On May 28, 2025, the Birch Glacier detached from its bed, triggering a massive ice-rock avalanche that destroyed
the village of Blatten in the Swiss Alps. This collapse was initiated by the overload of rock and debris on the glacier,
resulting from several days of continuous rock slope failures from Kleines Nesthorn. In this study, we use the finite-
element code Elmer/Ice to investigate the mechanisms that lead to the glacier’s failure. A first diagnostic simulation
reveals that the rock overload alone is sufficient to cause an increase of basal shear stress beyond levels typically
sustainable by hard bedrock across large portions of the glacier, even in the absence of basal water pressure. We
then conduct prognostic simulations using a rate-and-state friction law that accounts for transient cavity opening,
allowing us to model stress redistribution from tangential friction to normal force on larger scale bed topography,
and from overstressed areas to zones where basal shear can initially support the full local driving stress. By
testing a range of Coulomb friction coefficients C, we find that for lower values of C (C ≲ 0.4), this redistribution
results in rapid acceleration of the entire glacier, reaching velocities of the order of 10 m/day or more within a few
days. Additionally, we show that when basal shear cannot fully accommodate the driving stress, the imbalance is
compensated by a sharp increase in compressive normal stresses at the bed, which tend to concentrate upstream of
medium-scale (∼ 30 m) bedrock undulations, while ice tends to detach from the bed on the lee sides of these bumps.
Once a critical area of the glacier base becomes detached, the overall force balance can no longer be maintained,
ultimately resulting in the collapse of the glacier.