Anatomy of the Blatten rock-collapse debris-ice avalanche (28 May 2025): Insights from a local Quaternary geologist

In May 2025, a catastrophic debris-ice avalanche with a volume of approximately 10 million cubic meters led to the complete destruction of the village of Blatten (Valais, Switzerland). Despite tremendous destruction, and due to extensive monitoring and mitigation measures of the authorities, the residents of Blatten could be evacuated in time. The aim of this talk is to review the dynamic processes associated to the catastrophe, and the implications to the population of Blatten.

Accelerated deformation of the rock slope of Kleines Nesthorn around two weeks before the debris-ice avalanche was the first process in a cascade of events. The rock collapse was followed by 1) rock debris accumulating and loading Birch Glacier, 2) glacier collapse followed by two-phase debris-ice avalanche on 28 May 2025, 3) debris deposition as thick as 32 m onto the village of Blatten, 4) river damming in the main valley, 5) lake formation up-valley, and 6) outflow of impounded water and formation of a new river bed. The role of the inherited geology and climate on preconditioning and triggering of enhanced rock-collapse activity with a subsequent debris-ice avalanche is still debated and focus of ongoing research. Kleines Nesthorn, consisting of various metamorphic bedrock types (interlayering of jointed granitic gneisses, amphibolites, and biotite-sericite gneisses), has a complex geologic origin. The exposition and altitude of the rock flanks indicate that those bedrocks very likely were affected by the presence of permafrost. The unfavourable geology in combination with melting permafrost (and increased hydrostatic pressures) are most likely the main causes of the natural disaster. Meteorologic conditions prevailing in May 2025, such as the heavy precipitation on 28 May 2025, most likely were saturating the collapsed debris that was temporarily accumulated on Birch Glacier, resulting in a higher water content of the collapsed debris and partly explaining the runout of the event.

The village of Blatten existed since at least 1433, and through the past centuries, the population of Blatten has learnt to live with the threat of a variety of geohazards. Historic documentation shows that the two most common geohazards are snow avalanches and floodings, with recurrence intervals of 2 and 16 years for the Lötschental Valley. These records, however, lack any documentation about rock avalanches, highlighting the absence of a baseline for this type of hazard. The geohazard map of Blatten has been updated in November 2025, and its results allow to build a safe Blatten 2.0 following a well-defined roadmap, with land for building available in moderated areas, and return of residents by 2029.

The event is unprecedented for the Swiss Alps both in terms of the dynamics of collapse and its devastating impacts, and highlights that disasters can happen even on very low probabilities. Due to timely evacuation and avoided loss of life, financial support from the insurance companies and donations, and specific regulations for the case of Blatten, the mood and willingness to return to Blatten is rather high, indicating that a Blatten 2.0 has a “prosperous” future if time schedules are kept.