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On August 26, 2026, an ice-and-rock avalanche tore down the Himalayas near Mount Langtang Lirung, along the Nepal-Tibet border. At about 10:50 a.m., a massive section of bedrock and glacier ice collapsed from 5,200 metres, hurtling downhill at an estimated 167 km/h. It slammed into the Lendey Khola (also Lhende or Lende), a tributary of the Bhotekoshi, temporarily damming the river before the barrier burst. In just seven minutes, the debris surge travelled 22 kilometres, unleashing a wall of mud, rock and water through the Bhotekoshi-Trishuli corridor. Villages, roads, bridges and the Gyirong border checkpoint were destroyed. It has claimed over 1,000 lives and thousands are still missing.
What led to this has not been so clear. But in the past week geologists, geomorphologists and GIS experts have shone more light. They tell us that this was not the result of a single sudden event but the outcome of a geological process that has been unfolding for millions of years. The landscape, the rocks and the glaciers had all combined to create the conditions for the avalanche. The immediate trigger may have been much more recent, but the foundations of the disaster were laid deep in the geological history of the Himalaya.
The story begins millions of years ago, when the Indian tectonic plate collided with the Eurasian. The collision was slow and continuous, but it subjected the rocks to enormous pressure. This process created faults, fractures and other breaks in the Himalayan rocks. These weaknesses became important because they provided potential planes along which rock could eventually fail. The tectonic forces therefore helped create the structural conditions for the later collapse of the mountain slope.
The rocks at the site are thought to be high-grade metamorphic rocks, probably gneiss. Their distinctive banding was produced when the rocks were squeezed and transformed under the intense pressures associated with the collision between India and Eurasia. They are estimated to be about 34 to 17 million years old. In places, they also contain a light-coloured granite called leucogranite. The granite appears to be about the same age or somewhat younger than the surrounding metamorphic rocks.
The structure of these rocks is important in understanding the collapse. The layers in the exposed rock face run roughly perpendicular to the cut surface. This suggests that the layers themselves may not have been the main weak zone responsible for the failure. Layering in metamorphic rocks can sometimes provide a plane of weakness along which a slope gives way, but the evidence here does not appear to support that explanation. Instead, the failure may have occurred along a fracture or fault that developed within the rock.
While tectonic forces created the fractured and weakened rock, glaciers were reshaping the mountain above it. Over roughly the last 10,000 years, glacial erosion carved the Himalayan landscape, particularly the northern face of the mountain. The glaciers cut deep cirques and created steep slopes and sharp ridges. In doing so, they continually modified the shape of the mountain.
Glacial retreat then altered the balance of these steep slopes. As glaciers withdrew, the ice that had once pressed against and supported some of the rock faces disappeared. This removed part of the support that had helped hold the slopes in place. With the ice gone, the rock could expand slightly, while water could enter the joints and fractures within it. Over time, these processes could weaken the rock further.
Climate may have provided the final contribution. The summer before the avalanche brought some of the highest recorded temperatures across this part of Asia. The warmer conditions would have produced substantial meltwater. The possibility raised by the evidence is that this meltwater entered the fractures and weakened the rock, helping trigger the eventual collapse. However, the precise cause of the rock failure remains uncertain. Gravity was the fundamental force driving the collapse, but the particular fracture or weakness that finally gave way has not been conclusively identified.
The crucial point is that the event was not simply a glacier collapsing. Evidence from the exposed slope indicates that a large section of bedrock failed beneath the ice. The glacier was attached to the underlying rock, so when the rock broke away, the ice went with it. The event is therefore better described as an ice-rock avalanche initiated by a bedrock failure, rather than simply a glacial collapse. The rock in the bedrock is perhaps thick layer of schist overlayered above and below granites of different densities and strength. Fissures on the bedrock have widened and when it finally gave way, the glacier and rock mass dropped roughly 1,200 metres, onto the lower glacier. The enormous fall generated the seismic signal that was initially interpreted as an earthquake. The impact also melted a large amount of ice and mixed water with the shattered rock and debris. This transformed the moving mass into a more fluid mixture and sent it down the tributary valley towards the Lhende river.
The landscape then amplified the event. From the upper slope to the valley below, the terrain has several thousand feet of relief. After descending the first thousand metres, the material travelled another kilometre or so before reaching the Lhende river. As it moved downhill, the rocks moved, uprooted trees and swept them like a slurry, from the valley floor and sides of the hills. This is where the process known as ‘entrainment’ comes into play. Entrainment is the moment when a moving fluid, whether water, wind or ice, gathers up loose sediment and carries it along. As the fluid moves, it exerts drag, pushing grains forward, and lift, pulling them upward. Gravity, meanwhile, holds them firmly against the ground. When the fluid’s forces overcome gravity and the friction anchoring a grain in place, the grain breaks free. In streams that form on steep slopes, this turbulence can give it an extra push. Sudden, swirling bursts of water can lift particles from the bed and sweep them into the current.
