Rock Avalanches
Authors: Davies Tim 
Published: February 2018

Rock avalanches are very large (greater than about 1 million m3) landslides from rock slopes, which can travel much farther than smaller events; the larger the avalanche, the greater the travel distance. Rock avalanches first became recognized in Switzerland in the 19th century, when the Elm and Goldau events killed many people a surprisingly long way from the origin of the landslide; these events first posed the “long-runout rock-avalanche” problem. In essence, the several-kilometer-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 intense research in recent decades this phenomenon 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. Even more intriguing 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 revealed a number of consistencies in deposit and behavioral characteristics: 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 submicrometer size; that many of these fragments are 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 covered by hummocks of a range of sizes; and that, as noted above, runout distance increases with volume. 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; in addition, they can trigger extensive and long-duration geomorphic hazard cascades.

Although large rock avalanches are rare (e.g., in a 10,000 km2 area of the Southern Alps in New Zealand, research showed that events larger than 5 × 107 m3 occurred about once every century), studies to date show that the proportion of total landslide volume involved in such large events is greater than the proportion in smaller, more frequent 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, for example by blocking rivers and forming landslide dams; these either fail, causing large dam-break floods and 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 underestimation of hazard risk and misinterpretation of paleoclimate.

Rock avalanche runout behavior poses fundamental scientific questions, and rock avalanches have important effects on a wide range of geomorphic processes, which in turn pose threats to society. Better understanding of these impressive and intriguing events is crucial for both geoscientific progress and for reducing impacts of future disasters.