Unraveling the history of the Brenndalsbreen rock avalanche 

by Jacob Yde, June 4, 2024

Rock avalanches are dramatic and potentially dangerous events where tons of rocks fall from mountain sides onto the landscape below. However, rock avalanches onto glaciers are rare in Norway. A new study by Engen and colleagues explores what happened at Brenndalsbreen when tons of rock fell onto the glacier surface in 2010.

Rock avalanches onto glaciers are characterized by high speeds and long transport distances of the rock debris, as the material slides on the low-friction glacier surface, and in some cases the rock debris continues to the ice-free landscape down-valley before it is finally deposited. Several processes may contribute to the failure of the rock slope. These include pressure-release on the rock slopes as the glacier ice thins and disappears, changes in rock surface temperature from constantly low temperatures when the rock is ice-covered to highly variable diurnal and seasonal temperatures when the rock becomes ice-free, permafrost thaw in the rock slopes, freeze-thaw cycling, and glacial erosional undercutting of vertical rock slopes. Often the actual triggering of the rock avalanche is associated with a heavy precipitation event; other times it just happens suddenly.

The debris-covered front of Brenndalsbreen in September 2021 (photo: J. Yde).

The new study, published in the journal Landslides, provides new insights into what happened at Brenndalsbreen, a regenerated outlet glacier on the western side of Jostedalsbreen ice cap, when 300 000 tons of rock fragments were deposited on the glacier surface. The investigation revealed that the event most likely happened during the spring of 2010, when the glacier surface was snow-covered. The rock was detached from an almost vertical rock slope in the inner part of the valley Brenndalen above the lower part of Brenndalsbreen. The location of the detachment area was supported with modelling simulations of the rock avalanche event. While the rock deposit on the upper part of the glacier rapidly disappeared into the glacier as it became snow and ice-covered, the lower part of the rock deposit remained on the glacier surface. Through the years following the rock avalanche, the rock deposit gradually moved with the ice flow towards the glacier front, where it is currently being deposited onto the ice-free land in front of Brenndalsbreen, as the glacier front is retreating.

At Brenndalsbreen, it is likely a combination of several long-term processes that have destabilized the rock slope since it became ice-free. The triggering event may have a heavy snow and rainfall event in March 2010, which causes many landslides in western Norway.