Ancient asteroid impact dwarfed the one that killed the dinosaurs

Ancient asteroid impact dwarfed the one that killed the dinosaurs

When the subject of asteroid impacts comes up, especially in movies, it seems that there’s a need to produce one of mind-boggling proportions in order for it to be impressive enough to end all life on Earth. However, a new study is showing us that, in reality, it doesn’t take a Texas-sized rock to devastate the planet.

The asteroid that exploded 30 kilometres above Chelyabinsk, Russia back on Feb. 15, 2013 could easily fit inside a school gymnasium, yet the blast wave from it shattered windows throughout the city, damaged buildings and injured about 1,500 people. The rock that hit what is now the Yucatan Peninsula 65 million years ago set off a global firestorm and megatsunami that devastated the planet and ended the reign of the dinosaurs. However, if you hovered that asteroid over of the city of Toronto, its noon-day shadow would only stretch across the 10-kilometre span from the Don Valley Parkway to High Park. Going even further back in time, though, there was apparently another impact, in the northeastern part of what is now South Africa, that put these other two examples to shame.

Studying a region known as the Barberton greenstone belt, to the east of Johannesburg, South Africa, Stanford University geologists Donald Lowe and Norman Sleep have reconstructed an impact that happened around 3.26 billion years ago, that would have caused devastation unlike anything we’ve ever witnessed or even considered for other ancient impact events. Their findings show that an asteroid, estimated at somewhere between 37 and 58 kilometers wide, travelling at around 20 kilometres per second, slammed into the Earth there. The force of the impact would have produced a shock greater than a 10.8 magnitude earthquake, setting off seismic waves that stretched for hundreds of kilometres through the Earth’s crust and sending a tsunami that likely measured several kilometres high sweeping through the world’s oceans.

“We knew [the asteroid] was big, but we didn’t know how big,” Lowe said in an American Geophysical Union press release.

The crater that formed from this impact would have been over three times the diameter of the Chicxulub crater (the one that was formed by the asteroid that killed the dinosaurs).

According to Lowe and Sleep, this impact happened during the very end of the Late Heavy Bombardment. This is the period of Earth’s early history when it suffered thousands of impacts from asteroids and comets, very likely flung this way by Jupiter, Saturn, Uranus and Neptune as they migrated further out in the solar system.

This is the first time a study has modeled an impact this old, which was possible because the region of the Barberton greenstone belt has some of the oldest rocks on Earth — dated at between 3.6-2.5 billion years old. This region (part of the Kaapvaal Craton), the Pilbara Craton of Western Australia, and parts of the Canadian Shield are some of the only places in the world that we know of that have rocks this old. Erosion and plate tectonic activity elsewhere has buried or erased other evidence from that time period.

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The crater that this asteroid would have caused, estimated at around 478 kilometres wide, is nowhere to be seen these days, and it’s unlikely that we’ll ever find it. Enough time has passed since then for the Earth’s processes (wind, water and plate tectonics) to erode it away. So, it won’t show up on any maps, but the formations of rock in the Barbertone greenstone belt — which were likely thousands of kilometres from where the asteroid hit — still hold evidence of the ancient impact.

Also, according to the study, we may be seeing the effects of this impact even today, as the researchers suggest that it may have initiated the modern plate tectonic system in the area. They don’t go beyond that, but it’s interesting to speculate — given that Earth’s entire modern plate tectonics system supposedly started to develop at roughly the same time as this impact (3.2 billion years ago) — about whether this asteroid might have helped shape the entire face of our planet.

(Images courtesy: American Geophysical Union)

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