Catastrophic mass movements

Catastrophic mass movements are large and rapid mass wasting events such as landslides, rockslides, and rock avalanches. Although they are often believed to occur with no warning, catastrophic mass movements are often preceded by subtle changes such as rock creep that foreshadow their occurrence. Because of their speed and size, catastrophic mass movements are often fatal events.

One of the most notable catastrophic mass movements to have occurred during recorded times was associated with the May 18, 1980 eruption of Mount St. Helens. Magma movement produced a bulge on the north side of the volcano that failed as a series of three large landslides during a magnitude 5.2 earthquake on May 18, and was immediately followed by the well-known eruption. The volume of the material removed by the landslides is estimated to have been about 2.3 km³. The landslides broke apart as they began to move and traveled downhill as a rock avalanche , which is a common form of catastrophic mass movement .

Other rock avalanches buried the towns of Frank, Alberta in 1903 and Elm, Switzerland in 1881; both of these events were triggered by miners undercutting steep slopes above the towns. In 1963 a large rockslide that traveled into the Vaiont reservoir in Italy produced a wave that overtopped the dam and killed many downstream residents. The Vaiont landslide was triggered by changes in the reservoir level as it was filled and emptied each year after its completion in 1960, which affected the groundwater pressure within the adjacent slopes. Earthquakes can also trigger catastrophic landslides without human intervention, but catastrophic landslides rarely appear to be triggered by rainfall. A notable example of an earthquake-triggered catastrophic landslide during recent times occurred in 1959, when a magnitude 7.5 earthquake triggered a landslide that dammed the Madison River in Montana, killing 26 people and creating Earthquake Lake. The geologic record contains evidence of even more catastrophic events, including prehistoric rock avalanches that involved as much as 20 km³ of rock and traveled tens of kilometers from their points of origin.

One of the most perplexing aspects of catastrophic rock avalanches is that in many cases they begin as normal landslides but travel much longer distances than would be predicted by solving the simple physics problem of rock sliding along rock. The coefficients of friction necessary for rock avalanches larger than 10⁶ m³ to have traveled their observed distances decrease significantly as a function of the avalanche volume. A typical coefficient of friction for one piece of rock sliding past another might be about 0.55, and this is a value calculated for many small rockslides; in large rock avalanches, however, the coefficient of friction necessary to explain the travel distance can be as low as 0.05–0.10. In essence, rock avalanches move as though they are fluids rather than solid masses of rock and often run up the opposing sides of valleys before coming to rest. One early explanation of this phenomenon, based upon studies of the prehistoric Blackhawk landslide in California, was that

rock avalanches glide atop pockets of air trapped beneath the rock mass. Other proposed friction reducing mechanisms have included the frictional heating of water to generate steam that would fluidize the avalanche and the melting of rock to produce a layer of liquid glass along the base of the avalanche. The discovery of rock avalanche deposits on Mars and the Moon , however, cast doubt on mechanisms such as air entrapment and steam generation because neither air nor water would have been available. A process known as acoustic fluidization has been proposed to explain the behavior of rock avalanches without requiring air pockets or steam generation. Acoustic fluidization occurs when elastic waves travel through a rock avalanche as it moves downhill, breaks into pieces, and is jostled by the underlying topography .

Because catastrophic rock avalanches are so rare and short lived, and because their remnants contain little evidence of the dynamic processes that occurred during movement, it is likely that explanations of their unusual mechanical behavior will always be inferences based largely on theoretical possibilities rather than empirical observations.

See also Catastrophism; Debris flow; Lahar