Gravity Takes Its Toll: Mass Wasting - Geology - page 164

Groundshaking events may also shake sediments until they begin to act like li-
quids. This process is called liquefaction. When liquefaction occurs, sediment
particles begin to flow as if they were saturated by water, but no water is involved.
The shaking of the sediments moves each particle away from its surrounding
particles. If the particles aren't touching, no friction exists to compete with grav-
ity, and thus mass wasting occurs.
Removing vegetation
The root systems of plants help to keep loose sediment and rocks in place; they stabilize
the soil. In the absence of vegetation — such as when trees and plants are decimated by
a forest fire — the soil and rocks are more likely to move downslope.
Vegetation may be removed by fires, overgrazing by livestock, or urban development.
Mudslides are common in the winter rainy season in California, especially in areas that
experience fire or extensive grazing during the prior summer. Humans also create condi-
tions for mudslides by clear-cutting forested slopes for timber. In the last decade, Haiti
has experienced devastating mudslides during hurricane season due to its deforested
hillsides. (The wood is used for charcoal fuel.) Devastating flood damage in Haiti is
made worse when followed by mudslides that can bury or wash away homes and
people.
Moving Massive Amounts of Earth,
Quickly
When the pull of gravity overcomes the force of friction, a mass wasting event is
triggered. The term landslide is casually used to refer to any movement of earth materi-
als by gravity. However, geologists use specific names to describe how rock and sedi-
ment move downslope. In this section, I describe the different types of mass wasting
that occur quickly.