adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Mass-wasting is the down-slope movement of Regolith (loose uncemented
mixture of soil and rock particles that covers the Earth's surface) by the
force of gravity without the aid of a transporting medium such as water,
ice, or wind. Still, as we shall see, water plays a key role. Mass-wasting
is part of a continuum of erosional processes between weathering and
stream transport. Mass-wasting causes regolith to move down-slope where
sooner or later the loose particles will be picked up by another
transporting agent and eventually moved to a site of deposition such as an
ocean basin or lake bed. In order for regolith to move in a mass wasting
process it must be on a slope, since gravity will only cause motion if the
material is on a slope.
Gravity is a force that acts everywhere on the Earth's surface, pulling
everything in a direction toward the center of the Earth. On a flat
surface, parallel to the Earth's surface the force of gravity acts
downward. So long as the material remains on the flat surface it will not
move under the force of gravity.
On a slope, the force of gravity can be resolved into two components: a
component acting perpendicular to the slope, and a component acting
tangential to the slope.
The perpendicular component of gravity, gp, helps to hold the object
in place on the slope.
The tangential component of gravity, gt, causes a shear stress
parallel to the slope and helps to move the object in the down-slope
On a steeper slope, the shear stress or tangential component of
gravity, gt, increases, and the perpendicular component of gravity,
Another force resisting movement down the slope is grouped under the
term shear strength and includes frictional resistance and cohesion
among the particles that make up the object.
When the sheer stress becomes greater than the combination of forces
holding the object on the slope, the object will move down-slope.
Thus, down-slope movement is favored by steeper slope angles (increasing
the shear stress) and anything that reduces the shear strength (such as
lowering the cohesion among the particles or lowering the frictional
The Role of Water
Although water is not directly involved as the transporting medium in
mass-wasting processes, it does play an important role. Think about
building a sandcastle on the beach. If the sand is totally dry, it is
impossible to build a pile of sand with a steep face like a castle wall.
If the sand is somewhat wet, however, one can build a vertical wall. If
the sand is too wet, then it flows like a fluid and cannot remain in
position as a wall.
Dry unconsolidated grains will form a pile with a slope angle determined
by the angle of repose. The angle of repose is the steepest angle at which
a pile of unconsolidated grains remains stable, and is controlled by the
frictional contact between the grains. In general, for dry materials the
angle of repose increases with increasing grain size, but usually lies
between about 30 and 37 degrees.
Slightly wet unconsolidated materials exhibit a very high angle of repose
because surface tension between the water and the grains tends to hold the
grains in place.
When the material becomes saturated with water, the angle of repose is
reduced to very small values and the material tends to flow like a fluid.
This is because the water gets between the grains and eliminates grain to
grain frictional contact.
The down-slope movement of material, whether it be bedrock, regolith, or a
mixture of these, is commonly referred to as a landslide. All of these
processes generally grade into one another, so classification of
mass-wasting processes is somewhat difficult. We will use the
classification used by your textbook, which divides mass wasting processes
into two broad categories and further subdivides these categories.
Slope Failures- a sudden failure of the slope resulting in transport of
debris down hill by sliding, rolling, falling, or slumping.
Sediment Flows- debris flows down hill mixed with water or air.
Slumps - types of slides wherein downward rotation of rock or regolith
occurs along a curved surface. The upper surface of each slump block
remains relatively undisturbed, as do the individual blocks. Slumps leave
arcuate scars or depressions on the hill slope. Heavy rains or earthquakes
usually trigger slumps.
Rock Falls and Debris Falls - Rock falls occur when a piece of rock on a
steep slope becomes dislodged and falls down the slope. Debris falls are
similar, except they involve a mixture of soil, regolith, and rocks. A
rock fall may be a single rock, or a mass of rocks, and the falling rocks
can dislodge other rocks as they collide with the cliff. At the base of
most cliffs is an accumulation of fallen material termed talus. The slope
of the talus is controlled by the angle of repose for the size of the
material. Since talus results from falling large rocks or masses of debris
the angle of repose is usually greater than it would be for sand.
Rock Slides and Debris Slides - Rock slides and debris slides result when
rocks or debris slide down a pre-existing surface, such as a bedding plane
or joint surface. Piles of talus are common at the base of a rock slide or
Sediment Flows Sediment flows occur when sufficient force is
applied to rocks and regolith that they begin to flow down slope. A
sediment flow is a mixture of rock, regolith with some water. They can be
broken into two types depending on the amount of water present.
