The 1997 Mount Munday landslide (British Columbia) and the behaviour of rock avalanches on glacier surfaces

Rock avalanches onto glacier surfaces, involving volumes 1 Mm 3 or more, are common in the glacier environments of NW North America. We analyse the Mount Munday rock avalanche (British Columbia) which occurred in July 1997. It involved the initial movement of 3.2 Mm 3 of granitic gneiss that underwent a high degree of fragmentation as it was emplaced on Ice Valley Glacier as a thin 2.6-km 2 debris sheet. The total height of the path was 850 m, and its length was 4,163 m yielding a fahrböschung of 10°, suggestive of a long runout in relation to volume. Potential energy expended in the movement was calculated as 4.33 × 10 13 J and its specific energy was estimated at 5,204 J/kg. A simulation of the movement using 2D DAN-W and DAN 3D strongly supports the idea that debris sheet geometry (runout and thickness) and behaviour (velocity profile) resulted from movement on a low friction surface (glacier ice). Our analysis of the debris sheet geometry of 23 unconstrained rock avalanches on glacier surfaces in NW North America indicated that the debris sheets are distinct from those in non-glacial environments in that they are (a) longer in relation to volume and (b) more extensive in area in relation to volume. These two effects result in a very thin supra-glacial debris sheet. Using image analysis software, we found that ∼85 % of the initial source rock volume was fragmented to fragment sizes less than 4.7 m 3 in volume during emplacement, and that within the debris sheet, the highest degree of fragmentation is associated with the thinnest debris. In the emplacement of rock avalanche debris sheets on glacier surfaces, the low friction glacier surface drives debris sheet thinning through spreading, which in turn results in the fragmentation of its entire thickness. We thus propose low friction surface-driven fragmentation as a process that contributes to long runout of rock avalanches on glacier surfaces and explains their distinctive debris sheet geometry.