Mass wasting movie. Courtesy of Anneberg
, URL <http://www.learner.org/resources/series78.html>. Requires
Windows media Player. Sign in and view#*16 Mass Wasting.
Types and Processes, adapted from July 2004, U.S. Geological
Survey Fact Sheet 2004-3072, Version 1.0. URL: http://nationalatlas.gov/articles/geology/a_landslide.html.
101, U.S. Department of the Interior, U.S. Geological
Survey, Landslide Hazards Program
images, U.S. Department of the Interior, U.S. Geological
Survey, Landslide Hazards Program
||Terms: Mass wasting (mass movement), landslide,
angle of repose, cohesion, shear stress, collluvium, heave,
regolith, debris, earth, flow, slide, slump, topple, creep,
lahar, debris, avalanche, solifluction, gelifluction, permafrost,
active layer, talus
Mass Wasting (mass movement) is the down-slope transfer of rock
and debris by gravity. It is an important slope process and is responsible
for transporting large quantities of sediment into streams. In addition
it's annually responsible for causing millions of dollars worth
of damage. Movement can occur as slow as a few millimeter/year (creep)
to more that a hundred kilometers/hour (avalanche). Moderate to
fast moving mass-wasting phenomena are commonly referred to as landslides.
The force driving mass movement is gravity. The
down-slope component of gravity is determined by the weight of
the material and the angle of the slope or plane
of failure (fig. 1).
|Figure 1. SLOPE FORCES: The force of gravity (Fg)
is resolved into two forces, one parallel to the slope (Fp)
and one normal to the slope (Fn). The normal force acts to hold
material on the slope whereas the parallel force acts to pull
material down the slope. Imagine what will happen to the relative
magnitude of each of these forces as slope angle is increased
Forces that resist movement
1. internal friction, the mechanical resistance between
masses. These factors tend to increase resistance in rock
- Rock: roughness along internal surfaces, such as joint or
- Sediment: grain angularity, size, shape, sorting and consolidation
2. Cohesion (shear strength related to how well
the material is bonded)
- Rock: influenced by lithology, degree of fracturing and weathering
- Sediment: influenced by clay and water content, and cementing
agents, such as calcite or iron oxide
In summary when the driving forces (e.g. shear stress)
overcome resisting forces (e.g. shear strength)
the slope will fail. The
maximum angle that a material will hold without failure is called
it's angle of repose. Sand has an angle of repose
of around 35° whereas a bedrock cliff may have an angle of
repose of nearly 90.
- Shear stress: the down-slope component of gravity
- Shear strength: the normal component of gravity
+ frictional resistance + cohesion
- Slope Related
- Heave: ratchet-like movement resulting
from expansion and contraction
- Flow: deformation takes place throughout
the moving body
- Slide: movement occurs along planar surfaces
(translational) with little internal shearing of
the moving mass.
- Slump: Rotational slide
- Fall: Material falls through the air
- Topple: Rotational fall where
the top moves faster than the base
- Lateral Spread: lateral extension
resulting from failure of underlying weak zone
- Surface Related (many may not consider
this a mass wasting phenomenon)
- Subsidence: vertical movement
of the surface resulting from removal of underlying
support. Common in karst regions and in areas where
water or oil is being pump.
Figure 2. Heave. Ice formation pushes pebbles and
sediment upward perpendicular to the slope. When
the ice thaws the sediment contracts vertically downward
resulting in a slight displacement downslope.
||Figure 3. Earthflow in a thick regolith-mantled
slope in California.
- fast(km/hr) or slow (mm/year) rates of movement
3. Composition (debris, earth, rock, ice)
- Debris: regolith where 20-80% is greater than 2mm
- Earth: regolith where 80%<2mm
4. Water content
Water content can range from dry to saturated. In
contrast to streams where sediment is carried by water, mud and
debris flows are as viscous as concrete.
1. Slow movement
- soil or rock creep is
accomplished by heave or flow. Because the upper layers
move faster than the layers beneath creep will
cause the down-slope tilting of anything, such as a tree
(fig. 4), post, or monument, embedded in the soil.
- solifluction is the
shallow flow of a saturated layer over an impermeable, typically
frozen zone. The
concentration of water along the surface of the impermeable
layer facilitates flow. Solifluction or gelifluction (fig.
6) occurs in permafrost or periglacial regions
where the saturated active layer,
which melts during the summer months, flows over the still
zone. Affected slope surfaces are covered
with small overlapping flows called solifluction
In tropical regions a similar phenomenon occurs when
an impermeable horizon rich in clay and iron-oxides becomes
2. Moderate to fast moving mass-wasting events (Landslides)
- Earthflows are
slow pasty flows in soil or regolith(Fig. 3)
- Slumps form when regolith
fails along an arcuate rupture. The various
elements of a slump include the headscarp,
the head, which is rotated counter
clockwise into the slope, the body of
the slide that moves along the rupture, and the toe. The
toe is an earthflow that extends beyond the slide rupture
onto the adjoining lower slope.
