Geology Reference
In-Depth Information
Sometimes when earth materials move through mass wasting, they move very slowly. In
the previous section, I describe fast mass wasting movement, which occurs at the rate
of meters per second or faster. In this section, I describe slow mass wasting, measured
in millimeters per year.
Soil creep is so slow it can't be observed; it can be measured only over a peri-
od of time.
Soil creep
occurs as small amounts of soil are shifted downslope by the
pull of gravity. Figure 11-6 illustrates that the upper layers of soil usually move
more quickly than the deeper layers, so objects like trees, utility poles, or grave-
stones that are stuck in the soil begin to tilt downslope. The movement is so slow
that trees will readjust their direction of growth and develop bent trunks in re-
sponse to the creeping soil.
Figure 11-6:
As soil
creep occurs, ob-
jects in the soil be-
gin to tilt downhill.
In regions where the ground freezes in the winter, soil creep moves a little faster.
Solifluc-
tion,
or
soil flow,
occurs when the ground freezes for part of the year. In the summer, the
upper layers of soil thaw while the deeper layers stay frozen (this deep frozen layer is
called
permafrost
). The upper layers then become saturated with water because the wa-
ter can't be absorbed by the deeper, frozen soils. The saturated layer is prone to move-
ment due to all the water weight and reduced friction between particles.