So, what happened in the Lhende began as a rock-and-ice avalanche leading to a huge debris flow. The debris flow travelled extremely rapidly, reaching an estimated speed of up to 160 kilometres per hour. It covered the distance between the seismic event and the border crossing, roughly 20 to 30 kilometres, in about seven to eight minutes. Images and clips from trusted sources show how the geological structure of the Himalaya, long-term glacial erosion, the retreat of ice, meltwater and the steep terrain combined to turn a local bedrock failure into a much larger and faster-moving disaster.
The event did not necessarily end with the main avalanche. About three hours later, a tremor set off by a rockfall sent a second seismic signal, equivalent to a magnitude 4.2 event. Analysis indicated that it was consistent with another landslide or mass-wasting event. A smaller slope failure was also captured on video near the river confluence. One possible explanation is that the enormous debris flow undercut the slope as it moved through the valley, causing this second failure. This remains an interpretation rather than a confirmed explanation.
The Lhende avalanche therefore illustrates how a catastrophic event can have a long geological history. The collision of India with Eurasia, which happened in spurts starting around 50 million years or so ago, created fractured and metamorphosed rocks. The sustained push by the Indian Plate continued to crumple the rocks. Then the monsoons etched and eroded the Himalaya’s young granite. Then, about 750,000 years ago, when permanent glaciers covered the higher parts of the Himalayas, they exerted their weight on these rocks. Their meltwater and the monsoon’s rain carved the rocks to give each mountain an exceptionally steep summit. As the glaciers retreated, some of the rock faces lost the support provided by the ice, while fractures became vulnerable to water. Recent warmth may have supplied additional meltwater and helped destabilise the slope. Finally, gravity brought the weakened bedrock down. The falling rock carried ice with it, melted and fragmented the ice, and transformed the mass into a massive debris flow as it descended towards the river.
The high Himalayas are no strangers to catastrophe. Often the action of ice and rock does not reach habitation and goes unnoticed. When the ice-rock have broken loose from the mountains, they have unleashed torrents of water, mud and boulders downstream. The Bhotekoshi Valley alone has suffered at least two such disasters in the past four decades. On 22 July 1996, at Larcha in the upper Bhotekoshi Valley, a catastrophic debris flow tore through the village, killing 54 people. Heavy rainfall and runoff, combined with the undercutting of the stream, destabilised the bedrock. The resulting landslide dammed the Bhairab Kunda rivulet, and very quickly the dam suddenly failed setting free the water and debris.
The pattern has been repeated across the Himalayas. In 1985, the Dig Tsho glacial lake in Khumbu, Nepal, suddenly burst through its moraine dam. The flood destroyed a hydropower station and swept away bridges and infrastructure along nearly 80 kilometres of river valley. On 3 September 1998, an avalanche from a hanging glacier crashed into Tam Pokhari in the Hinku Valley of eastern Nepal, generating waves that overtopped and breached the lake’s moraine dam. About 19.5 million cubic metres of water escaped, producing a devastating glacial lake outburst flood that swept down the Dudh Koshi. In 2012, the Seti River near Pokhara became a torrent of mud, rock and water, killing about 72 people and devastating settlements along the valley. Scientists are not sure what triggered the flood or have been able to find its precise origin. In 2016, a rain-triggered slope failure above Gongbatongsha Lake in Tibet generated a flood that crossed into Nepal’s Bhotekoshi basin, damaging the highway and the Upper Bhotekoshi hydropower plant. In 2017, a rockfall from Saldim Peak struck a glacial lake in the Barun Valley of eastern Nepal, demonstrating again how a sudden impact can destabilise a mountain lake and release its stored energy. Then, on 7 February 2021, a colossal mass of rock and ice collapsed from Ronti Peak in Chamoli, Uttarakhand. It transformed almost instantly into a roaring debris flow that tore down the Rishiganga and Dhauliganga valleys, destroying two hydropower projects and killing or leaving missing more than 200 people.
What appeared to be a sudden disaster was therefore the final stage of a much longer geological story. This process has and will speed up and become more mercurial as the planet warms and heats up the rock faces of Himalayas unevenly.
The Lhende-Trishuli tragedy reminds us that these mountains are not still. Processes that began millions of years ago continue to shape the landscape beneath our feet. The rocks store tectonic memory and orogenic power since the time the continents collided. The scars of glaciers and the force of water will find new ways to combine and find expression in sudden and devastating ways. To understand such disasters, we must look beyond the moment of collapse. We need to delve deeper into the history of the landscape that created them. Knowing their deep past will allow us to predict disasters better. It can help us anticipate the forces at work, understand where vulnerabilities lie, and prepare better for what the mountains, rivers and clouds may do next.
Pranay Lal is a natural history writer
Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth
