Scientists Reconstruct the Blatten Disaster — And Discover Why the Giant Rock-Ice Avalanche Traveled So Far

The avalanche descended through a narrow gorge, crossed the valley bottom and ran more than 200 metres up the opposite Weissenried slope.

Deposits on the valley floor reached thicknesses of up to roughly 34 metres, and the debris dammed the Lonza River, creating a lake that flooded parts of the village.

Those numbers alone explain the scale of the disaster.

But they do not explain its extraordinary mobility.

Why Did the Avalanche Travel So Far?

Large landslides and avalanches do not behave like rigid blocks simply sliding down a slope.

Once millions of tonnes of fractured material begin moving, the interaction between rock fragments, ice, water, terrain and enormous impact forces can transform the way the mass flows.

For Blatten, ordinary dry-rock friction was not enough to reproduce what actually happened.

The researchers tested several numerical descriptions of avalanche motion and compared the results with two independent observations:

• the forces reconstructed from seismic waves generated by the avalanche
• the final geometry and distribution of the deposits

Models using friction values typical of dry granular landslides consistently underestimated the observed mobility.

The avalanche simply traveled too far.

The Models Needed Extremely Low Effective Friction

A conventional volume-dependent friction relationship for a dry avalanche of approximately 9.3 million m³ predicts an effective friction angle near 16°.

But simulations using ordinary dry-landslide values could not reproduce the Blatten runout.

Even constant-friction models that pushed the friction angle down to around 8° to 11.8° could approximate parts of the runout but failed to reproduce the full deposit geometry.

The best results came from a model in which friction changed during the avalanche.

In those simulations, the lower-bound effective friction angle fell to approximately:

1°–2° during critical phases of motion.

That is extraordinarily low.

Importantly, the authors do not claim the avalanche literally became frictionless.

The value is an effective bulk friction parameter used by the model. Several real physical processes can produce the same apparent reduction in resistance, and the available observations cannot determine exactly how much each mechanism contributed.

The Avalanche Unfolded in Four Major Stages

The researchers reconstructed the main failure as a sequence of four kinematic stages.

As the mass accelerated away from its source, it entered the narrow gorge below Birch Glacier.

The confined terrain forced the moving debris into complex interactions with the valley walls.

Numerical simulations indicate that part of the flow could even detach temporarily from the terrain as it crossed strongly curved topography.

The front then struck the gorge wall and climbed across it before spreading onto the valley floor.

By about 92 seconds after the main motion began, the avalanche’s center of mass was decelerating and the remaining material transitioned toward slower lateral spreading.

That entire violent sequence lasted only minutes.

The Disaster Began More Than Two Weeks Earlier

One of the most valuable aspects of the study is that it does not begin with the catastrophic May 28 collapse.

The mountain was already sending signals.

Rockfall activity from the Kleines Nesthorn intensified on May 14, 2025, more than two weeks before the main avalanche.

Those events were recorded continuously by regional seismic stations.

The growing instability prompted authorities to evacuate approximately 300 residents of Blatten before the final collapse.

That evacuation was crucial.

When the catastrophic failure eventually occurred, most of the population was already outside the danger zone.

Millions of Cubic Metres of Rock Fell Before the Final Collapse

The seismic record allowed researchers to estimate how much material was already being transferred from the mountain before May 28.

By May 23, approximately 3.75 million m³ of rock had already fallen from the Kleines Nesthorn, according to the seismic reconstruction.

That estimate agrees closely with independent calculations based on changes in digital elevation models.

The cumulative sequence showed an accelerating, S-shaped progression rather than a single isolated failure.

The debris accumulated on Birch Glacier below.

And that created a second problem.

The Falling Rock Loaded and Destabilized Birch Glacier

Birch Glacier became progressively buried beneath millions of cubic metres of rock debris falling from the mountain above.

As that load increased, the glacier itself began moving faster.

Before the final collapse, ice velocities reached tens of metres per day.

