Why in news?

Flash floods devastated Nepal's Rasuwa district and parts of Tibet this week. Officials initially suspected an earthquake. Experts and the US Geological Survey later confirmed the seismic signal was a result of a glacial collapse — not its cause.

This raised a key question: what exactly is a glacial collapse, and how frequent are such events?

What’s in Today’s Article?

• What Is a Glacial Collapse?
• How a Glacier Forms and Collapses?
• Role of the Glacier's Base
• How Common Are Glacial Collapses?
• The Climate Change Question

What Is a Glacial Collapse?

• A glacial collapse is the sudden detachment of a large mass of ice, rock, and water from a glacier.
• It occurs on steep mountain slopes. Sometimes millions of cubic metres of material break away. This mass can transform into a fast-moving, ice-rich avalanche or debris flow.
• The term is not a formal scientific definition in glaciology. It is, however, widely used to describe such events.
• The journal Earth System Science Data describes this broader category as gravity-driven failures of glacier ice. It includes ice avalanches, glacier detachments, and rock-ice avalanches.

How a Glacier Forms and Collapses?

• A glacier forms when snow builds up and survives several melting seasons. These layers compress into firn — a stage between fresh snow and solid glacial ice. Over time, firn recrystallises into ice, becoming part of the glacier.
• Once the glacial mass grows heavy enough, it begins to flow outward. Steep mountain gradients cause this outward flow, which can trigger collapse.
• Warming temperatures add to the risk. Melted water collects in channels beneath the glacier. When the glacier's lower portion collapses, these channels open suddenly, releasing water in large volumes.
• As the ice mass moves over uneven terrain, stress builds up. This stress can exceed the ice's structural strength, causing fractures.
• Surface melting adds further weakness: water seeps into cracks, and repeated freeze-thaw cycles force these cracks apart until the ice splits through.

Role of the Glacier's Base

• What lies beneath a glacier also matters. Many Himalayan glaciers rest on soft mud or clay — known as "wet-base" glaciers.
• In these cases, water collects in large quantities at the glacier's base, sometimes connected through internal tunnels.
• When such glaciers collapse, the sudden rush of water mixes with loose dirt, rock, and riverbed material. This debris-laden torrent can race down narrow valleys at very high speed.
• Collapsed ice and rock can also block river channels, forming temporary natural dams.
• When such dams break under pressure, they trigger a second wave of flooding — exactly what happened in Nepal this week.

How Common Are Glacial Collapses?

• Glacial collapses remain rare and highly unusual, according to scientists and geologists.
• Only a handful of such events have been documented over the past decade — though Tibet has emerged as a hotspot for some of the world's largest recorded cases.
• Notable past instances include:
  • July 2016, Aru Range, Tibet: A valley glacier's lower section detached, releasing about 68 million cubic metres of ice and killing nine herders and hundreds of animals. A nearby glacier collapsed two months later, releasing about 83 million cubic metres — NASA called it a rare occurrence.
  • 2018, Sedongpu Basin, Tibet: Two ice-rock avalanches spilled debris into the Yarlung Tsangpo River (upper Brahmaputra), blocking it and forming a dangerous temporary lake.
  • November 2022, Bukadaban Feng glacier, Kunlun Mountains: About 40 million cubic metres of ice detached, crashing into a lake below and creating a 13-metre wave. It went unnoticed initially due to the remote location.
  • 2018, Ronti Peak, Uttarakhand, India: About 27 million cubic metres of rock and glacier ice collapsed, killing over 200 people — though this was classified more as a rock avalanche coupled with ice.
  • May 2025, Blatten, Switzerland: A cascade of rock, soil, and ice buried much of the village. Experts linked the collapse to climate change in the Alps.

The Climate Change Question

• Experts noted that climate change in the Himalayas shows increased heatwaves, glacial retreat, and permafrost reduction — all of which can drive landslides and glacial outburst floods.
• Glaciers across the Hindu Kush Himalaya region — spanning Afghanistan, Bangladesh, Bhutan, China, India, Myanmar, Nepal, and Pakistan — lost ice 65% faster between 2011-2020 than in the previous decade.
• Nepal's glaciers alone have lost nearly one-third of their ice volume over roughly three decades, according to a 2023 UN assessment.
• Hydrologists flagged a critical warning-system challenge: the absence of rainfall before such events defeat most operational flood warning systems, which typically rely on precipitation data.
• By the time a mud flow becomes visible, it is often too late to escape — the fast-moving mixture behaves "like liquid concrete."

Conclusion

Glacial collapses remain rare geological events, but their consequences can be catastrophic — as seen in the Nepal-Tibet floods. While scientists resist attributing any single event directly to climate change, the broader pattern of accelerated Himalayan glacial melt raises the risk of such collapses recurring.

This underscores the urgent need for improved seismic and satellite monitoring, early-warning systems attuned to non-rainfall triggers, and greater regional cooperation to protect downstream communities.