Scientists led by Professor Kerry Sieh, from the Earth Observatory of Singapore, are poring over complex data. They’re hunting for the impact crater of a massive meteorite that slammed into the Earth’s surface around 800,000 years ago. Frustratingly, the crater’s location has eluded scientists for over a century. But Sieh and his researchers believe they finally have an answer to this enduring mystery.
Even without a known crater, researchers know that a large meteorite crashed into Earth some 790,000 years ago. And they can say this with confidence because traces left behind by the impact provide concrete evidence. These remnants left from the meteorite’s impact are called tektites.
And tektites are created by the massive blast when a meteorite hits our planet’s crust. For the impact generates temperatures so high that the earth’s own rock can be transformed into molten liquid. This melted rock is then shot upwards into the air before cooling and falling to the ground. These formations are called tektites, and they scatter widely when coming back down to earth.
Crucially, scientists can trace tektites and calculate their age. The distribution of the tektites can also indicate the likely location of an impact. So, for example, researchers know that a meteorite hit Earth around 790,000 years ago and that its size was more than one mile across. But, until very recently, they have been unable to pin down exactly where this massive rock from space landed.
Of course, meteorites and asteroids have been smashing into Earth from outer space for millions of years. In case you don’t know, an asteroid is a large lump of rock in space, which orbits the Sun. A meteoroid is a smaller rock, also spinning round the Sun. But a meteor is a meteoroid that has disintegrated in Earth’s atmosphere – also known as a shooting star. And a meteorite is what lands on Earth’s surface after a large meteoroid (or small asteroid) fails to fully vaporize.
Now, a meteor is largely harmless since it falls apart before hitting the ground. On the other hand, a meteorite can cause huge disruption to the planet if it’s big enough. And asteroids, or comets, can be something else altogether. For instance, the asteroid (or comet) that scientists believe landed 66 million years ago was so violent that it led to the extinction of the dinosaurs, as well as many other animals and plants.
That giant rock, estimated to have been anywhere between six and 50 miles across, left a crater more than 90 miles wide. And that crater is called Chicxulub and was discovered in the 1970s during an oil exploration operation in Mexico’s Yucatan Peninsula. As recently as 2016, analysis confirmed the identity of the crater to the satisfaction of most researchers.
But the meteorite that caused our mystery crater was far smaller than the rock that caused the Chicxulub crater, as well as more recent. As we’ve learnt, it landed only 790,000 years ago and is estimated to have been a bit more than a mile across. However, even this much smaller rock would have caused devastation in the region where it hit.
Indeed, when that meteorite crashed into the ground, it cast tektites across the planet’s Eastern hemisphere, over the lands of Australasia. In fact, the impact was such that the tektites from the blast were distributed over some 20 percent of the Eastern hemisphere. That’s equivalent to around ten percent of the Earth’s entire surface, a huge area.
Anyway, Dr. Sieh and his team described the problem they faced in a paper published in the academic journal Proceedings of the National Academy of Sciences in January 2020. They wrote, “A field of black glassy blobs, strewn across about 20 percent of Earth’s Eastern Hemisphere, resulted from the impact of a large meteorite about 790,000 years ago.”
And the authors continued, “The large crater from which these tektites originated has eluded discovery for over a century…” But the distribution of the tektites provided essential clues. As the researchers explained, “Evidence has long pointed to a location somewhere within Indochina, near the northern limit of the strewn field.”
So Sieh and his colleagues had limited the area of search for the crater. But that still left an enormous amount of territory to scan – a large chunk of Indochina. Those tektites, millions of them, had covered an even wider area, stretching from Antarctica to Southeast Asia and also landing across much of the Pacific and Indian Oceans. So narrowing it down to Indochina was already a worthwhile achievement.
Now, this spread of tektites across the planet’s surface is known as the Australasian strewn field. And strewn field is the scientific term for the area covered by tektites from a meteorite impact, with this one reportedly being the youngest on the planet. That makes our meteorite the biggest impact on Earth over the last million years or so.
So just how often do significant meteorites smash into our planet? In fact, material from outer space is constantly raining down on us, although we are blissfully unaware since no harm is caused. According to NASA, around 100 tons of particles the size of a sand grain and other dust comes into our atmosphere each day.
And once a year on average, a rock the size of a car zooms through earth. But even that is not dangerous since a meteor that size will entirely burn up in the atmosphere. However, around every 2,000 years, a meteoroid 300 feet across will penetrate the atmosphere, hit the Earth and cause local damage.
But as we go up the scale, the impact of a rock – regardless of its classification – becomes increasingly dramatic. And NASA reckons that once every few million years one large enough to wipe out civilization is likely to hit us. Just think of the fate of the dinosaurs 66 million years ago. And recall that our meteorite from 790,000 years ago was probably a little over a mile across.
As we’ve seen, tektites help researchers to identify crater sites. Interestingly, the word comes from the Greek tektos, and it was Austrian geologist Franz Eduard Suess that first used the term. In fact, tektites can be confused with glass-like pebbles formed by volcanic action. But careful analysis identifies them by their unique chemical make-up and by their extremely low water content.
Although the term tektite was coined relatively recently, people have spotted these strange pebbles for thousands of years. You see, the first recorded instance of them comes from China around 900 B.C. One Liu Sun described what were almost certainly tektites calling them “inkstones of the Thundergods,” according to the University of Texas at Austin website.
Speaking to CNN in January 2020 about the tektites from our meteorite, Prof. Sieh said, “Their existence means that the impacting meteorite was so large and its velocity so fast that it was able to melt the rocks that it hit.” However, as we’ve seen, despite all of the evidence from the tektites the precise location of the crater has remained elusive.
