What ETH researchers know about the collapse of the Birch Glacier

On Wednesday, May 28th, the Birch Glacier collapsed under the weight of rock and debris from rockfalls on the Kleine Nesthorn. Following an on-site inspection, ETH Zurich researchers explain the background to the collapse that buried the village Blatten in an updated fact sheet.

A collapse of the magnitude of the Birch Glacier, which buried the village of Blatten in Valais on Wednesday, is unprecedented for the Swiss Alps. While a rock-ice collapse at Piz Scerscen in April 2024 did cause a similar amount of ice and debris to fall – estimated at eight to nine million cubic metres – as in the Lötschental valley this week, the collapse in the Engadine region did not result in as much damage as in Blatten.

Daniel Farinotti, Professor of Glaciology at ETH Zurich and the Federal Institute for Forest, Snow and Landscape Research (WSL), asserts that the exact causes of Wednesday’s glacier collapse are far from fully understood. In particular, Professor Farinotti states, it is not yet conclusive whether external factors or primarily reactions inside the glacier itself were decisive in ultimately triggering the collapse. However, a field inspection of the Lötschental break-off area on 1 June 2025 provided the researchers with new insights, which they have included in an updated fact sheet - this summarises the current state of knowledge from the perspective of glacier research.

However, since authorities and researchers have been monitoring the glacier for years, there are clear indications that several rock avalanches and terrain movements on Kleines Nesthorn, the mountain above the glacier, were the primary cause behind the glacier’s collapse.

“We know there were already several rock avalanches before the collapse on Wednesday and that rock debris accumulated on the glacier as a result,” says Professor Farinotti, who has been working with ETH and WSL researchers Matthias Huss and Mylène Jacquemart to compile the fact sheet.

“Our aim is to use this data and our available knowledge to support the authorities, who are doing incredible work in extremely difficult circumstances to maintain an overview of the conditions and improve the situation for the people,” explains Professor Farinotti.

A combination of factors led to the collapse

The accumulated rock debris – estimated by the authorities to exceed 10 million tonnes – significantly increased the pressure and stress exerted on the glacier ice. This, in turn, encouraged the formation of meltwater, both at the base and inside the glacier, leading to a rise in water pressure. Combined with the incoming rain and additional stress from unstable mountain sections, these factors destabilised the Birch Glacier and accelerated the flow of ice, ultimately causing it to break off. The final impetus for the collapse came when a large section of the flank of the Kleine Netsthorn slid off, as the authorities had already communicated.  

The unusual valley movement of the Birch Glacier had already attracted the attention of researchers and authorities some time ago, prompting closer observation of the movements around Kleines Nesthorn.

With glaciers generally retreating throughout Switzerland, ETH researchers – currently documenting glacier developments within the GLAMOS glacier monitoring network together with the universities of Fribourg and Zurich – noticed that the front of the glacier in the lower part of the Birch Glacier had advanced by approximately 50 metres since 2019.

Observational data also shows that ice thickness at the glacier snout advanced by up to 15 metres between 2017 and 2023, while the upper reaches of the glacier thinned. This increase is due in part to the rock debris resting on the glacier, as this prevented the ice from melting at the surface.

Comparison with other ice and rock collapses

Kleines Nesthorn and the Birch Glacier have been under close observation since the 1990s, when two snow and ice avalanches affected part of the local infrastructure. Part of the Birch Glacier’s development can also be seen in external page satellite images.

In the updated fact sheet, the researchers state that a direct link between the permafrost degradation and the collapse of the Birch Glacier has not yet been conclusively established without further investigations. However, it is considered likely that rising temperatures have contributed to the thawing of the subsoil, an increase in rockfalls, and – paradoxically – the unusual advance of the glacier.

The glacier collapse above Blatten has some parallels to the rock avalanche at Piz Cengalo in 2017, when around three million cubic metres of rock fell onto a small glacier, causing it to be partly swept away and triggering a flow of debris. This caused severe damage to infrastructure in the village of Bondo. Eight people were killed during the incident.

On 2 September 2002, the Kolka–Karmadon glacier collapsed in the Russian Caucasus, resulting in 100 million cubic metres of ice travelling up to 19 kilometres down the valley. The avalanche buried the village of Nizhniy Karmadon and claimed 125 lives.

In Valais, systematic surveillance and the precautionary evacuation of the village of Blatten largely prevented such a tragedy. "Our thoughts remain with the community of Blatten and with the incredible work carried out on the ground by the local authorities and other actors," concludes Daniel Farinotti.

This article was first published on 4 June by ETH Zurich.