Variations in runout of rock avalanches controlled by fragmentation, not basal friction

Rock avalanches are large rockslides consisting of highly fragmented materials that display exceptionally long runouts, which are found to correlate with their volume. Such volume-dependent runouts are conventionally attributed to dynamic lowering of the effective basal friction. However, even for similar volumes, the runouts are seen to span several orders of magnitude suggesting additional controlling factors. Here, we perform analogue models of fragmenting rockslides and compare them to natural rock avalanches. We show that for a given low basal friction, the runout of rock avalanches varies over two orders of magnitude and is determined by their degree of fragmentation. The fragmentation is observed to cause spreading, but also to increased mechanical interactions between fragments. Consequently, the runout’s dependence on fragmentation appears to be determined by the competition between spreading and internal friction. This shows that variation in degree of fragmentation can explain the large variation of runout of rock avalanches.