On May 27 — one day before the main disaster — a smaller ice avalanche involving several thousand cubic metres detached from the glacier terminus and stopped roughly 400 metres above the Lonza River.

The mountain and glacier were therefore not failing independently.

They had become part of a cascading system:

rockfall → glacier loading → glacier acceleration → rock-ice failure → catastrophic avalanche.

Ice Probably Helped Make the Avalanche More Mobile

About one-third of the total avalanche volume consisted of glacier ice. That makes the Blatten disaster an unusual crossover between a rock avalanche
and the processes involved in snow and ice avalanches.

That matters because mixtures of rock and ice can behave very differently from dry rock.

Experimental and numerical studies cited by the researchers show that adding substantial ice to a granular mixture can reduce its bulk friction.

For an ice fraction close to that estimated at Blatten, ice alone could plausibly lower the effective friction angle significantly.

But the researchers found that ice friction reduction by itself was not sufficient to explain the entire observed runout and deposit pattern.

Something more complicated was likely happening.

Fragmentation May Have Helped the Avalanche Flow

The collapsing rock mass experienced enormous stresses as it fractured, accelerated and repeatedly struck steep, irregular terrain.

Those impacts can shatter large blocks into progressively smaller fragments.

Dynamic fragmentation changes the internal structure of an avalanche and can help increase its mobility.

The models indicate particularly strong frictional weakening during failure, passage through the gorge and collision with the opposite slope — precisely the stages in which severe fragmentation and impact loading would be expected.

Meltwater May Have Reduced Resistance Even Further

Another possible mechanism involves water.

Friction and violent impacts generate heat.

Because millions of cubic metres of glacier ice were mixed into the moving debris, part of that energy could have melted ice during the avalanche.

The researchers estimate that frictional and impact heating may have generated roughly 88,000 to 221,000 m³ of meltwater under their energy-balance assumptions.

Water already present in the source material could have added to that amount.

If water pressures increased within parts of the moving debris, they could have reduced the effective stresses between particles and lowered resistance to motion.

Field surveys shortly after the disaster documented wet avalanche deposits, consistent with the possibility that elevated pore-water pressures contributed to the weakening.

But Scientists Cannot Yet Identify One Single Cause

This is where the scientific nuance matters.

The study does not conclude that one mechanism explains Blatten’s extraordinary mobility.

Instead, several processes may have acted together:

• reduced friction from the large volume of glacier ice
• meltwater generated during rapid motion
• elevated pore-water pressures
• dynamic fragmentation of the rock mass
• segregation and rearrangement of particles within the moving debris

Different combinations of these processes can produce similar effective friction values in numerical models.

The researchers therefore describe the low friction as an effective bulk behavior, rather than proof that one specific physical process dominated the avalanche.

The Avalanche Was So Powerful It Registered Like an Earthquake

The Blatten collapse produced seismic waves that traveled hundreds of kilometres.

The ground motion generated by the avalanche was equivalent to approximately a local-magnitude 3.1 earthquake.

A seismic station around 5 km from Birch Glacier recorded roughly 100 seconds of strong avalanche-generated motion.

These signals became one of the key tools used to reconstruct the movement of the mass.

Instead of merely detecting that something had happened, researchers could use the low-frequency seismic waves to estimate the changing forces generated as millions of tonnes of debris accelerated, struck terrain and slowed.

Seismic Signals Revealed the Disaster Before and During the Collapse

The seismic record is especially important because it captured two very different parts of the event.

Before the catastrophe, it documented the accelerating sequence of rockfalls and smaller glacier failures.

During the main avalanche, it recorded the forces generated by the moving mass.

Combined with high-resolution terrain models and the final deposit, those observations allowed the researchers to test whether simulated avalanches actually behaved like the real one.

That is far more powerful than reproducing runout distance alone.

Why the Blatten Disaster Matters Beyond Switzerland

The study is not only about one Alpine village.

