GEO 101 Lecture Notes - Lecture 24: Regolith, Landslide, Mass Wasting
24 Jun 2018
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Types of Mass Wasting
Rockfalls and rockslides. Rockfalls occur when pieces of rock break loose from a
steep rock face or cliff. These result from the rock face being undercut by rivers or wave
action. Frost wedging may also eventually loosen large blocks, causing them to fall. The
accumulation of rock debris at the base of a steep slope is called talus.
Rockslidesusually follow a zone of weakness, such as a bedding plane or foliation
plane. Separation of the rock is more likely along these planes because of their reduced
shear strength. Water also tends to be channeled along these planes, which increases
slippage. Collisions down the slope generally break the rock mass into rubble that
eventually comes to rest. If steep slopes are involved, a fast moving‐ rock
avalanche may result. The rockslide or rock avalanche loses energy and speed as it
moves across more level terrain.
Debris flows. Debris flows are defined as mass wasting events in which turbulence ‐
occurs throughout the mass. Varieties of these are called earthflows, mudflows, and
debris avalanches.
When earth material moves down a hillside as a fluidlike mass, it is called
an earthflow. These flows typically occur in humid areas on steep slopes with thick,
clay rich soil that becomes saturated with water during storms. The earthflow usually ‐
leaves a steep scarp behind where it separated from the hillside. Earthflows can be fast
(a few hours) or slow (a few months). Velocities range from 1 millimeter per day to
meters per day. Intermittent activity can continue for years as the earthflow continues to
settle and stabilize. Earthflows typically have rounded, hilly fronts. A common trigger for
an earthflow is the undercutting of the slope by erosion from wave action or rivers or by
construction projects.
A variety of earthflow called solifluction is the flow of watersaturated earth material
over an impermeable surface such as permafrost. It occurs frequently in bitterly cold
regions such as in Alaska or Canada. Springtime temperatures thaw only the first few
feet of the frozen ground (the active layer), which becomes saturated quickly and slowly
flows over the ever frozen permafrost below. Solifluction can occur on even the gentlest‐
of slopes. Not forceful enough to break apart the surface vegetation, the migrating
material drags it along like a wrinkled green rug. The soil finally settles on level ground
at the base.
A mudflow is a liquidy mass of soil, rock debris, and water that moves quickly down a
well defined channel. Generally viscous and muddy colored, it can be powerful enough ‐
to move large automobiles and buildings. Mudflows occur most often in mountainous
semiarid environments with sparse vegetation and are triggered by heavy rainfall that
saturates the loose soil and sediment. They are also the natural result of volcanic ash
build ups on flanks of volcanoes and of forest fires that have exposed the soil to rapid ‐
erosion. A mudflow originating on a volcanic slope is called a lahar.
The deadliest variety of debris flow is the debris avalanche, a rapidly churning mass of
rock debris, soil, water, and air that races down very steep slopes. It has been theorized
that trapped air may increase the speed of an avalanche by acting as a cushion
between the debris and the underlying surface.
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Document Summary
Rockfalls occur when pieces of rock break loose from a steep rock face or cliff. These result from the rock face being undercut by rivers or wave action. Frost wedging may also eventually loosen large blocks, causing them to fall. The accumulation of rock debris at the base of a steep slope is called talus. Rockslidesusually follow a zone of weakness, such as a bedding plane or foliation plane. Separation of the rock is more likely along these planes because of their reduced shear strength. Water also tends to be channeled along these planes, which increases slippage. Collisions down the slope generally break the rock mass into rubble that eventually comes to rest. If steep slopes are involved, a fast moving avalanche may result. The rockslide or rock avalanche loses energy and speed as it moves across more level terrain. rock. Debris flows are defined as mass wasting events in which turbulence occurs throughout the mass.
