Geography Reference
In-Depth Information
Specific causes of landslides are usually divided into two groups: (1) internal con-
dition of the rocks and (2) external factors affecting slopes. The first group includes
such factors as weak rock formations, steeply dipping rock with bedding planes, joints,
fault zones, and steep slopes; the second group includes climatic factors, erosion, and
other types of disturbances, such as earthquakes (Howe 1909). It is difficult to point
to any single factor as being responsible for a landslide. The Turtle Mountain slide at
Frank, Alberta, is a good example of the interaction of several slide-causing factors. The
mountain stood 940 m above the valley and was composed of massive limestones over-
thrust across softer sandstones and shales. This framework formed a natural zone of
weakness, further exaggerated by coal mining in an interlying seam, which decreased
the cohesiveness of the material. Finally, several earthquakes had disrupted the area
over the previous two years. The precise trigger that caused the mountain to collapse
is unknown, but this combination of factors surely led to its eventual release. Over
30,000,000 m
3
of rock buried the town and railroad and killed 70 people. However, only
part of the mountain fell. The northern shoulder still stands as an ever-present threat to
the citizens of the rebuilt town. The mountain may stand for centuries with no further
problems, but may only require a slight trigger to release another tremendous land-
slide.
With statistics, remote sensing, and GIS, landslide hazard analysis has been im-
proved. Guzzetti et al. (2005) used a temporal sequence of aerial photographs, stat-
istics, and morphological, lithological, and land-use data to predict the location, fre-
quency, and size of landslides. Ayalew and Yamagishi (2005) used a logistic regression
to produce a landslide susceptibility map of central Japan and found that lithology,
bedrock, slope, lineaments, aspect, elevation, and road networks played major roles in
occurrence and distribution. Logistics regression with stepwise backward procedures
produces lower error rates and best generalizations for landslide prediction (Brenning
2005). Electrical resistivity field measurements can help discern discontinuities
between landslide materials and underlying bedrock, to help understand areas that may
reactivate (Lapenna et al. 2005). Airborne laser altimetry is effective at differentiating
landslide components and activity (Glenn et al. 2006).
Features of Mass Wasting
Many surface forms result from the aforementioned mass wasting processes. Some oc-
cur as specific features, such as stone stripes or solifluction lobes, while other less dis-
tinct features are simply identified as various sorts of deposits such as mudflow or land-
slide deposits. Two features deserve special mention because of their distinctive char-
acter and importance in the alpine landscape: talus and rock glaciers.
Talus
Talus is an accumulation of rock debris of various sizes transported from a mountain
valley wall by gravity, rain-wash, snowmelt, or avalanching snow (White 1981). It results
primarily from rocks breaking off and falling until they come to rest to form a ramp or
rock apron (Fig. 5.16). Talus accumulations are best developed above treeline, where
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