Rock-ice avalanches are becoming an increasingly important hazard in high mountain environments where glaciers are retreating, permafrost is thawing and steep slopes are adjusting to rapidly changing cryospheric conditions.

The authors point to mountain regions from the European Alps to Alaska, the Himalayas and the Andes where complex cascading failures can threaten communities far below unstable slopes.

That makes understanding runout critically important.

Knowing that a slope might fail is only part of the hazard problem.

Emergency planners also need to know:

How far could the debris travel?

At Blatten, ordinary dry-landslide assumptions would have underestimated that distance.

Extreme Mobility Changes the Hazard Zone

This is perhaps the most practical result of the new research.

A large avalanche does not become dangerous only because of its volume.

Its effective friction determines how much of the initial gravitational energy is dissipated and how much remains available to carry the mass farther across the landscape.

If ice, water, fragmentation or other processes dramatically lower resistance, a rock-ice avalanche can travel well beyond the runout expected from a conventional dry-rock model.

For communities beneath glaciers and unstable mountain slopes, that difference can determine whether a hazard remains confined to a steep valley or reaches inhabited terrain.

Blatten Was a Cascading Disaster

The catastrophe also demonstrates why mountain hazards cannot always be separated into simple categories.

This was not merely a rockfall.

It was not merely a glacier collapse.

And it was not merely a landslide.

A destabilizing mountain slope deposited millions of cubic metres of rock onto a glacier.

The glacier accelerated.

Rock and ice then collapsed together.

The resulting avalanche transformed as it moved through steep terrain, possibly generating meltwater and extreme fragmentation while its effective friction dropped.

Finally, the deposits blocked a river and produced flooding.

One instability triggered the next.

More Than Two Weeks of Warning — Then About 100 Seconds of Catastrophe

The timing of the Blatten disaster is extraordinary.

The mountain showed escalating instability for more than two weeks.

Authorities observed enough danger to evacuate the village.

Seismic instruments recorded thousands of individual signals from the developing slope failure.

Then, once the main collapse began, much of the catastrophic motion unfolded in roughly a minute and a half.

That contrast is one of the most important lessons from Blatten:

catastrophic mountain failure can develop slowly — and then move with astonishing speed.

What Scientists Learned From Blatten

The new reconstruction provides one of the most detailed datasets yet for understanding a catastrophic rock-ice avalanche from its precursor phase through final deposition.

It shows that:

• the slope instability accelerated for more than two weeks before failure;
• millions of cubic metres of rock accumulated on Birch Glacier;
• the loaded glacier accelerated dramatically;
• the final collapse mobilized about 9.3 million m³ of material;
• around 3 million m³ of that volume was ice;
• standard dry-landslide friction could not reproduce the observed runout;
• the best models required extreme effective frictional weakening;
• ice, fragmentation, meltwater and pore pressure may all have contributed.

The Most Important Number May Be 1°

The enormous volume of the Blatten avalanche is visually spectacular.

But scientifically, one of the most revealing numbers in the new paper may be much smaller.

One degree.

During some critical phases of the best-performing three-dimensional simulation, the calibrated lower limit of the effective friction angle approached approximately 1°.

That does not mean the avalanche literally lost all friction.

It means that to reproduce what nature actually did, the model had to behave as though resistance within the moving mass had fallen to an extraordinarily low level.

The reason may have been a combination of ice, water, fragmentation and the intense mechanics of millions of cubic metres of debris rushing through a narrow Alpine valley.

The Blatten Avalanche Was Not Just Huge — It Changed How It Moved

That is what makes the new study especially important.

Blatten was not devastating simply because a huge volume of mountain collapsed.

The moving mass appears to have changed its mechanical behavior during the descent.

As friction weakened, the avalanche became far more mobile than a comparable dry granular collapse.

That helped it cross the valley floor, climb the opposite slope and spread across the area where Blatten had stood.

The mountain did not merely collapse.

The collapsing rock, ice and water transformed into an exceptionally mobile flow — and that transformation is what scientists are now beginning to reconstruct.