Why don’t mountains grow forever? Colliding tectonic plates continuously push mountains up, but what pulls them back down?
Among other erosive processes, colossal landslides may take down the highest peaks, geologist Jérôme Lavé, from the Centre National de la Recherche Scientifique, and his colleagues showed in a paper recently published in Nature.
“A landslide of this size is unheard of.”
The researchers provided evidence that in medieval times, around 23 cubic kilometers of sediment fell from the Himalayan peak Annapurna IV—about 10 times the volume of Lake Winnipesaukee and 1,000 times greater than the largest recent landslides.
“A landslide of this size is unheard of,” said glacial geologist Ann Rowan from the University of Bergen. But the study laid out convincing evidence that such events shape the highest regions of Earth once every few hundred thousand years, she and other experts said.
Teflon Peaks Escape a Glacial Buzz Saw
Few sites on Earth compare to the Annapurna massif in Nepal, where peaks soar thousands of meters above the nearby city of Pokhara.
A force scientists call a “glacial buzz saw” curtails the growth of some parts of this rugged landscape. As glaciers flow slowly downhill, they pull the underlying land with them. Debris caught in the glaciers also abrades the mountains below, keeping peaks from rising forever.
Annapurna IV is a “Teflon peak,” however; ice and snow slide off its steep, smooth sides, just as pancakes slide off a Teflon pan, letting it grow to above the altitude where glaciers form. Permafrost makes this mountaintop even more difficult to destabilize.
Despite the stability of some high Himalayan peaks, sediment from a landslide did fill the valley below Annapurna IV, researchers realized back in the 1980s. Until now, they had been unsure of when the landslide occurred and whether the sediment came down all at once or in chunks. Lavé and his colleagues went to the Himalayas to reconstruct the ancient processes that shaped the region.
The group used a helicopter to reach a remote valley between Annapurna IV and a nearby peak called Annapurna III. There they sampled sediment from the landslide; then Lavé and two pilots continued on to collect more samples from above, where the air was almost too thin for the helicopter to maintain altitude. Lavé’s colleagues stayed below to lessen the weight of the aircraft.
Back at the lab, the researchers reconstructed Annapurna IV’s historical peak by analyzing the sediment samples in three ways. First, they tested how long the sediment they’d collected had been exposed to cosmic rays—particles produced during supernovas that constantly bombard Earth. Cosmic rays interact with a common mineral called calcium carbonate to produce an isotope called chlorine-36, which scientists can count using a mass spectrometer to get an idea of how the mineral has been exposed to the atmosphere. They supplemented these measurements with carbon dating, which revealed how long ago organic matter in the sediment was alive, and by analyzing quartz to see when it was last deformed by an event such as a landslide.
A Decapitation in the Time of King Richard I
Around 1190, the top of Annapurna IV came crashing down all at once, the researchers found. If the city of Pokhara—Nepal’s second most populated metropolitan area—had existed at the time, it would have been decimated. Instead, the landslide created the flat surface that allowed people to settle in the region.
“It’s a really high-quality paper,” Rowan said.
The results raised new questions; for example, what triggered this massive landslide? Lavé and his colleagues suggested that the Medieval Climate Anomaly, during which Earth’s climate warmed by more than 1°C, may have thawed and weakened the base of Annapurna IV. Without a strong foundation, the peak toppled. “That is a potential trigger,” Rowan said.
Alternatively, an earthquake may have caused the collapse. There’s no record of an earthquake in 1190, however, leading Lavé and his colleagues to conclude that this scenario is unlikely. But geological dating methods have uncertainty, and the earthquake record may be incomplete, so some scientists aren’t ready to rule out the possibility.
The exact sequence of events is “simply very difficult to determine with the methods that we have on hand.”
The exact sequence of events is “simply very difficult to determine with the methods that we have on hand,” said geomorphologist and sedimentologist Anne Bernhardt from Freie Universität Berlin, who was not involved in the study.
Researchers also have debated how the sediment that fell from Annapurna IV moved down the mountain’s slope after the initial collapse. Lavé and his colleagues suggested that the sediment flowed downhill slowly, at a rate of about 1 meter per year. But geomorphologist Wolfgang Schwanghart from the University of Potsdam and his team, who were not involved in the new study, have found sediment deposits that are meters thick downstream of the initial collapse, as well as deposits that were pushed kilometers upstream. “That doesn’t happen gradually,” he said. “That must be catastrophic.”
Today, loose sediment from the medieval landslide still sits on the mountain slope above Pokhara, Bernhardt said. Some of that material could pose a modern-day geohazard. Any high-rainfall event or earthquake could cause it to flood into a heavily populated area, where it could damage critical infrastructure or cut off transportation routes. “The aftermath of these events goes on and on,” she said.
—Saima May Sidik (@saimamaysidik), Science Writer