a hammer blow so strong it may have created the geology of our world.
Scientists have played around with the idea of enormous asteroid hits in the early development of Earth, much much earlier than the impact that may have eliminated the dinosaurs 65 million years ago. Now a new study of the geological features in a South African region known as the Barberton greenstone belt points to an ancient crash some 3.26 billion years ago.
According to the study, the colossal asteroid would have been nearly as wide as Rhode Island, three to five times larger than the one that wiped out our favourite ancient reptiles, and would have hit Earth at 20 kilometres a second.
The collision punched a crater into the planet’s surface nearly 500 kilometres across, sending out seismic waves larger than any recorded earthquakes to shake the planet for half an hour at a time – six times longer than the huge quake that struck Japan three years ago. Those seismic waves broke off rocks, set off further shocks and launched tsunamis thousands of metres deep across the world.
In the aftermath, the sky would have become red hot, the atmosphere would have filled with dust and the tops of the oceans would have boiled. Vaporised rock from the impact would have rocketed into the atmosphere, where it would have condensed, first into liquid, before solidifying and raining back down onto the surface.
Lowe first guessed at the asteroid when he discovered telltale rock formations in the Barberton belt a decade ago. The new research models just how big the asteroid was and the effect it had on the planet – including the possibility that it was partly responsible for the modern plate tectonic system seen in the region.
As if that wasn’t enough, the asteroid may have been just one of dozens of similarly gigantic space rocks that scientists believe hit the Earth during the end of the Late Heavy Bombardment period early in our world’s history.
As incredible as it seems, most of the sites of impact have since been destroyed by erosion, the movement of Earth’s crust and other forces, leaving no evidence of the epic explosions behind. But some regions in South Africa and Western Australia still bear signs of the impacts.
“We can’t go to the impact sites. In order to better understand how big it was and its effect we need studies like this,” said Lowe.
The Baberton greenstone belt is an area 100 kilometres long and 60 kilometres wide that is east of Johannesburg near the border with Swaziland. It contains some of the oldest rocks on the planet.
The team used physics modelling and data from other earthquakes and asteroid impact sites on Earth and the Moon to calculate the strength and duration of the collision and figure out how the resulting seismic waves travelled to Barberton and caused its geological formations.
Reconstructing the impact not only helps scientists form a picture of the geology of the early Earth and how it the planet formed, it could also help researchers to better understand how early life evolved. Environmental changes triggered by the impact may have wiped out microscopic organisms, for example, allowing others to evolve and flourish.
“We are trying to understand the forces that shaped our planet early in its evolution and the environments in which life evolved,” Lowe said.
The research has been accepted for publication in Geochemistry, Geophysics, Geosystems, a journal of the American Geophysical Union.