After a century, researchers may finally have solved a prehistoric meteorite mystery.
Imagine the scene: One fine day, about 790,000 years ago in the late Pleistocene, an asteroid 2 km (1.2 miles) across split the sky over modern-day Southeast Asia. The gigantic space rock came in at a shallow angle, gouging out a crater 17 km (10.6 miles) wide and showering the surrounding region with debris. While the impactor was 30 times smaller than the one that caused the Chicxulub extinction event 65 million years ago, it was 100 times bigger than the more recent bolide that exploded over Chelyabinsk, Russia, in 2013.
Nevertheless, while the impact was not on the level to cause an extinction event, it was significant enough to have left a sizeable crater. But there was no obvious crater in sight. All geologists had to go off of were a multitude of molten-glass fragments called tektites, which rained down all across the Indian Ocean, Australia,and Southeast Asia after the impact. Many of these glass beads take on a teardrop shape, known as splash-form tektites (see image below). Others take on an ablated "pancake" form after cooling.
This region where tektites are found is known as the Australasian tektite strewnfield. It's the largest of its kind, covering a tenth of the planet’s surface; some of these tektites have even been found as far away as the coast of Antarctica.
Yet despite the preponderance of these signs of an ancient impact, the location of the crater from which these fragments were thrown out has long remained a mystery — despite more than a century of searching. Previous claims had resorted to suggesting that multiple craters could be responsible, some as distant as Kazakhstan.
Now, researchers think they've found the crater responsible buried under the Bolaven Plateau, a sandstone and mudstone upland in southeastern Laos that's covered in dried lava up to 300 meters (1,000 feet) thick.
The proposed Bolaven impact “is likely the largest crater (on Earth) formed within the past million years,” says study lead Kerry Sieh (Earth Observatory Singapore). A 31-km crater recently discovered under Hiawatha Glacier in Greenland is larger, he adds, but its age is still unclear.
Evidence of an Ancient Impact
In a study appearing December 30th in the Proceedings of the National Academy of Sciences, Sieh and colleagues laid out four lines of evidence pointing to the existence of a crater under the Bolaven Plateau as the source of the Australasian strewnfield.
The researchers first tested the tektite fragments themselves, to see if they contained any basalt, which would indicate their origin on the volcanic plateau. Then they tested the proposed impact site, dating the basalts there and mapping the region's density to look for evidence of a buried impact crater. Indeed, mapping of gravitational anomalies in the area showed hints of a 17-by-13-km crater buried beneath the volcanic plain in southeastern Laos.
Finally, the team identified small tract of oddly rilled terrain located 10 to 20 km southeast of the volcanic plateau's summit. This is one of the few surface exposures not covered with lava flows, and there the researchers found what appears to be a thick layer of ejecta from the impact.
"Now that we know the site of the impact, we can begin to investigate how the surrounding areas were affected by the impact,” says Sieh. “For example, how thick and coarse was the ejecta blanket 10, 50, 300 kilometers away?” From this, he says, we can understand the effect of the shock wave that blasted outward after the impact.
Intriguingly, the sedimentary layer where geologists find Australasian tektites, which is 790,000 years old, overlaps with the time frame of the sediments in which the 770,000-year-old Peking Man (Homo erectus) was found. Maybe early hominids witnessed this catastrophic event!
Even though geologists have known about the Australasian strewnfield for more than a century, the exact location of its source crater has eluded detection until now. Makes you think: What other secrets does the Earth hide just below its surface?