If you think the asteroid that made the Chicxulub Crater was terrifying, and no doubt nearby dinosaurs did, consider the force of an object weighing up to 30 times as much smashing into Earth. Evidence for just such an event has been found, and geologists have been able to date it with unusual precision.
Around 3.8 billion years ago the Moon was bombarded with huge asteroids, leaving craters that subsequently filled with lava and became what we now call “seas”. Earth must have experienced a similar set of impacts, but a combination of tectonic forces and erosion from wind and waves have eliminated all traces of craters older than three billion years.
This, however, doesn't mean we cannot find evidence of such events. Materials thrown up in the collision, like droplets spraying when a stone hits a pond, can leave a distinctive trace long after the impact crater is gone. This is what Dr. Andrew Glikson of the Australian National University found when he re-examined a drill core taken from Marble Bar, Western Australia, in some of the oldest continental crust on Earth.
Glikson told IFLScience the core was originally collected a decade ago for other purposes, but he checked it for the glass beads known as spherules that are created by large impacts. Objects that looked like spherules were found, and their metallic composition confirmed asteroidal origin. And these turned out to be more interesting than Glikson could have reasonably hoped.
The size of these spherules indicates they were formed in a truly enormous impact. Andrew Glikson/ANU
For one thing, they were unusually large. Glikson explained to IFLScience, “Other researchers have shown that the larger the impact, the larger the spherule.” Based on the 1-2 millimeter diameters of the spherules, Glikson concluded the asteroid would have been 20 to 30 kilometers (12 to 19 miles) in diameter.
"The impact would have triggered earthquake orders of magnitude greater than terrestrial earthquakes, it would have caused huge tsunamis and would have made cliffs crumble," Glikson said in a statement.
Moreover, the spherules were found wedged between two volcanic layers, the upper of which Glikson told IFLScience might have been triggered by the impact itself. Uranium-lead isotope analysis of volcanic crystals enabled Glikson to date the collision with unusual precision to 3.45 to 3.46 billion years ago. This makes it the second oldest known impact, as well as one of the largest in Earth's history.
"Exactly where this asteroid struck the earth remains a mystery," Glikson said. "This is just the tip of the iceberg. We've only found evidence for 17 impacts older than 2.5 billion years, but there could have been hundreds. Asteroid strikes this big result in major tectonic shifts and extensive magma flows. They could have significantly affected the way the Earth evolved."
The finding was published in Precambrian Research. Glikson and his co-authors confirm the spherules have a different origin from those found in sediments just 40 kilometers (25 miles) to the west that come from an impact 3.47 billion years ago.
These sediments at Marble Bar, Western Australia, contain tell-tale signatures of an epic asteroid impact. Andrew Glikson/ANU