Mass wasting

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Mass wasting
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What is mass wasting?
Movement of material down a slope Due to
gravity - affects any slope Occurs when shear
stress gt shear strength Sometimes fast,
sometimes slow Affected by presence of water
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Angle of repose
Highest angle at which loose material will be
stable. Here, rock debris makes a shallow fan in
Death Valley
Surface of fan slopes gently
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Some material can hold a steep slope angle.
Other material is less strong and will make
shallow slopes.
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When a slope is steeper than the materials angle
of repose, it is said to be oversteepened.
Oversteepened material will eventually slide
down until it reattains its angle of repose.
This movement may be fast or slow.
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As waves undercut this hillside on the Oregon
coast, it is oversteepened and material tends to
slide down.
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A little water will add strength to slope
materials. More will weaken the slope,
potentially aiding slope failure. In areas prone
to mudslides, the slopes are at the angle of
repose for dry material heavy rains that
saturate the soil can cause it to fail.
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Diagram of features of a landslide
Head
Toe
Scarp fault along which material moved
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A large landslide above a subdivision in La
Conchita, CA. The steep hills of California are
prone to mass wasting. For some reason, people
still consider them to be desirable places to
live.
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This slide damaged I-90 outside Sheridan, WY in
1999.
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Rockfall, probably encouraged by freeze-thaw
cycles
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In 1903, the Frank slide in Alberta, CA, buried
part of the town of Frank
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Lahar (volcanic mudflow) at Mt St Helens
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Hazards of mass wasting
This lahar at Nevado del Ruiz illustrates some of
the dangers of rapid mass wasting. The high
water content of this flow allowed it to move
down-slope rapidly, giving people little time to
get out of its way.
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Thistle Slide, Colorado 1983. The debris dam
caused a destructive flood.
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Landslides typically produce hummocky terrain
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The slump terraces seen in complex craters are a
form of mass wasting. The crater walls formed by
the impact are too steep for the material to hold
up, so material falls downslope to make the slope
shallower. These crater walls are now at the
angle of repose.
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A great deal of mass wasting on Mars takes place
near the boundary between the highlands and
lowlands. Some also occurs in the deep rift
valleys like Valles Marineris.
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Mass wasting is common in the steep-sided valleys
of Martian fretted terrain
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Rockfall and slide deposits in the Martian
fretted terrain. Cracks in the plateau rock
suggest where new failure might occur.
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This large landslide in Valles Marineris has cut
into a crater. Debris and scarps from older
slides are also visible.
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The dark streaks in the right-hand picture
(arrows) may be new landslides on this Martian
crater wall. They are not visible in the image
to the left, taken 21 months earlier.
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Landslides (left) and talus cones (above) on
Venus.