At 8:37 a.m. Nepal time, on August 26, a huge chunk of glacier perched on the Langtang Lirung peak on the Nepal-Tibet border, sheared off. It is one of the worst catastrophes to hit Nepal.

Breaking off at the lower end, at a height of about 5,200 metres, the epic journey of the avalanche of ice and rock began. It took along a lot of the bedrock. The energy released would have melted the upper parts of the glacier too, triggering mini avalanches of snow.

“In the satellite pictures you can see the place from where this chunk breaks off. The cut is very neat,” Vaibhav Kaul says. A mountain geographer with a PhD in hydrological extremes and geo-hazards in the High Himalayas, he is intimate with the terrain there. He is helping us piece together what might have happened there. But he is speaking of probabilities, not certainties. This is not a forensic analysis, which will come in the months and years ahead.

This is a glacier collapse, which means a glacier or a slab of glacier crumbles. This is different from Glacial Lake Outburst Floods (GLOFs), sudden discharges of water from a lake formed by melting glacier ice and dammed only by loose stony rubble.

As Kaul sees it, the falling mass starts disintegrating, pulling in stuff from the bedrock, from around the glacier, from the sides of the valley. All that material now starts cascading down from about 5,200 metres. Another lot of material that would have been added is dead ice, bodies of ice that were once part of the glacier but are now disconnected from it. As the whole mass hurtles down , “Let’s say,  150-200 km per hour because of the very steep slope,” it smashes everything in its path and drags it along. It has become bigger now, more voluminous and taller, carrying rapidly thawing pulverised ice, shattered rocks, meltwater, mud, boulders and eroded stuff.

The valley of the Lende River is at an elevation of 3,000 metres, and the debris flow smashes into it. It then, by taking a turn, barrels down the Rasuwagadhi border crossing, at about 1,900 metres.

Kaul thinks the combined volume of the viscous mix of meltwater, debris from shattered bedrock, and sediments of all sizes from moraines—would easily be 100 billion litres. If this were drinking water, it would be enough to supply the entire population of India for about 25 days.

Kaul says, “The velocity of moving mass between  the rupture at 5,200 metres, the point where the glacier collapses and where it hits the Gyirong-Rasuwagadhi border crossing is definitely above 100 km per hour.” In particularly steep places it might have reached 200 km per hour.

The debris flow would have been taller than 150 metres (or 500 feet high, a multi-storeyed building) initially, about half that height when it hit the border crossing, and about a quarter by the time it got to the downstream town of Nuwakot. It would still have been around 35 metres high.

Kaul, who is familiar with the territory, says upstream of the Gyirong-Rasuwagadhi border crossing, the river is called Lhende Khola . Khola means “stream”. The Lhende Khola marks the boundary between Nepal and Tibet (China). At the Rasuwagadhi border crossing, Lhende Khola joins Gyirong/Kerung Tsangpo/Khola (from Tibet).  The combined waters of these two rivers are known as the Bhote Koshi. Further downstream, after receiving a tributary from Gosainkunda, Bhote Koshi is called Trishuli. The Trishuli eventually joins the Narayani River—known as Kali Gandaki in its upper reaches—in the Chitwan area of the Terai. The Narayani is known as the Gandak upon entering India. The Gandak is a major tributary of the Ganga.

The simplest way to think of a glacier is as a tower of ice. The difference is that it’s a moving tower, a stupendously dynamic environment, masses of its ice in constant motion.

Snow falls on top, keeps piling up, gets squeezed into a dense, solid mass by its own weight, layer upon layer, year after year, decade upon decade. Long-lasting expanses of snow and ice are called snowfields. This is where snow builds up. Then there is the process of melting and evaporation.

In the body of a glacier, the part where there is net gain of ice—the area where the stockpiling of ice is greater than the loss due to melting and evaporation—is called the accumulation zone. Generally, much of the accumulation zone consists of snowfields. Snowfields are, essentially, troves of snow and ice. The part of the glacier where there is a net loss of ice through melting and evaporation rather than a net gain is known as the ablation zone.

These zones can have small or large pools of meltwater. Those pools can be sitting underneath the glacier, subglacial pools; as pockets within the glacier, englacial pools; or, on the surface of the glacier, supraglacial pools; or pools in front of the glacier, proglacial pools.  Sometimes these pools are abruptly drained.

Then there are crevasses—linear ruptures or cracks—and moulins—cylindrical holes or shafts—which transport meltwater and heat into the interior of the glacier, sometimes all the way to its base, lubricating its under-surface.

In Himalayan glaciers, the thickness of ice can range from a few dozen metres in its ablation zone, its lower parts, to a few hundred metres in its accumulation zone at the highest elevations.

While snowfields—and accumulation zones—sit on top of the glacier, the places of melting and evaporation—the ablation zones—typically are in the lower reaches. In fact, the lower third of the glacier is an ablation zone. This is where it may shed huge chunks and blocks of itself, in a process called calving. Trekkers, mountaineers, and pilgrims can attest to hearing the sound of glacier calving in the form of reverberating, muffled booms and eerie cracks of giant walls breaking.

