• Offshore earthquake May
1970
• Triggered rock and ice slide
on 6,700 m Nevado
Huascaran
• Rock and ice slide plunged
nearly 1 km
Make a list…
Mass
Wasting:
•  What do you know about mass
wasting???
•  What controls the “wasting” of “mass”???
•  Examples…are there any nearby?
• Hurricane force winds
ahead of the slide
• 20,000 people in Yungay
and Ranrahirca, Peru
were killed
• The avalanche traveled
over 14 km (9 miles)
An Introduction
Mass wasting and
landform development
Landslide
La Conchita, CA
January 2005
Definition of Mass Wasting
Movement of rocks and soil (debris)
down-slope due to gravity without a
flowing medium.
Controlling Factors
Gravity (g) is resolved into two
components, gn (normal) and gs (shear).
The greater the slope, the greater the
shear force.
•  Mass wasting downslope movement of
rock, regolith, and soil
under the influence of
gravity
•  Role of mass wasting
–  Geologic process that
often follows
weathering
–  Combined effects of
mass wasting and
running water
produce stream
valleys
Forces acting on a rock on a hill slope.
Mass wasting and
landform development
•  Slopes change through time
–  No minimum angle required for mass wasting
–  Most rapid and spectacular mass-wasting events
occur in geologically young mountains
–  Mass wasting and erosional processes slowly
lower the land surface
–  Plate tectonic processes continuously lift the land
If it were not for plate tectonics
the Earth’s surface would
eventually become flat
Resistance to Movement
•  Cohesive strength-the resistance of
an object to move downhill
– Cohesion, stickiness of particles
– Other forces holding a particle in place
(plant roots, cementation)
Movement caused when
gs > g
n
+ cohesion + friction.
A block’s movement will be controlled by
friction at its base.
1
Secondary Controls
Factors that either weaken cohesion
forces or increase downslope force
1.
2.
3.
4.
5.
Saturation of the material
Slope Modification
Over-steepening of the slope
Ground vibrations
Expansion/contraction cycles of soil/
regolith
Saturation of Material
The Role of Water
•  Too much water:
•  Lubricates the
material (decreases
cohesion)
•  Adds weight
(increases downslope
force)
•  Increases pore
pressure (increases
downslope force and
decreases cohesion)
•  Water may act to increase or decrease
cohesive strength
– Dry regolith or soil has little or no cohesion
– Damp regolith or soil is sticky
– Saturated regolith or soil flows easily
Controls and triggers
of mass wasting
The Vaiont
Dam Disaster
•  Engineers did not consult
geologists!
•  Built in 1960
•  265 m tall, crossing Vaiont
Canyon in the Italian Alps
•  Bedrock in canyon dipped
steeply towards the lake
•  Bedrock is highly fractured
limestone with numerous
cavities and clay layers
•  As the reservoir filled the rocks
became saturated and the clays
swollen
The Vaiont
Dam Disaster
–  The mountain started
to moved down into
the lake at 1 cm, then
10-20 cm per day
–  Finally the
mountainside failed,
filling the gorge to 150
m above the original
lake level
–  The lake exploded
over the dam sending
a 90 m wall of water
down the valley, killing
2600 people
•  The steepest angle
material can create
yet remain stable
http://www.eng.ucalgary.ca/CSCE-Students/images/dams/VaiontDam/CrossSection.jpg
Hurricane Mitch, 1998 Central America – devastating mud flows
Angle of Repose
•  Important factors include
–  Oversteepening of slopes –
slope angle
•  Stable slope angle (angle of
repose) is different for
various materials
•  Oversteepened slopes are
unstable
•  Causes: Stream undercutting
a valley wall, waves pounding
against a cliff, silly humans
•  Angle of repose – the
steepest angle at which a
material is stable – 25-45o
(depending on grain shape)
•  Decreased when wet
•  Vegetation
•  Ground vibrations
•  Function of
–  Particle size
–  Particle shape
–  Moisture Content
From http://www.geocities.com/geogsoc2000/Vaiont1.htm
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Slope Modification
•  Removal of Vegetation
–  Roots hold soil/rock
together
–  Plants and trees remove
water from the soil
–  Removal decrease
cohesive force
Controls and triggers
of mass wasting
•  Plants protect soil from erosional effects of raindrops
–  Landslide in Menton, France caused by replacing Olive trees
(deep roots) with Carnations (shallow roots) - 11 dead
•  Building of structures
–  Depending upon the
structure and the location,
either a decrease in
cohesive force &/or
increase downslope force
due to added weight will
occur
Ground Vibrations
•  Earthquakes – decrease cohesion forces
–  Initiates movement (it is easier for a particle to
move than it is to begin to move)
–  Liquefaction induced slides
•  Human induced – blasting for construction,
large equipment, etc.
Controls and triggers
of mass wasting
•  Landslides without triggers
–  Slope materials weaken over
time
–  Random events that are
unpredictable
–  Sacred Falls, Hauula, Oahu
Controls and triggers
of mass wasting
Controls and triggers
of mass wasting
•  Important factors
–  Earthquakes as triggers
•  May cause extensive
property damage
•  Can cause liquefaction –
water saturated surface
materials behave as fluidlike masses that flow
•  1994 Northridge
earthquake caused
thousands of landslides
•  In some places 75% of
slope areas were affected
by landslide
Classification of mass wasting
processes
How would you classify the landslide?
•  Important factors
–  Loss of vegetation
through wildfires may
result in debris flows
– Glenwood Springs,
CO, 1994)
–  Fires may “bake” the
ground, resulting in
increased runoff
Expansion/Contraction Cycles
•  Movement of material due to
–  Wetting and drying cycles
–  Freeze-thaw cycles
Make a list…
•  What do you know about mass
wasting???
•  What controls the “wasting” of “mass”???
•  Examples…are there any nearby?
•  Rocks plunged 150 meters
down a cliff into a pool below
•  10 people dead
•  No trigger – perhaps just long
term weathering
3