Planetary scientists have released evidence they say establishes where an asteroid hit Earth 250,000 years ago. Although evidence of the magnitude and timing of the earthquake has been found extensively, identifying the possible crater in southern Laos has been a long process.
Southeast Asia and even Australia are filled with rocks that were formed when an asteroid hit Earth 789,000 years ago. This precisely dated black glassy rock known as tektite forms the largest droplet of debris from an extraterrestrial impact, reaching as far as Madagascar and Antarctica.
Older, larger impacts, such as the one that caused Chicxulub crater, may have produced even larger sprays, but geological processes have long buried that evidence. The tektites known as the Australasian disseminated field are recent enough and can still be seen in many places, but they reveal the presence of a collision that must have been the largest since at least Zamanshin.
For the tektites to have spread so widely, the impact would have created a large crater, but nothing is obvious. Scientists have been searching for it for decades and gradually reached the Bolaven Plateau in southern Laos.
A combination of geological conditions, dense rainforest and one of the most politically isolated countries in the world is probably why this place has remained hidden for so long. The research team, led by Nanyang Technological University professor Kelly See, said unexploded ordnance “hindered field operations for decades” after the war in the region. Geologists may explore exploding volcanoes because they love their field, but bombs and mines left behind are another matter.
Nevertheless, some secrets cannot be kept forever. Gravel deposits dating back to 1983 was attracting attention It lies just below the Vietnamese tektite specimen. The layer of dust on top, called “catastrophe loess” by Thai geologists, was thought to have fallen from the sky, kicked up by the impact. Using these clues, Shi and his colleagues zoomed in and eventually located the site using a layer of pebble- to boulder-sized rock known as bolaven diamicton. They believe that diamicton was formed when ancient sandstone and basaltic lava broke apart and was hurled into the air by the force of the impact.
The idea that the crater lies within the Bolaven volcanic field has been proposed before, but ultimately there is no better place to hide it, but the evidence was never strong enough to convince anyone.
The location of the Bolaven volcanic field in Southeast Asia and the location of estimated impact craters and tektites of suitable age within it.
The research team is trying to convince skeptical geologists using five separate datasets that all claim to show the same location.
For example, although tektites of suitable age are found thousands of kilometers apart, they are most abundant at or above the diamicton. Furthermore, two of his properly aged tektites, MN and “splash form”, have been found in comparable numbers in most areas of the region. However, within 200 kilometers (124 miles) of the proposed site, MN-type tektites predominate, indicating that there is something unique about the site.
This research is published open access Proceedings of the National Academy of Sciences.