Slurry Flows- are sediment flows that contain between about 20 and
40% water. As the water content increases above about 40% slurry flows
grade into streams.
Granular Flows - are sediment flows that contain between 20 and 0%
water. Note that granular flows are possible with little or no water.
Fluid-like behavior is given these flows by mixing with air.
Each of these classes of sediment flows can be further subdivided on the
basis of the velocity at which flowage occurs.
Solifluction - flowage at rates measured on the order of centimeters
per year of regolith containing water. Solifluction produces
distinctive lobes on hill slopes . These occur in areas where the soil
remains saturated with water for long periods of time.
Debris Flows-these occur at higher velocities than solifluction, and
often result from heavy rains causing saturation of the soil and
regolith with water. They sometimes start with slumps and then flow
down hill forming to lobes with an irregular surface consisting of
ridges and furrows.
Mudflows- a highly fluid, high velocity mixture of sediment and
water that has a consistency of wet concrete. These usually result
from heavy rains in areas where there is an abundance of
unconsolidated sediment that can be picked up by streams. Thus after a
heavy rain streams can turn into mudflows as they pick up more and
more loose sediment. Mudflows can travel for long distances over
gently sloping stream beds. Because of their high velocity and long
distance of travel they are potentially very dangerous.
Creep- the very slow, usually continuous movement of regolith down
slope. Creep occurs on almost all slopes, but the rates vary. Evidence
for creep is often seen in bent trees, offsets in roads and fences,
and inclined utility poles .
Earthflows - are usually associated with heavy rains and move at
velocities between several cm/yr and 110s of m/day. They usually
remain active for long periods of time. They generally tend to be
narrow tongue-like features that begin at a scarp or small cliff
Grain Flows - usually form in relatively dry material, such as a
sand dune, on a steep slope. A small disturbance sends the dry
unconsolidated grains moving rapidly down slope.
Debris Avalanches - These are very high velocity flows of large
volume mixtures of rock and regolith that result from complete
collapse of a mountainous slope. They move down slope and then can
travel for considerable distances along relatively gentle slopes. They
are often triggered by earthquakes and volcanic eruptions.
Mass-Wasting in Cold Climates
Mass-wasting in cold climates is governed by the fact that water is frozen
as ice during long periods of the year. Ice, although it is solid, does
have the ability to flow, and freezing and thawing cycles can also
contribute to movement.
Frost Heaving - this process is large contributor to creep in cold
climates. When water saturated soils freeze, they expand, pushing
rocks and boulders on the surface upward perpendicular to the slope.
When the soil thaws, the boulders move down vertically resulting in a
net down slope movement.
Gelifluction - Similar to solifluction, this process occurs when the
upper layers of soil thaw during the warmer months resulting in water
saturated soil that moves down slope.
Rock Glaciers - a lobe of ice-cemented rock debris (mostly rocks
with ice between the blocks) that slowly moves downhill
Mass wasting processes also occur on steep slopes in the ocean basins. A
slope failure can occur due to over-accumulation of sediment on slope or
in a submarine canyon, or could occur as a result of a shock like an
earthquake. Slumps, debris flows, and landslides are common.
Triggering of Mass-Wasting Events
A mass-wasting event can occur any time a slope becomes unstable.
Sometimes, as in the case of creep or solifluction, the slope is unstable
all of the time, and the process is continuous. But other times,
triggering events can occur that cause a sudden instability to occur.
Shocks - A sudden shock, such as an earthquake may trigger a slope
instability. Minor shocks like heavy trucks rambling down the road,
trees blowing in the wind, or man made explosions can also trigger
Slope Modification - modification of slope either by humans or by
natural causes can result in changing the slope angle so that it is no
longer at the angle of repose. A mass-wasting event can then restore
the slope to its angle of repose.
- streams eroding their banks or surf action along a coast can undercut a
slope making it unstable.
Exceptional Precipitation - heavy rains can saturate regolith reducing
grain to grain contact and reducing the angle of repose, thus triggering a
mass-wasting event. Volcanic Eruptions - produce shocks like explosions
and earthquakes. They can also cause snow to melt or empty crater lakes,
rapidly releasing large amounts of water that can be mixed with regolith
to reduce grain to grain contact and result in debris flows, mudflows, and
Submarine Slope Failures - these can be caused by rapid deposition of
sediment that does not allow water trapped between grains to escape, or by
generation of methane gas from the decay of organic material, which
increases pressure between unconsolidated grains and thus reduces grain to