Published December 5, 2023
• 9 min read
In 2011 geologist Kerry Sieh and his husband were shopping in Ho Chi Minh City, Vietnam, when they popped into a little jewelry shop to check out the country’s famous rubies and spinels. But once inside, a set of glassy, black blobs on the wall behind the counter caught Sieh’s eye.
“Tektites,” announced the cashier, handing over a photocopied paper that indicated they formed in a mysterious meteorite strike—Earth’s last major impact, in fact, a catastrophic collision that could have been witnessed by ancient human relatives. As Sieh read about a colossal debris field covering 20 percent of the planet’s landmass, from China to Antarctica, he realized a key detail was missing.
“I thought, oh my gosh. How could it be that nobody’s found the hole?” Now, a dozen years later, Sieh is convinced he has.
After his jewelry store encounter, Sieh studied the scientific literature and searched satellite images across the region for geological features of interest. He began to suspect that the missing crater, formed some 800,000 years ago, could be in the Bolaven Plateau in southern Laos, buried beneath a young but extensive lava field.
This region is known today for stunning, 300-foot waterfalls and numerous coffee and tea plantations. But Sieh’s latest research, published today in the Proceedings of the National Academy of Sciences, suggests the area had a cataclysmic history. The new study identifies a pattern of thickening ejecta deposits, continuing from the lowlands of southern Laos and eastern Thailand to the center of the plateau itself—pinpointing a possible impact site.
These rocky deposits, up to 30 feet thick near the plateau’s center, contain pebble and boulder gravels at their bases, which Sieh believes are the detritus of a land-stripping impact blast. The tektites, a type of natural glass formed when meteorites hit sandy surfaces, were found near the tops of these gravels and beneath a thicker ashy deposit, possibly evidence of a giant plume sent skyward by the meteorite strike that later settled over the landscape.
These geologic sequences were recorded and measured at hundreds of sites across a 310-mile-wide region on and around the plateau, revealing a radial pattern of thickening deposits that converges on the plateau center. Sieh describes his case for the Bolaven Plateau as now “all but indisputable.”
“To have a rubbly, poorly sorted deposit made up of stuff we think is from where the crater was, [and] to have it thicken and coarsen toward the source, toward the plateau … offer me some other explanation,” he says.
But not all scientists are yet convinced. Fred Jourdan, a geochronologist at Curtin University in Perth, Australia, who has used chemical clues from the tektites to date the impact to about 788,000 years ago, says Sieh’s proposal is “very well possible,” though he believes the new findings only offer indirect evidence. He says the study is “not a demonstration of the absolute location,” pointing out that many volcanically active places across Southeast Asia also have sandy surface geology that could have produced the tektites in an impact.
Scientists are already proposing return trips to the region, which could confirm beyond doubt whether the plateau of southern Laos is truly the site of Earth’s last great impact.
Shards of impact debris
Tektites, such as the ones in Sieh’s jewelry store, form when fragments of molten material thrown out by a meteorite strike are launched into the air where they solidify and rain back down over a wide area. In the absence of an impact crater, tektite strewn fields provide the next best record of Earth-shattering meteorite strikes.
Jourdan’s 2019 dating of the tektites, found today across much of Australia and Southeast Asia, made it the youngest of Earth’s four major tektite strewn fields. The date fueled speculation that ancient Asian hominins such as Homo erectus could have witnessed the explosive event. While a limited fossil record makes that difficult to prove, this planetary collision was clearly best viewed from afar—Jourdan’s analysis showed the tektites formed at temperatures of up to 7,200 degrees Fahrenheit.
“I can imagine animals just getting blown away then vaporized,” says Sieh. Indeed, the late Thai geologist Sangard Bunopas documented evidence in petrified forests and fossil pits of widespread fire, mega-floods, regional extinctions, and mass animal mortality caused by the impact.
The tektites also offer clues about the missing crater itself. In 2007 Shyam Prasad, a scientist at India’s National Institute of Oceanography, modeled the impact event required to produce the distribution of tektites. He proposed a crater between 20 and 75 miles wide, though further research suggested the lower end of the range was more likely.
This lower estimate is similar to predictions based on iridium concentrations by Gerhard Schmidt, a geochemist now at the University of Heidelberg. Schmidt’s measurements of this rare metal, which is famously enriched in meteorites and the geologic boundary that marks the dinosaurs’ extinction, led him to propose a 1.5-billion-ton impactor hitting the planet and forming a 9- to 12-mile-wide crater.
Searching for the hole
Other proposals for the location of the impact, like Tonle Sap, Southeast Asia’s largest freshwater lake, fall in a region where tektites are bigger, more abundant, and less streamlined by journeys through the atmosphere.
But Sieh suspected that some of the key evidence of the impact may be hidden from sight. In a 2020 paper, he established layers of lava on the Bolaven Plateau as sufficiently wide (up to 62 miles across), thick (985 feet deep in places), and young to bury the Australasian impact crater. Some of the lavas Sieh dated were much older that the impact, up to 16 million years old, while later eruptions occurred as recently as 30,000 years ago. Sieh argues the pre-impact lavas can help explain why the tektites contain traces of volcanic as well as sandy material, before later eruptions filled and covered up the crater.
Additional evidence included bouldery outcrops on one side of the plateau that Sieh linked to the initial land-stripping impact blast. And results from 400 gravity surveys around the plateau revealed a large subsurface density anomaly, which Sieh ascribed to a buried 10-mile-wide crater infilled with less dense rock fragments.
That 2020 paper had its critics. Geochemist Jiri Mizera of the Czech Academy of Sciences claimed the Bolaven lavas lack some of the chemical signatures found in the Australasian tektites, questioning the link between the two. Now he says the new paper makes assumptions about the origins of the impact deposits.
“It could easily be impact ejecta connected with an impact elsewhere and deposited in a much shallower layer, and then transported from the top of the plateau to its foothills,” Mizera says.
However, Sieh’s ideas have found support from river geologist Paul Carling at the University of Southampton in England, helping make sense of some unusual tektite-rich rock sequences in northeastern Thailand. In a 2022 paper, Carling, Toshihiro Tada of the University of Tokyo, and colleagues reported an outcrop there featuring the key sequence of tektites and ejecta deposits above a course gravel base, all below a silty ‘loess’ deposit. This upper layer was again linked to a collapsed impact plume that had a devastating effect on regional flora and fauna.
Additionally, all three layers from that Thai outcrop contained shocked quartz, a deformed mineral also present inside the famous Barringer and Chicxulub impact craters. “Shocked quartz is quartz grains which experience shock waves, recorded as shock metamorphic features such as fractures or feathers,” says Tada—additional evidence that these deposits were formed in a major meteorite strike.
Seeking more clues
While Carling’s paper focuses on just one site, he claims to have since mapped this “impact ejecta sequence” across Thailand, southern Laos, Vietnam, and northern Cambodia. He also says that this field work has demonstrated a thickening in these deposits toward southern Laos, supporting Sieh’s case for the Bolaven Plateau as an impact site.
Carling hopes to publish his team’s wider field analysis next year, adding more clues to this geological puzzle. For Jourdan, though, the final proof can only be found by searching deep underground. “To rally the entire impact community behind the case, they need to drill down to where they think the crater is located,” he says.
Carling believes an exploration down to about 600 or 700 feet might suffice to find other features like shatter cones, shocked minerals, and melt rocks—all signs of a major impact. “We might also find fragments of the meteor itself,” he says.
Now that would be something to display on a jewelry store wall.
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