Mass Wasting Chapter
Prepared by Iggy Isiorho for
Dr. Isiorho
Mass Wasting
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Mass Wasting
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Mass Wasting – Movement, caused by gravity, in
which bedrock, rock debris, or soil
moves downslope in bulk.
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Landslides – The general term for a slowly to very
rapidly descending mass of rock or
debris.
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Classification of Mass Wasting
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The classification system used here and
summarized in Table 9.1 is based on (1) rate
of movement, (2) type of material, and (3)
nature of the movement.
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Rate of Movement
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A landslide like the one in Peru clearly
involves rapid movement. Just as clearly,
movement of soil at a rate of less than a
centimeter a year is slow movement.
Between these extremes is a wide range of
velocities.
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Type of Material
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Mass wasting processes are usually distinguished on the
basis of whether the descending mass started as bedrock
(as in a rockslide) or as debris. The term debris, as
applied to mass wasting processes, means any
unconsolidated material at the earth’s surface, such as soil
and rock fragments of any size.
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The amount of water (or ice and snow) in a descending
mass strongly influences the rate and type of movement.
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Type of Movement


In general, the type of movement in mass wasting can be
classified as mainly flow, slide, or fall (Fig. 9.1). A flow implies
that the descending mass is moving downslope as a viscous
fluid. Slide means the descending mass remains relatively
coherent, moving along one or more well-defined surfaces. A
fall occurs when material free-falls or bounces down a cliff.
Two kinds of slip are shown in Fig. 9.1. In a translational slide,
the descending mass moves along a plane approximately parallel
to the slope of the surface. A rotational slide involves
movement along a curved surface, the upper part moving
downward while the lower part moves outward.
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Controlling Factors in Mass Wasting
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Gravity

Shear force – In mass wasting, component of
gravitational force that is parallel to
an inclined surface.
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Shear strength – In mass wasting, the resistance to
movement or deformation of
material.
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Water

Water is a critical factor in mass wasting. When debris is saturated
with water (as from heavy rain or melting snow), it becomes heavier
and is more likely to flow downslope. The added gravitational shear
force from the increased weight, however, is probably less important
than the reduction in shear strength. This is due to increased pore
pressure in which water forces grains of debris apart.
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Paradoxically, a small amount of water in soil can actually prevent
downslope movement. When water does not completely fill the pore
spaces between the grains of soil, it forms a thin film around each
grain (Fig. 9.3). Loose grains adhere to one another because of the
surface tension created by the the film of water and shear strength
increases.
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Common Types of Mass
Wasting
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Creep

Debris Flow

Rockfalls and Rockslides
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Creep
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Creep is very slow, downslope movement of soil or unconsolidated
debris. Shear forces, over time, are only slightly greater than shear
strengths.
Two factors that contribute significantly to creep are water in the soil
and daily cycles of freezing and thawing. As we have said, watersaturated ground facilitates movement of soil downhill. What keeps
downslope movement from becoming more rapid in most areas is the
presence of abundant grass or other plants that anchor the soil.
Although creep does take place in year-round warm climates, the
process is more active where the soil freezes and thaws during part of
the year.
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Debris Flow

The general term debris flow is used for
mass wasting in which motion is taking
place throughout the moving mass (flow).
The common varieties Earthflow, Mudflow,
and Debris avalanche are described in this
section.
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Earthflow
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In an earthflow, debris moves downslope as a viscous fluid; the
process can be slow or rapid. Earthflows usually occur on hillsides
that have a thick cover of debris, often after heavy rains have saturated
the soil.
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A landslide may be entirely an earthflow, as in Fig. 9.7A, with debris
particles moving past one another roughly parallel to the slope.
Commonly, however, rotational sliding takes place above the
earthflow as in Fig. 9.7B and Fig. 9.8. This examples is a debris slide
(upper part) and an earthflow (lower part). In such cases, debris
remains in a relatively coherent block or blocks that rotate downward
and outward, forcing the debris below to flow.
Debris Flow
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Solifluction and Permafrost
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Solifluction – Flow of water-saturated debris over
impermeable material.

Permafrost – Ground that remains permanently
frozen for many years.
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Mudflow
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Mudflow – A flowing mixture of debris and water, usually
moving down a channel.
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Mudflows are most likely to occur in places where debris is not
protected by a vegetative cover. For this reason, mudflows are
more likely to occur in arid regions than in wet climates. A
hillside in a desert environment, where it may not have rained
for many years, may be covered with a blanket of loose material.
With sparse desert vegetation offering little protection, a sudden
thunderstorm with drenching rain can rapidly saturate the loose
debris and create a mudflow in minutes.
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Debris Avalanche
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The fastest variety of debris flow is a debris avalanche, a
very rapidly moving, turbulent mass of debris, air, and
water.
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Some geologists have suggested that in very rapidly moving
rock avalanches, air trapped under the rock mass creates an
air cushion that reduces friction. This could explain why
some landslides reach speeds of several hundred kilometers
per hour. But other geologists have contended that the rock
mass is too turbulent to permit such an air cushion to form.
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Rockfalls and Rockslides
Rockfall – Rock falling freely or bouncing down a
cliff.
 Talus – An accumulation of broken rock at the base
of a cliff.
 Rockslide – Rapid sliding of a mass of bedrock
along an inclined surface of weakness.
 Rock avalanche – A very rapidly moving, turbulent
mass of broken-up bedrock.
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