Mass Movements
Mass movement- The down slope movement of
earth material under the influence of gravity.
Figure 10.3
Key role in mass movement is gravity
In order for the material to move down slope, it
must overcome
1. Friction –surface resistance to relative
motion.
2. Inertia – tendency of an object at rest to
remain at rest.
Internal Causes of Slope Failure
Inherently weak earth materials
1.
Clay – most earth failures in the
soil zone are due to clay.
Clay layer in a sedimentary rock beach cliff
2.
Pyroclastic materials – Light colored tuff from
Santorini produced 3500 year ago in Minoan eruption.
3.
Shale – sedimentary rock made of clay
and silt particles
Bedding planes, friend or foe?
Internal problems caused by water
1. Adds weight to a slope
8.34 lb/gallon
54 gal. = 457.36 lb!
54 gal.
barrel
2.
Saturation increases the pore-water pressure
Low pore water pressure
High pore water pressure
3.
Dissolves the cementing material in rock
4.
Causes “piping”, the subsurface erosion of
earth material
The Vaiont Dam Failure
Vaiont, Italy 1963
Setting: Dam on river in a glacially carved
canyon.
Some Factors Leading to Slope Failure
1.
2.
3.
4.
Valley is a syncline
Cavernous weathering of limestone rock
Valley downcutting by river
Slope saturation - water from reservoir and heavy rains.
Massive
landslide and
~3,000 dead.
Falls
Occurs when one or
more rocks fall down
steep slopes.
Rockfall,
South
Island,
New
Zealand
Moonlight
Beach,
Encinitas
Yosemite 1996. 162,000-ton rock mass fell.
Yosemite
rockfall
1996
A common cause of rock fall is frost wedging.
Talus cones
Slide
Occur when a more or less coherent mass
moves down slope.
Rotational slide- slump
1. Curved failure plane.
2. Head moves down,
toe moves up and
out.
Don’t move far or fast
In California, triggered by heavy winter rain.
Pacific Palisades, CA
Another cause of slumps – road cuts.
Elks Lodge near Lewiston, Idaho
Translational slide –
landslide
1. Mass moves
down and out.
2. Moves far and
fast.
3. Primary causes:
heavy rains or
earthquakes.
Landslide on Big Sur Coast
Madison River Canyon Landslide 1959
Madison River Canyon from top of slide
View from debris dam and visitors center
Flow
Mass loses coherence and moves as a viscous fluid.
Earthflow- Downslope movement of a fluid layer
of mainly soil and weathered rock material
that is saturated with water.
Earthflows
often begin
as slumps.
Usually, like
slumps, they
do not move
very fast.
Road cut caused slump/earthflow
Debris flow- Downslope movement of a fluid
mixture of mud and larger rock debris.
Venezuela 1999
Key differences between earthflows and debris
flows:
1. A debris flow moves faster; meters/second.
2. A debris flow carries larger rock material.
Debris flows can carry large boulders
because their density is 1.5 – 2.0 times
that of water.
http://www.youtube.com/watch?v=8mKC3eID0
74
Causes of debris flows:
1. Most commonly heavy rains or rapid snow
melt.
2. Volcanic eruptions – a lahar is a type of
debris flow.
3. Earthquakes
Debris flow Alps 2004
Debris flow after rains in an area previously
hit by wildfires in Utah.
Debris flow in San Bernardino after 1980
wildfires
Amago Creek debris flow after Oct. 2007 fires
Debris Avalanche- A very fast moving debris
flow occurring on steep slopes.
La Conchita
1995
slump/earthflow
2005 debris
avalanche
• Debris avalanches
http://www.youtube.com/watch?v=QOUH4da
O4-4
http://www.youtube.com/watch?v=BKTG58Bp
ziw&feature=fvwrel
Small mass movements usually move
horizontally less than twice their vertical
distance of fall.
With very large mass movements they may
move up to 25 times the distance of their
vertical fall.
Sturzstroms- Long-run out debris flows.
Blackhawk sturzstrom 300 million m3
Drop .75 mi.
Flow 5.6 mi.
Nevado Huascaran
1970 subduction zone
earthquake
100 million m3 of rock, ice
glacial sediments, and
water.
Speed 175 - 210mph
Devastated area 8.7 mi2
18,000 deaths
What kind of movement is this?
• http://www.youtube.com/watch?v=ZVYGJYnJT
i0&feature=related
Landslide Thistle, Utah 1983
Landslide 1903 in Frank, Alberta