Geoscience Reference
In-Depth Information
Wet
conditions
3. Mudflow
2. Earthflow
1. Solifluction
5. Debris flow
4. Slump
6. Debris slide
Dry
conditions
8. Rockfall
7. Soil creep
Slow speed
Fast speed
Figure 14.16 Major processes and forms associated with mass wasting. Movements to the right of this figure are relatively fast,
whereas those to the left are relatively slow. Movements near the top of this figure are triggered by wet conditions, such as heavy rain; those
near the bottom of the figure can occur under dry conditions.
environmental settings and individual mass-wasting processes.
These processes are subdivided on the basis of:
most common in places where extensive rock outcrops occur on
steep hillslopes or canyon walls (Figure 14.17a). Frost wedging
and large temperature changes contribute significantly to rockfall
because these processes gradually split outcropping rocks apart
from one another. Eventually, the gravity threshold is passed and
stones and boulders break free to fall onto the lower part of the
hillslope. Rockfall also occurs in association with differential
weathering in places where relatively resistant rocks such as lime-
stone and sandstone overlie softer rocks like shale. In these situ-
ations, the shale surfaces gradually retreat into the cliff through a
combination of weathering and erosion, leaving the resistant rock
as a ledge that juts out over the shale. Sooner or later the force
of gravity exceeds the strength of the rock and it collapses. This
process is very common in places like the Grand Canyon.
Extensive deposits of fallen rock, called talus (or scree ),
often accumulate through rockfall. These deposits can collect
in relatively small amounts along the base of individual, small
hillslopes or in larger quantities along high cliffs. Typically,
1. The kinds of Earth materials being moved, such as rock or
regolith, and their association with the presence of water.
2. Their physical properties, including whether they are
hard, plastic, or fluid.
3. The kind of motion; for example, whether the material is
falling or flowing.
4. How quickly the process occurs.
The following discussion focuses on the processes outlined in
Figure 14.16.
Rockfall
The easiest kind of mass-wasting process to envision is rock-
fall. As the name implies, rockfall occurs when rocks fall quickly
down a hillslope under the force of gravity. Rockfall is usually
Rockfall A mass-wasting process in which rocks break free
from cliff faces and rapidly tumble into the valley below.
Talus A pile of rock fragments and boulders that accumulates
below a cliff due to rockfall.
 
 
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