So we know that the tektites from that meteorite impact around 790,000 years ago came pouring down from the sky over a vast area covering thousands of miles. But by analyzing the spread of the tektites it should be possible to come up with a likely epicenter for the impact. However, in this case the size of the field of tektite landings has made it very difficult to pinpoint.
You might have thought that a meteorite more than a mile across would have created a crater that was quite easy to spot with the naked eye, or at least with satellite scanning. And the crater’s size had been estimated at a minimum of several miles across with a depth of hundreds of feet.
Now, Aaron Cavosi, a scientist with Curtin University in Perth, Australia, hit the nail on the head in an interview with The New York Times in January 2020. As Cavosi put it, “That’s a very difficult size hole to make go away.” But there are reasons why this crater may be so difficult to find.
You see, when a new “impact” crater is first formed on the Earth’s surface it will be easier to spot. But Earth’s surface is not a static body. Over millennia, it can change drastically. For instance, the movement of tectonic plates over the earth’s mantle can drastically reshape entire continents given enough time.
Furthermore, volcanoes can substantially remodel the planet’s surface as can large earthquakes. Remember, it wasn’t until the 1970s that the Chicxulub crater was found. And when it was, it was quite impossible to see with the naked eye. But in the case of our meteorite’s crater, the timescale is in hundreds of thousands of years rather than tens of millions. However, that’s still enough time for significant change.
And to add to the mystery, Southeast Asia, the most likely location for the crater, is a region that generally has low rates of surface change caused by erosion. Yet it’s still been extremely difficult to find this crater. And researchers had looked far and wide for it without success – until very recently.
As Prof. Sieh explained to CNN, “There have been many, many attempts to find the impact site and many suggestions, ranging from northern Cambodia, to central Laos, and even southern China, and from eastern Thailand to offshore Vietnam.” So it seems that those telltale tektites were not enough on their own to uncover the whereabouts of the missing crater.
So Sieh and his colleagues had to look at other strands of evidence in their latest hunt for the crater. Now, Prof. Sieh himself had spent many years looking for it. But until recently his searches have ended up in a series of frustrating blanks. So a new approach using different techniques was clearly called for.
As Prof. Sieh went on to explain to CNN, “Our study is the first to put together so many lines of evidence, ranging from the chemical nature of the tektites to their physical characteristics, and from gravity measurements to measurements of the age of lavas that could bury the crater.” So applying different scientific methods was the key to Sieh’s new research.
Indeed, using these different methods allowed Sieh and his colleagues to make a major breakthrough. You see, they’ve actually identified a site that they now believe is the location of the impact crater. And it’s in the Southeast Asian country of Laos at a place called the Bolaven Plateau. Their favored spot is in the south of the country, not far from the Mekong River.
And the detective work that’s allowed Prof. Sieh and his team to settle on this spot is fascinating. First of all, the plateau they’ve identified was covered in a heavy layer of volcanic lava, up to 1,000 feet deep. So although much of Southeast Asia has not been subject to drastic change on its surface due to erosion forces, a coating of lava could nevertheless easily cover a large crater.
The next step was to compare tektites from the Laos site with others from different locations in the Australasian strewn field. Well, researchers found that there was a good match, indicating that the Laos tektites were the product of the same meteorite strike as those in the wider strewn field.
Then Prof. Sieh’s team went on to calculate the dates of the lava flows at the site of the crater. Now, some of those flows had happened before the meteorite had crashed into the Bolaven Plateau. But others would’ve been younger than the date of the meteorite strike and therefore could’ve helped bury it .
After this, the experts pursued another line of evidence. Yes, they analyzed the strength of the gravitational field around the site of the suspected meteorite strike. And they did that because research has established that the gravity field over a crater exhibits a measurable weaker strength than elsewhere. Indeed, they did end up detecting a lower level of gravity.
That diminished gravitational field comes about because, over time, a crater is filled with fractured material that’s not so closely packed as the geology around the impact site. In fact, the results were consistent with an oval-shaped crater some 11 miles in length and eight miles in width. And Prof. Sieh and his colleagues calculated that the crater was around 300 feet deep.
So the research team was definitely on the track of the mystery crater. Their next move was to search around the location they’d identified in Laos for the remnants of the meteorite. After all, the impact of a rock of that magnitude would certainly have scattered a large amount of debris around its crater.
It turned out that the scientists were in luck. For construction workers had dug into the flank of a hill within a few miles of the suspected impact site. And in doing so, they had unwittingly exposed just what the geologists wanted to examine. The workers revealed a pile of sandstone boulders which Sieh told The New York Times fitted together “like a jigsaw puzzle.”
One of Prof. Sieh’s colleagues, Vanpheng Sihavong, a geologist with the Laos Ministry of Energy and Mines, told The New York Times, “We calculated that they [the rocks] were ejected from the crater and landed at about 1,500 feet per second, fast enough to shatter them upon impact.”
And as a final test, the scientists analyzed some of the grains of quartz in the sandstone rocks. You see, they were looking for evidence of fracturing in the particles. Prof. Sieh told the Times, “We think we’ve found that.” Fractures like these are generally seen as evidence of the tremendous forces that can be generated by a meteorite impact.
Therefore, Prof. Sieh and the other scientists on his team had now examined multiple strands of evidence. And it was increasingly clear that they had, at the very least, identified a highly plausible location for this crater that people had been searching for over a century. So was this the definitive answer to the mystery of the crater’s location?
In fact, it’s not quite that yet. And Prof. Sieh himself is not ready to claim absolute certainty at this point. He told CNN that researchers must next “drill down a few hundred meters to see if the rocks below the lavas are indeed the rocks you’d expect at an impact site – that is, lots of evidence for melting and shattering.”