When an immense and inexorable flow of rock, mud and water barreled through northern Nepal on August 26, it killed more than 1,200 people and left thousands still missing. The magnitude of the event caught even experts on the region’s geology off guard.
“The victims, their pain and all the ruined towns … it is almost impossible to believe,” says geological engineer Ranjan Dahal of Tribhuvan University in Kathmandu, Nepal, who has studied geohazards in the Himalaya for nearly 30 years. “I don’t think even Hollywood could make such a movie.”
Satellite imagery helped researchers piece together a rough picture of the event’s origin: An enormous slab of bedrock and ice falling from a mountain called Langtang Lirung spawned a debris flow that hurtled downstream toward India. That dense torrent reached speeds over 160 kilometers (100 miles) per hour and destroyed villages along the way.
“I’ve never seen anything remotely as large as this,” says mountain geographer Alton Byers of the University of Colorado Boulder, who studies the impacts of climate change in the Himalaya.
What caused the bedrock to break in the first place is not entirely clear. But research and insights from scientists suggest that certain geologic, glacial and climate factors probably played a role in the disaster. Those insights underscore the potential threat to other high mountain regions, and how the risk of catastrophe may increase over time.
Human-caused climate warming is melting the ice that holds high mountains together, says glaciologist Ulyana Peña, also of the University of Colorado Boulder. “And it’s not something that’s just limited to Nepal,” she says. “The mountains are crumbling globally.”
Why the bedrock broke
There was early speculation that a collapsing glacier caused the debris flow. But images taken by satellite soon after the event showed a freshly exposed, uniform slope had appeared on Langtang Lirung’s north face, at an elevation of around 5,150 meters. In reality, a slab of bedrock roughly 2.2 square kilometers wide had detached from the mountain, carrying the overlying ice down into the valley below.
At this point, scientists estimate that the detached chunk of glacier and bedrock amounted to a volume somewhere around 110 million cubic meters. That’s roughly 40 times the volume of the Great Pyramid of Giza.
A swath of factors probably contributed to the breaking of bedrock on August 26, scientists say. First, it’s possible that the magnitude 7.8 Gorkha earthquake that hit central Nepal in 2015 may have damaged the bedrock, mountain hydrologist Walter Immerzeel of Utrecht University in the Netherlands said at a Sept. 16 briefing about the factors behind the event. The temblor’s shaking triggered a landslide on Langtang Lirung that killed around 200 people in Langtang Valley.
There’s also the matter of climate warming. The ancient tectonic uplift of the Himalaya is thought to have left fractures now sealed with ice known as permafrost in rocks at the altitude where the bedrock failure occurred. “It’s that cryospheric glue that, for millennia, has held together the glaciers and the rocks and the soils and the boulders up at high altitude,” Byers said September 1 during an online panel discussing the avalanche’s connection to climate change.
Human-caused climate warming has softened much of the permafrost in these regions, he said. “It’s like taking a stick of butter out of the refrigerator and putting it on the counter — it becomes mushy.”
Such softening may have contributed to the August avalanche, Byers and others speculate. Climate change has increased the annual average temperature on the slope and in the region more generally by about 2 degrees Celsius since preindustrial times, according to a report released by World Weather Attribution on September 17. “That’s much faster than the average rate of global warming,” said climate scientist Ben Clarke of Imperial College London during the Sept. 16 briefing.
On top of that, “the period right before the collapse was exceptionally warm,” Immerzeel said. During August 24 to 25, the average temperature at the elevation of the avalanche was about 5° C, with daily maximum temperatures reaching over 10° C. Historically, temperatures at that time of the year average around 2.7° C and don’t typically go above 6° C.
Immerzeel also pointed out that the overlying glacier had retreated more than 300 meters from 2010 to 2026 due to warming, potentially shifting the stresses on the rock wall.
