Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 260

Landslides and Debris Avalanches
Because of media sensationalism, many types of mass movements are included in the
general term “landslide.” According to geomorphologists, the term refers to movements
where there is a distinct zone of weakness that separates the slide material from stable
underlying material. Sudden, rapid movement of a cohesive mass not saturated with
moisture occurs above this boundary. Translational landslides occur when a displaced
surface moves parallel to the slope angle, and rotational landslides slip and rotate along
a concave stable surface. Debris avalanches involve the catastrophic falling and tum-
bling of rock, debris, snow, ice, and soil from melting or release of ice. The distinction
is difficult to make in practice, however, since sliding, falling, and flowing are gener-
ally all involved to various degrees. For our purposes, they will be considered jointly,
and simply called landslides, recognizing that the movement is not restricted to sliding
alone.
Landslides are the most spectacular form of mass movement (Fig. 5.14). Steep
slopes, great local relief, and sufficient room for cascading rock to accelerate provide
ideal conditions for their formation. Most major landslides occur in the 5 percent of the
steepest land surfaces on Earth where relief is close to the proposed upper strength
limit of the rock; in other areas that lack relief, failures are attributed to soft rocks,
low-angle discontinuities, high rates of fluvial incision, tectonic uplift, and slope loading
(Korup et al. 2007).
Some landslides reach velocities of over 100 m s −1 , moving horizontally for several
kilometers, and even ascend nearby slopes. In the Pamir Mountains of Tajikistan, a
massive landslide blocked the Bartang River, causing a 60 km lake to form (Alford et al.
2000). If the dam breaks, nearly 5 million people could be affected, but with proper lake
monitoring, the potential for a disaster could be averted. Although media propaganda
exaggerated the risk since the likelihood of dam failure was so slight, this illustrates
the potential effects of landslides on human settlements. When examining landslides,
natural and artificial effects, as well as the time scales associated with response and re-
covery, must also be examined (Chang and Slaymaker 2002). Although a landslide may
last only a few minutes, it disturbs and disrupts other systems, such as fluvial processes
at the foot of a slide, which have a longer and potentially greater impact on the system
(Hewitt et al. 2008).