Mass-Wasting is defined as the down slope movement of rock and regolith
near the Earth's surface mainly due to the force of gravity.
Mass-wasting is an important part of the erosional process, as it moves
material from higher elevations down to lower elevations where
transporting agents like streams and glaciers can then pick up the
material and move it to even lower elevations. Mass-wasting
processes are occurring continuously on all slopes; some mass-wasting
processes act very slowly, others occur very suddenly, often with
disastrous results. Any perceptible down slope movement of rock or
regolith is often referred to in general terms as a landslide.
However, as we will see, landslides can be classified in a much more
detailed way that reflects the mechanisms responsible for the movement and
the velocity at which the movement occurs.
As human populations expand and occupy more and more of the land surface,
mass-wasting processes become more likely to affect humans. The
table below (taken from your text) shows the impact of mass-wasting
processes on human life over the last century.
Earthquake triggered landslide
Flood triggered landslide
Earthquake triggered landslide
Earthquake related debris avalanche
Mudflow related to volcanic eruption
Earthquake related landslide
In a typical year in the United States, landslides cause over $1.5 billion
in damages and 25 to 50 deaths. In other countries, especially less
developed countries, the loss is usually higher because of higher
population densities, lack of zoning laws, lack of information about
mass-wasting hazards, and lack of emergency preparedness.
Knowledge about the relationships between local geology and mass-wasting
processes can lead to better planning that can reduce vulnerability to
such hazards. Thus, we will look at the various types of mass-wasting
processes, their underlying causes, factors that affect slope stability,
and what humans can do to reduce vulnerability and risk due to
Types of Mass-Wasting Processes
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 such
processes is somewhat difficult. We will use the classification in your
textbook, dividing mass-wasting processes into two broad categories.
Slope Failures - a sudden failure of the slope resulting in
transport of debris down hill by sliding, rolling, falling, or
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 concave-upward 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. Slumps can be isolated or may occur in large complexes
covering thousands of square meters. They often form as a result
of human activities, and thus are common along roads where slopes have
been oversteepened during construction. They are also
common along river banks and sea coasts, where erosion has under-cut
the slopes. Heavy rains and earthquakes can also trigger slumps.
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, vegetation, 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.
Because this process involves the free fall of material, falls
commonly occur where there are steep cliffs. At the base of most
cliffs is an accumulation of fallen material termed talus.
Slides - Rock slides and debris slides result when rocks or debris
slide down a pre-existing surface, such as a bedding plane, foliation
surface, or joint surface (joints are regularly spaced fractures
in rock that result from expansion during cooling or uplift of the
rock mass). Piles of talus are common at the base of a rock slide or
debris slide. Slides differ from slumps in that there is no
rotation of the sliding rock mass along a curved surface.
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, and/or regolith with some water or air. They can be broken into
two types depending on the amount of water present.
- are sediment flows that contain between about 20 and 40% water. As the
water content increases above about 40% slurry flows grade into
streams. Slurry flows are considered water-saturated flows.
- are sediment flows that contain between 0 and 20% water. Note that
granular flows are possible with little or no water. Fluid-like behavior
is given these flows by mixing with air. Granular flows are not
saturated with water.
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 (see figure. 6.4 and
6.5 in your text). These occur in areas where the soil remains
saturated with water for long periods of time.
- these occur at higher velocities than solifluction, with
velocities between 1 meter/yr and 100 meters/hr 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 lobes with an irregular surface consisting of ridges
and furrows (see figure 6.4 in your text).
- these are a highly fluid, high velocity mixture of sediment and
water that has a consistency ranging between soup-like and wet
concrete. They move at velocities greater than 1 km/hr and tend to
travel along valley floors. 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. As we have seen,
mudflows can also result from volcanic eruptions that cause melting
of snow or ice on the slopes of volcanoes, or draining of crater
lakes on volcanoes. Volcanic mudflows are often referred to as
lahars. Some lahars can be quite hot, if they are generated as
a result of eruptions of hot tephra.
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 (see figure 6.9 in your text).
Earthflows - are usually associated with heavy rains and move at
velocities between several cm/yr and 100s 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
(see figure 6.11 in your text)
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.