Warming may have facilitated the bedrock breakup in other ways too. If meltwater from the glacier or permafrost trickled deep into fractures in the rock and then refroze, it could have wedged the cracks open even more, says geologist Jeffrey Kargel of the Planetary Science Institute in Tucson, Ariz. Additionally, meltwater may have chemically weathered the bedrock and formed weak, flaky minerals known as phyllosilicates inside of the fractures, he says, which would have made it easier for the rock to slip.
Fragile rock
The type of rock may have also contributed to the destruction downstream. Kargel notes that the scarp left behind appears to consist of rocks that began as hardened layers of mud and sand. These sedimentary rocks were later squeezed by heat and pressure into metamorphic rocks. But the resulting “metasedimentary” rocks still contain their relic layers, he says, which may act as weak zones.
Those weak zones could have helped the rock crumble more easily as it tumbled down the mountain, causing more collisions between pieces of rock and ice during the fall and upon impact with another glacier at the base of the slope, Kargel says. More collisions would mean more frictional heating of ice into water, which would have been crucial in mobilizing the solid debris into the fluid freight train that swept down into the Lhende Khola river valley. It would have picked up more water and sediment on its way downstream, he says.
Weak zones in similar metasedimentary rocks appear to have been involved in another rock-ice avalanche that caused a devastating debris flow in Chamoli, India, in 2021, Kargel and colleagues reported in a 2021 study in Science. More than 200 people were killed or went missing in that event.
But in the case of the August Nepal disaster, the weak zones don’t seem to explain why the slab broke away in the first place. The mountainside doesn’t appear to have fractured along the metasedimentary layers, Kargel says. So the breakoff itself may have had more to do with preexisting fractures and the consequences of warming.
The link to climate change is hard to deny, Byers said. “If you believe that warming temperatures melt ice, then I think you can make the connection.”
Beyond Nepal
A similar event could occur in many other parts of the Himalaya, Kargel says. Anywhere that has a combination of fractured and weathered metasedimentary rocks, glaciers and steep topography would be at risk.
Unfortunately, current geologic maps don’t have the detail needed to home in on locations where these rocks pose the biggest threat to people. “We need more mapping,” he says, especially as climate change continues to destabilize the Himalaya.
Ultimately, rock-ice avalanches are just one face of a many-headed beast, Peña says. As climate warming melts and thins glaciers and permafrost in high mountain regions, a variety of hazards are becoming more common.
Among the most notable are glacial lake outburst floods, or GLOFs. Since 1990, the thawing of high mountain glaciers has led to an increase in the number of glacial lakes, which form as meltwater accumulates behind a glacier or natural dam. When overfilled, degraded or otherwise disturbed, these lakes can unleash sudden and devastating floods on downstream communities. In 2023, researchers reported in Nature Communications that 15 million people live within 50 kilometers of a glacial lake and within 1 kilometer of a potential flood path.
The Andes are a prime example of another high mountain region where the risks to people are growing, Peña says. The 2023 study noted that the number of glacial lakes in the Andes has nearly doubled over the last two decades, reaching more than 1,600 by 2020, and that roughly 17 percent of the region’s population lives near glacial lakes.
Within the Andes, Peru’s Cordillera Blanca is a particularly concerning area, Peña says. As early as 1941, an outburst from Lake Palcacocha in Peru’s Cordillera Blanca destroyed a third of Huaraz, a city of 120,000 people, killing at least 1,800 people. Since that flood, the population of Huaraz has grown by more than 100,000, and from 1995 to 2018, the lake’s area grew by six times to roughly the size of Vatican City, raising the likelihood of an outburst flood, researchers noted in a 2021 study in Nature Geoscience.
The risk of glacial hazards is also increasing in the European Alps, Peña says. Just last year, a rock-ice avalanche fell from the Birch Glacier in southwestern Switzerland and buried most of the village of Blatten. In a May 2026 study, researchers noted that warming temperatures may have triggered the avalanche.
Dahal says that the Himalaya could experience another debris flow of similar size in the future. He hopes the international community can work together to install early warning systems across the Himalaya to help mitigate future fatalities. “Let’s work together,” he says, “and let’s save the lives of the people.”
