Mass Wasting - The Changing Earth: Exploring Geology and Evolution - page 285

◗ Figure 11.11 Rock Slides Rock slides occur when material moves downslope along a generally

planar surface. Most rock slides result when the underlying rocks dip in the same general angle as the
slope of the land. Undercutting along the base of the slope and clay layers beneath porous rock or soil
layers increase the chance of rock slides.

to slide. Percolating water from heavy rains wets subsurface
clayey siltstone, thus reducing its shear strength and help-
ing to activate the slide. In addition, these slides are part of a
larger ancient slide complex.
Not all rock slides are the result of rocks dipping in
the same direction as a hill's slope. The rock slide at Frank,
Alberta, Canada, on April 29, 1903, illustrates how nature
and human activity can combine to create a situation with
tragic results (

along joint planes, killing 70 people and partially burying
the town of Frank.

Flows
Mass movements in which material fl ows as a viscous fl uid
or displays plastic movement are termed fl ows. Their rate of
movement ranges from extremely slow to extremely rapid
(Table 11.2). In many cases, mass movements begin as falls,
slumps, or slides and change into fl ows farther downslope.
Of the major mass movement types, mudfl ows are the
most fl uid and move most rapidly (at speeds up to 80 km/hr).
They consist of at least 50% silt- and clay-sized material
combined with a signifi cant amount of water (up to 30%).
Mudfl ows are common in arid and semiarid environments
where they are triggered by heavy rainstorms that quickly
saturate the regolith, turning it into a raging flow of mud
that engulfs everything in its path. Mudfl ows can also occur
in mountain regions (

Figure 11.13).
It would appear at first glance that the coal-mining
town of Frank, lying at the base of Turtle Mountain, was
in no danger from a landslide (Figure 11.13). After all,
many of the rocks dipped away from the mining valley,
unlike the situations at Point Fermin and Laguna Beach.
The joints in the massive limestone composing Turtle
Mountain, however, dip steeply toward the valley and are
essentially parallel with the slope of the mountain itself.
Furthermore, Turtle Mountain is supported by weak lime-
stones, shales, and coal layers that underwent slow plas-
tic deformation from the weight of the overlying massive
limestone. Coal mining along the base of the valley also
contributed to the stress on the rocks by removing some
of the underlying support. All of these factors, as well as
the frost action and chemical weathering that widened the
joints, finally resulted in a massive rock slide. Approxi-
mately 40 million m 3 of rock slid down Turtle Mountain

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Figure 11.14) and in areas covered by
volcanic ash where they can be particularly destructive (see
Chapter 5). Because mudflows are so fluid, they generally
follow preexisting channels until the slope decreases or the
channel widens, at which point they fan out.
As urban areas in arid and semiarid climates continue
to expand, mudflows and the damage that they create are
becoming problems. Mudfl ows are common, for example, in

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