In geology, in contrast business, nothing is too big to fail. Mountains provide the most spectacular example. Because they were pushed up by the crumbling of the Earth’s crust as a result of tectonic plates colliding, they could theoretically continue to rise almost indefinitely. In practice this is not the case. A series of geological processes — including the grinding of glaciers, the gentle action of rain, and the violent fracturing of water freezing and thawing — are eroding them to their size.
In an article published in Nature Jérome Lavé, geologist at the University of Lorraine, describes another, much more spectacular mechanism. dr Lavé has collected evidence suggesting that around 1190, a massive landslide struck about 500 meters above the elevation of Annapurna IV, a mountain in the Himalayas that is now about 7,500 meters high. If he’s right, it would be one of the largest landslides ever recorded. The collapsing peak would have displaced up to 17 cubic miles of rock – about enough to bury all of Manhattan at about the height of the Empire State Building.
When the debris fell, the energy released was about six times that of the Tsar Bomba, the largest nuclear weapon ever detonated. “I didn’t think I could imagine what it would sound like,” says Ann Rowan, a geologist at the University of Bergen who did not attend Dr. Lavé’s work was involved.
dr Lavé’s suspicions were raised while he was conducting field research in the Ganga plains of Nepal in 2012. He noticed that the ground beneath his feet was of an unusual composition. A 50 meter core drilled from the rock showed an average limestone concentration of approximately 10%. But over a 4-meter stretch, concentration went up to almost 50%, “which is huge and totally abnormal,” he says.
This suggested that the stones in question had entered the Ganga plains from the Annapurna massif hundreds of kilometers away. This in turn indicated a massive landslide in the (geologically) recent past. After examining satellite imagery of the massif and a helicopter flight to see for himself, Dr. Lavé a large debris field that looked like it could have been created by the same event. So he visited the site the following year, becoming only the second known geologist to do so, and took some samples. As he examined the surrounding cliffs for signs of collapse, he noticed that a peak called Annapurna IV had a relatively smooth, sheer face that seemed to fit.
At home, he sent samples of the debris field, rock core, and other paths the landslide may have taken to be dated. If their ages are roughly the same, that would indicate that they were related to the same event. By measuring the presence of chlorine-36 (a radioactive isotope that accumulates in surface rocks and decays once buried) and carbon-14 (another isotope that accumulates in living matter and decays after death), his dated Colleagues date the samples to the later 12th century and are within a few decades of each other. That is within the accuracy limits of the dating techniques themselves.
dr Lavé’s work not only sheds light on a previously unknown catastrophe, but may also fill a gap in the prevailing explanation for why mountains stop growing, the so-called “glacier-buzz saw” hypothesis. According to this model, it is primarily the glaciers, which are extremely effective at carving glaciers out of the mountains, that are responsible for stemming their growth.
The problem with this theory, according to Dr. Rowan is that there are some peaks that manage to escape the erosive action of the glaciers and then rise so steeply that the glaciers can no longer cling to their slopes. “The question is,” she asks, “what keeps these mountains from getting bigger?”
Landslides could be one answer. While the exact trigger of the Annapurna landslide is unknown, Dr. Lavé theorizes that very tall mountains simply grow until their weight becomes too great for the lower slopes, where erosion is still occurring, since there is nothing to carry rock away from their tops for support.
To determine exactly how and when the tipping point will be reached, other rockfalls of this type need to be studied. Unfortunately, due to the action of glaciers and swelling rivers during the monsoon season, the debris from the Annapurna landslide is quickly disappearing. dr Lavé estimates that only about 10% of the detached material remains in place. Older rockfalls, if there were any, may no longer be able to be reconstructed. ■
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