Then there are places where the net gain and loss are in equilibrium, and these are known as equilibrium zones. The height on the glacier at which the mass balance—the gain and loss of ice are in equilibrium and so its mass balance is zero—is the equilibrium line altitude (ELA). Although a theoretical boundary, it tells an awful lot about the glacier: local and regional climate; glacier dynamics, the health of the glacier, among others.

Glaciologists are always on the lookout for the most important things: the thickness of a glacier and its mass balance. It’s these two that lead to insights into the behaviour and viability of a glacier.

For the western Himalayas—Jammu and Kashmir, Ladakh, Himachal Pradesh, Uttarakhand, including parts of Pakistan and western Nepal—the average ELA is about 4,600 metres.

For the eastern Himalayas—Nepal, Bhutan, and Sikkim—the average ELA is about 5,200metres. What it indicates is that you have to reach that high to get to zero degrees—the freezing level—where ice forms and accumulates. It indicates that the glacier gains ice above that height. Below that, not only does it not gain ice, but loses it through melting and evaporation.

Last year, 50 people climbed above 5,000 metres to commemorate the dying Yala Glacier in Langtang in Nepal, according to an ICIMOD report.

The Yala Glacier lost 66 per cent of its ice and shrank by hundreds of metres. A new study, published in the journal Global and Planetary Change in September 2026, paints a grim picture for glaciers in the Langtang Catchment in Nepal. On average, the ELA for the glaciers is about 175 metres above the ELA that typically obtains there. The Yala is the worst affected. The authors calculate that its ELA rose about 200–250 metres since the 1980s. That is, its ELA is now above 5,600 metres. It means the entire glacier is now in the melting zone.

With rising temperatures, the height at which ice forms on glaciers is increasing. Many Yalas dot the Himalayas, and many may give way, in the form of landslides, chunks simply tearing off.

Even so, Kaul thinks  the latest disaster may not have happened but for some other unrelated events. For instance, he said, the 7.8-magnitude earthquake that hit the Langtang Lirung mountain area on April 25, 2015, is in the same area where this glacier collapsed.

“It is  highly likely that the Langtang Lirung mountain was affected by the earthquake. Cracks would have emerged in its rocks. There would have been slope failures,” he says.

According to Kaul the earthquake 11 years ago likely introduced “slope instability” that had an impact on the “failure of the bedrock underneath the shrinking and increasingly unstable mass of ice which has now collapsed.”

The impact of climate warming on the stability of the bedrock underneath and around glaciers needs special attention. With accelerated climate warming, including wintertime warming, the intensified melting and fracturing of glaciers—such as those on Langtang Lirung—is likely to be undermining the stability of high-mountain rock slopes in several ways:

First, Kaul notes, it causes greater volumes of meltwater to seep into the bedrock, thereby weakening it by widening the cracks within it through repeated cycles of freeze and thaw.

Secondly, it exposes greater surface areas of bedrock to direct heating from the atmosphere as well as warm summer rainfall—including extreme-intensity events—escalating the melting of the permafrost within it, and allowing even more water to enter through cracks, which are then progressively widened through freeze-and-thaw cycles.

Thirdly, at locations where steep or overhanging sections of bedrock are kept in place and almost plastered by glacial ice, the loss of that supporting ice greatly destabilises them. This process also contributed to the Langtang Lirung ice-and-rock avalanche.

Kaul also says this year, as in the three previous years, the Langtang Himal region saw an abnormally warm winter followed by large spring snowstorms.

“This could have contributed to exceptionally high meltwater seepage into the bedrock, and rapid permafrost thawing and freeze-thaw cycles shattering within it,” he says.

So tectonics could have played a role; global warming helped thermal destabilisation, of taking the equilibrium zone higher and higher, which leaves more glaciers  in the melting zone.

“If you hadn’t had that kind of bedrock failure along with the glacier failure, the volume of the material and therefore the magnitude of the event may have been smaller,” he says.

Based on his personal observations and fieldwork, he says something similar might happen in the Kargil area in Ladakh. There are a few dozen small, shrinking, deeply fractured, disintegrating glaciers hanging precariously over the upper valley of the Suru River, especially upstream of Sankoo, between Tangole and Shafat. One of these could break off any time, culminating in an ice-rock avalanche that transforms into a debris-rich flood similar to the one in Nepal. Such a glacial flood, Kaul warns, along the Suru-Shingo-Indus will affect riverside habitations and settlements not only in India’s Kargil district but also in the Kharmang and Skardu districts on the other side of the LoC.

He flags another type of hazard. Along the Lachen Chu, a feeder of the Teesta River in northern Sikkim, there is the ever-looming threat of  lake outbursts (GLOFs). As a part of his PhD, Kaul studied disaster risk from one such lake, which is capable of catastrophically releasing billions of litres of water when hit by extreme rain, a landslide or an avalanche from the glacier hanging over it.

“The magnitude of hazards in the high Himalayas is now becoming increasingly difficult to cope with,” says Kaul.