- Soil, rock and debris slides occur
along an inclined glide plane, usually a soil horizon,
bedding plane, unconformity, or joint surface.
- Rock fall is
the vertical release of debris down a steep slope (figs.
5 and 7). Rock falls are comon where slopes have been
oversteepened by fluvial or glacial activity. Talus (fig.
the slope of accumulated rocky debris that forms at cliff
- Avalanches are extremely fast
flows that are often initiated by a fall or slide that
breaks up and accelerates. Collisions between particles
keeps the flow in motion.
- Mud and debris flows are
saturated flows typically confined to valleys. Debris
flows affecting the Los Angeles Basin travel down canyons
during floods. They are capable
of carrying large boulders and even vehicles.
- Lahars (volcanic
mudflows) are a special variety of mud flow composed
of saturated volcanic ash. Saturation can be
caused by preciptation during storms (e.g. Pinatubo,
1992) or by the melting of glaciers (e.g. Mt. St. Helens,
1980) commonly capping stratovolcanoes.
Figure 4. Surface creep is evidenced by the down-slope bending
of tree trunk. (Click to enlarge)
Figure 5. Rockfall of massive sandstone in Arches
National Park. (Click to enlarge)
|Figure 6. Solifluction (gelifluction) lobes covering
an Alaskan slope in the Tanana River Basin. The saturated
active layer flows over the permafrost zone.
|Figure 7. Rock fall from the jagged peaks of the Picos
De Europa, Northern Spain. The debris slope is called
talus, which can take the form of a fan or apron.
|A slope can be considered to
be in one of three stability states. A number of conditions,
such as those listed below, may set the stage for destabilization
(preconditioning and preparatory factors) or initiate failure
(triggers). Sustaining factors determine the behavior of the
|Figure 7. Stability states of slopes and destabilizing
factors. Modified from Croszier(2004)
Factors that reduce slope stability
- Removal of lateral or underlying support: Over-steepening,
undercutting, solution (Boulder,
Colorado landslide )
- Weakening of slope materials by weathering (fracturing, solution,
alteration to clay, etc.)
- Structure: inclined joints, cleavage, or bedding plane that
dip in the same direction as the slope
- Removal or change in vegetative cover: Preventing
slope failure (Anaheim)
- Addition of mass: Overloading a slope
influence water content and vegetative cover (Wildfires
and debrisflows in Southern CA)
- Water content: can increase normal stress or decrease it depending
on the degree of saturation. (Aberfan
disaster, South Wales) An increase in porewater or seepage
pressure will often cause a slope to fail.
- Vibration: earthquakes, traffic, etc. (1970
Ancash earthquake) - decreases internal strength
Factors 1-6 often set the state for failure. Factors 7 and
8 are the most common triggers.
- Tension cracks
- Tilted trees
- Lack of vegetation
- Poor soil development
- Hummocky or lobate topography
- Blocked drainage
Go to Kelso
Landslide site and view the Japanese video clip. URL:
http://www.nwgeoscience.com/kelso/. See if you can
answer the following questions:
- What kind of mass-wasting event is represented.
- Can you determine the rate of movement? If so
what is it?
- Do you notices any preconditioning factors that would
lead the the slope's instability?
- What evidence is there that the slope is unstable.
See also Features
that may Indicate Catastrophic Landslide Movement, Anaheim
Landslide Site, URL:http://anaheim-landslide.com/features.htm
- Underground mining
- Solution of underlying bedrock
- Over pumping of aquifers or oil well fields
Additional sites to explore:
- California Geological Survey Landslide site,
Movements on Venus :Preliminary Results from Magellan Cycle
I Observations, Michael C. Malin, Malin Space Science Systems,
Journal of Geophysical Research, v. 97, No. E10, 16337-16352,
1992. Copyright 1992 by the American Geophysical Union, URL:
- USGS Landslide
Hazards site, URL: http://landslides.usgs.gov/>USGS
Landslides in the News http://landslides.usgs.gov/recent/ls_news.php
Bloom, Arthur. 1998, Geomorphology, A systematic analysis of Late Cenozoic
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Elsevier, Amsterdam, p. 157-181.
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*Clark, M.J., and Small, R.J., 1982, Slopes and weathering: Cambridge
University Press, Cambridge, England, 112 p.
*Dalrymple, J.B., et al, 1968, An hypothetical nine unit landsurface
Easterbrook, Donald J., 1993, Surface Processes and Landforms:
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Mayer, Larry, 1990, Introduction to Quantitative Geomorphology:
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of Geological Sciences/Salem