The country is home to 14 “eight-thousanders,” peaks that top out at more than 8,000 meters or 26,247 feet above sea level. All these great mountains rise above the Himalayas, the highest place in the world.
But our planet is dynamic – could there have been other peaks like these since we lost it? “We wanted to know if the Earth and the Himalayas had another one 830 years ago,” says Jérôme Lavé, a geomorphologist at the National Center for Scientific Research (CNRS) and the University of Lorraine in France.
According to Lavý and his colleagues, the answer seems to be yes. In a new paper published Wednesday in Nature, they found evidence of an ancient landslide that reshaped the geography of South Asia — and linked it to the collapse of what would have once been one of the tallest mountains on Earth.
In search of missing mountains, these plains are prime ground for geomorphologists—scientists who study the evolution of the land beneath our feet (or, in this case, the land towering above all but the hardiest climbers). Rivers like the Narayani carry sediments downslope and these sediments can tell a lot about the mountains where they originated.
For example, Lavé and colleagues found medieval sediments with five times the average carbonate content. This mineral imprint indicated that something had disturbed the Narayani flow. “A giant landslide…seemed like the most obvious avenue to explore,” says Lavé.
They began to climb the hill to find out more. The Narayani flows through the city of Pokhara, nestled in a valley less than 3,000 feet above sea level. But this is one of the steepest landscapes on Earth: above Pokhara rises the Annapurna massif, part of the Himalayas. (The massif’s crowning jewel is its highest peak: it’s also called Annapurna, a proud member of the eight-thousander club.)
By studying images of the Annapurna massif, the team found geographic signs of an old landslide. In one subsection of the massif, called the Sabche cirque, they spotted strange formations like columns and pinnacles, signs of erosion.
Lavé and colleagues think this is what happened: A second 8,000-meter mountain once climbed from the Annapurna massif. Then it collapsed. The resulting rock slide thoroughly eroded the Himalayan landscape, dumping sediment into the valley that now contains Pokhara, from where the water carried it downstream. This event played a major role in eroding the rock and reshaping the massif closer to what we see today.
The paper suggests that large, dramatic landslides can be a significant driver of erosion at high elevations like this one. “This is a mechanism that still needs to be further investigated, but this hypothesis may open up new insights,” says Odin Marc, a geomorphologist at the CNRS who was also not involved in the research.
What caused the mountain to collapse is unclear. A warming medieval climate may have melted the permafrost on top of the mountain, which otherwise strengthens the peak. Schwanghart, who has also studied the geology of the region, believes earthquakes may be the answer. He says the chronology shows that three earthquakes hit Nepal around the time Lavé and colleagues suggested the mountain collapsed, and one of them may have caused the mountain to topple in the first place.
After all, Himalayan landslides are not a thing of the past. In 2021, an avalanche and rock slide rolled down a mountainside in Uttarakhand, India, about 300 miles northwest of Annapurna. The disaster breached the dam and the resulting flood left about 200 people dead or missing.
If such a rockslide occurred in Pokhara today, the results could be devastating. Pokhara is the second largest city in Nepal (after the capital Kathmandu) and home to over half a million people. In addition, evidence is mounting worldwide that a warming climate is increasing the risk of landslides. Just last month, the Fluchthorn Alpine peak, nestled on the Swiss-Austrian border, suddenly collapsed in an event scientists attributed to melting permafrost.
Landslides like these may be more common than we realize. “You’d find similar events in Alaska—but they often go unnoticed because there’s nobody there,” says Schwanghart.
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