This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Natural Hazard Science. Please check back later for the full article.
Rock avalanches are very large (> 1 million cubic meters) landslides from rock slopes, which can travel much farther across the landscape than smaller events; the larger the avalanche, the greater the excess travel distance. Rock avalanches first became prominent in Switzerland in the 1800s, when the Elm and Goldau events killed people a surprisingly long way from the origin of the landslide; these events first posed the “long-runout rock-avalanche” problem. In essence, the long runout of these events appears to require low friction beneath and within the moving rock mass in order to explain their extremely long deposits; but in spite of recently intense research, this low friction still lacks a generally accepted explanation. Large collapses of volcano edifices can also generate rock avalanches that travel very long distances, albeit with a different runout-volume relationship to that of non-volcanic events. Compounding the puzzle is the presence of long-runout deposits not just on land, but also beneath the sea and on the surfaces of Mars and the Moon.
Numerous studies of rock avalanches have yielded some consistencies of material and behavior, for example, that little or no mixing of material occurs within the moving debris mass during runout; that the deposit material beneath a meter-scale surface layer is pervasively and intensely fragmented, with fragments down to sub-micron size; that many of these fragments are themselves agglomerates of even finer particles; that throughout the travel of a rock avalanche, large volumes of fine dust are produced; that rock avalanche surfaces are typically hummocky at a range of scales; and that there are definite trends in plots of runout distance against volume from rock avalanches in different environments.
Since rock avalanches can travel tens of kilometers from their source, they pose severe, if low-probability, direct hazards to societal assets in mountain valleys. Compounding this is their effect in the triggering of extensive and long-duration geomorphic hazard cascades.
Although large rock avalanches are rare, magnitude-frequency studies show that the proportion of total volume involved in large events is greater than the proportion in small events, so that a large proportion of the total sediment generated in mountains by uplift and denudation originates in large rock avalanches. Consequently, large rock avalanches exert a significant influence on mountain geomorphology by, for example, blocking rivers and forming landslide dams; these either fail, causing large dambreak floods and intense, long-duration aggradation episodes to propagate down river systems; or remain intact to infill with sediment and form large valley flats. Rock avalanches that fall onto glaciers often result in large terminal moraines being formed as debris accumulates at the glacier terminus, and these moraines may have no relation to any climatic change. In addition, misinterpretation of rock avalanche deposits as moraines can cause serious underestimation of hazard risk and misinterpretation of paleoclimate; for example, a deposit 28 km long in Kyrgyzstan was originally thought to be of glacial origin, but it is now known to be a rock avalanche caused by coseismic failure of a mountain slope.
Rock avalanche runout behavior poses truly fundamental scientific questions, and rock avalanches have important effects on a wide range of geomorphic processes. Better understanding of these awesome events is crucial for both geoscientific progress and for reducing impacts of future disasters.