People in the Mountains - Mountain Geography: Physical and Human Dimensions - page 524

FIGURE 10.19 Accelerated erosion and slope failure, a common consequence of road construction in
the mountains, NH21, Pir Panjal Himalaya, Himachal Pradesh, India. (Photo by J. S. Gardner.)
Volcanic mountains that have a snow and/or glacier ice cover pose an additional haz-
ard and a high level of risk because eruptions may produce lahars (mud and debris
flows) that can extend some distance away from the mountains into densely populated
areas. Attention was drawn to this type of hazard by the 1985 eruption of Nevado del
Ruiz in the Colombian Andes (Sigurdsson and Carey 1986) when a summit area erup-
tion melted snow and ice, generating a lahar that devastated a downstream area for a
distance of 50 km. The greatest number of fatalities, approximately 20,000, was in the
town of Armero, a community at a distance of less than 50 km from the mountains. Sim-
ilar high-risk locations for this type of disaster are found through the western Cordillera
of the Americas, a good example being the Cascadia volcanoes of southwestern British
Columbia, Washington, and Oregon. The glacier- and snow-covered Mounts Hood, Rain-
ier, Baker, and Meager, among others, all with the imprint of prehistoric eruptions and
lahars, loom over the densely populated lower slopes and lowlands, including the great-
er metropolitan areas of Portland, Seattle, and Vancouver (Clague and Turner 2003).
Slope Instability
Mountain topography, being dominated by very steep and long slopes, is prone to slope
instabilities of several types, including large rock avalanches, smaller rockfalls, mass
movements of rock, soil, and vegetative debris, debris and mudflows, and slow soil
creep. Some occur very infrequently or very slowly, and therefore may not be perceived
as important hazards. Others occur frequently in known locations, and are more eas-
ily planned for and mitigated. Many are damaging to people, land, and structures and