An ancient meteorite impact from around 790,000 years ago scattered pieces of glassy debris from Asia to Antarctica, but the resulting crater has long gone unnoticed. According to the researchers, the meteorite fell on Earth with such force that the explosion covered around 10% of the planet with pieces of rocky debris.
For more than a century, scientists have been looking for evidence of this impact, but until now it has been impossible to solve this mystery. However, a geochemical analysis as well as local gravity measurements allowed the researchers to discover that the crater was located in the south of Laos, on the Bolovens plateau, and that the ancient impact was hidden under a field of cooled volcanic lava, covering almost 5000 square kilometers.
It should be noted that when a meteorite strikes the Earth, the terrestrial rocks at the site of the impact can literally liquefy (due to the intense heat) and then cool into glassy tectites. In order to locate an impact, scientists can notably analyze the abundance and location of these tectites, even if the original crater is eroded or hidden, explain the researchers in their study.
Tectites in abundance, so where is the crater?
According to the researchers, the force of the impact was so powerful that it created a plume more than 100 meters high. Impact tectites were most abundantly found in the eastern part of central Indochina. However, as the tectites spread over such a large area, previous estimates of the size of the crater ranged from 15 km in diameter to 300 km in diameter …
Not to mention the exact position, which remained very uncertain, even if many scientists have spent decades finding it. In the new study, they first investigated several promising eroded crater candidates: in southern China, northern Cambodia and central Laos, but quickly ruled out these possibilities. Indeed, in all of these cases, the alleged crater features turned out to be much older and were instead identified as erosion of rocks dating back to the Mesozoic (around 252-66 million years ago).
A buried crater?
On the Boloven Plateau in Laos, scientists discovered a site where volcanic lava fields may have hidden signs of an older meteoritic impact. In one of the regions the researchers targeted as a likely location for a crater, most of the lava flows were also in the appropriate age range: between 51,000 and 780,000 years.
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On this geological map of the volcanic field region, the blue ellipse marks the perimeter of the buried crater. Credits: Sieh et al./PNAS 2019
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The study’s authors scanned the surface of the lava, taking gravity readings at more than 400 different locations. The resulting gravity map demonstrated a “particularly interesting” area, with a gravitational anomaly: an underground area less dense than the volcanic rock that surrounded it.
According to the measurements, the crater would be elliptical and elongated, about 100 meters deep by 13 kilometers wide and 17 kilometers long. Together, all of these clues therefore suggest that “this thick pile of volcanic rock does indeed bury the site of the impact,” said the scientists.
source: Proceedings of the National Academy of Sciences
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