Rock avalanches are a form of hazardous long-runout landslide and leave fragmented deposits of complex sedimentology that, if studied in detail, can provide insight into their emplacement processes. Complexity arises due to the myriad overlapping factors known to contribute to the final deposit fabric, such as source structures, lithology (i.e. material properties), topographic feedback, substrate interaction and emplacement processes (i.e. internal factors), as well as our reliance on (un)suitable exposures. Herein, we present sedimentological data from two carbonate rock avalanche deposits (Tschirgant in Austria and Flims in Switzerland), where changes in lithology can be eliminated from the causal equation due to their largely mono-mineralic composition. We further eliminated the effects of external influences such as topography or substrate interactions by detailed facies mapping of the deposit interior. Since sedimentary properties locally vary within less than 1-m² outcrop area, emplacement processes are the only causes that remain to explain the different fabrics. Characteristic (fractal) grain size distributions of three distinctive sub-facies in the interior of these, and other, rock avalanche deposits—jigsaw-fractured, fragmented, and shear zone facies—can be linked to specific processes acting during emplacement. We suggest that a heterogeneously distributed and progressively increasing particle breakage in the moving granular mass best explains the ranges of fractal dimensions and associated features for the respective sub-facies, from simple breakage along pre-existing planes, through dynamic fragmentation which locally minimises coordination number, to zones of shear concentration. No exotic emplacement mechanisms (such as air-layer lubrication or fluidised substrates) are required to produce these features; continued, heterogeneous degrees of fragmentation of an initially intact source rock best explains the sedimentary record of rock avalanches.

The retreat of glaciers in response to global warming has the potential to trigger landslides in glaciated regions around the globe. Landslides that enter fjords or lakes can cause tsunamis, which endanger people and infrastructure far from the landslide itself. Here we document the ongoing movement of an unstable slope (total volume of 455 × 10⁶ m³) in Barry Arm, a fjord in Prince William Sound, Alaska. The slope moved rapidly between 2010 and 2017, yielding a horizontal displacement of 120 m, which is highly correlated with the rapid retreat and thinning of Barry Glacier. Should the entire unstable slope collapse at once, preliminary tsunami modeling suggests a maximum runup of 300 m near the landslide, which may have devastating impacts on local communities. Our findings highlight the need for interdisciplinary studies of recently deglaciated fjords to refine our understanding of the impact of climate change on landslides and tsunamis.