Gravity Takes Its Toll: Mass Wasting - Geology - page 163

Increasing the steepness of a slope increases the pull of gravity, which results in materi-
als moving downslope until they regain the angle of repose. What could change the
steepness of a slope? In many cases, water is the culprit.
Moving water in streams or rivers washes away or erodes sediments at the
base of a slope. This type of erosion is called undercutting, and it creates a much
steeper slope, which can trigger the movement of materials downslope.
Figure 11-3 illustrates how a stream or river washes away sediments that support the
slope. This erosion by stream or river water that leads to mass wasting is called under-
cutting the angle of repose.
Figure 11-3:
Stream erosion,
undercutting the
angle of repose,
leads to mass
wasting.
Is water the only factor that can change the slope angle? No — keep reading to find out
about some groundshaking events that can create the same type of change.
Shaking things up: Earthquakes and volcanoes
Earthquakes and volcanoes shake the ground and displace earth materials from their
stable positions. These groundshaking events may also result in uplift (a change in eleva-
tion due to crustal plate movements; see Chapter 9) or fracturing of the bedrock, thus
changing the slope angle, which may trigger mass wasting.