Gravity Takes Its Toll: Mass Wasting - Geology - page 160

Focusing on the Materials Involved
The relationship between gravity and friction determines when materials move
downslope or become unstable. Whether or not a slope is stable depends on what earth
materials are involved. Soils, loose sediments, and rocks are much less stable than large
sections of bedrock (solid rock). But even bedrock is susceptible to mass wasting under
certain conditions. In this section, I describe how loose materials and bedrock respond
differently to the pull of gravity.
Loose materials: Resting at the angle of repose
In any mass wasting movement, the material moves downslope until it
reaches its angle of repose, at which point it stops moving. The angle of repose is
the angle of the slope at which sediment and rock are stable and won't move
farther downslope. Repose means “to rest temporarily.” When loose sediments are
settled at this angle, for the time being they will not continue downslope; they will
stay in place. However, keep in mind that the repose is temporary. Eventually, the
conditions that support the current angle of repose are likely to change.
The angle of repose is different for different materials. Factors such as the sediment
grain size (I explain grain size and other sediment characteristics in Chapter 7), rough-
ness, and what the slope is made of determine the amount of friction — and, by exten-
sion, the angle of repose. Figure 11-2 illustrates how the angle of repose is different for
small (fine) grains of sand, larger (coarse) grains of sand, and large, angular pebbles.
Figure 11-2: Sedi-
ments of different
grain size have dif-
ferent angles of re-
pose.