Chapter 12
Mass
Wasting
MASS
WASTING
1.
2.
3.
4.
5.
6.
7.
8.
Ch. 12 Mass wasting
Landform development
Controls and triggers
Mass wasting processes
Rock slides
Earth creep
Earth flow
Debris and mud flow
Slump and debris slides
MASS
WASTING
1) Landform development
“Mass wasting”
 downslope movement of rock, regolith, soil
 from force of gravity
 one form of erosion
 follows weathering
 mass-wasting + running water = stream valleys
Mass wasting and streams
sculpt Earth’s surface
MASS
WASTING
2) Controls and triggers
controlling force: gravity
other important factors:
 water saturation
Water fills pore spaces between sediments,
reduces internal resistance, adds weight
 over-steepened slopes
Too steep = unstable
angle of repose = steepest stable angle of a slope
 vegetation removal
Plants add slope stability by
protection against erosion
 earthquakes
Strong ground vibrations
Talus slope, Wheeler Peak,
Great basin Natl Park
Angle of repose: Steepest slope that can remain stable
Sand has a small angle of repose, with increasing grain size and angularity
the angle increases
Moisture means larger angle of repose
Fig. Story 12.1
Gravitational force which is directed to Earth’s
center, it has two components (vectors).
Friction is a force directed
opposite to the downslope force.
Vector
diagram
Downslope force
Normal force
Angle of dip
If down slope force < friction,
rock stays put
Friction
Downslope force
Vector
diagram
Normal force
Angle of dip
If we steepen slope, down slope force > friction, rock
starts sliding.
The angle where that happens is the angle of repose.
Friction
Downslope force
Vector
diagram
Normal force
Angle of dip
Mass movements occur when the force of gravity (meaning
the downslope component) exceeds the strength (meaning
frictional adhesion) of the slope materials.
MASS
WASTING
3) Mass wasting processes
 Type of material
Soil/regolith -or- rock/bedrock
 Type of motion
Fall - free-fall of material
Slide - coherent material slides
along defined surface
Flow - material flows as viscous fluid
 Rate of movement
Fastest - rock avalanche:
floats (flows) on trapped air
Slowest - creep: inches per year
(or less)
Talus slopes
MASS
WASTING
4) Rockslide
Blocks of bedrock break loose, slide down slope
usually where strata are inclined
causes:
Base of slope is undercut
Rain or melting snow lubricates a sliding surface
MASS
WASTING
5) Earth creep
 Creep
Slow downhill movement of material.
From alternate expansion/contraction of material from
freezing/thawing or wetting/drying
MASS
WASTING
6) Earthflow
 humid areas
 hillsides
 rich in clay/silt
 slow rates
MASS
WASTING
7) Debris and mud flow
Flow of soil/regolith containing lots of water
Also called mudflows
Common to semi-arid regions, volcanoes (lahars)
Very hazardous
MASS WASTING
8) Slump and debris slides
 downward sliding of material
 moves as a whole section on curved surface
 usually slow, material doesn’t go far
 often involves multiple blocks
 commonly due to slope over-steepening
Fig. 12.6
MASS
WASTING
Some MC….
Which of the following statements is false?
A. Mass movements occur when the force of gravity exceeds the
strength of the slope materials.
B. Mass movements can be triggered by earthquakes and floods.
C. Materials can move down a slope very slowly or as a sudden,
catastrophic, large movement.
D. Mass movements require wind or running water.
MASS
WASTING
Some MC….
Which of the following is not an important factor in causing
mass movements?
A.
B.
C.
D.
amount of water in materials
nature of slope materials
steepness and instability of slopes
temperature of slope materials
MASS
WASTING
Some MC….
A hill consisting of loose, dry sand that slopes at the angle of
repose and has no vegetation ______.
A.
B.
C.
D.
is stable unless oversteepened by excavation
may flow if it becomes saturated with water
will be more stable if vegetation takes root on the hill
all of the above
MASS
WASTING
Some MC….
What is the difference between a rockslide and a rock avalanche?
A. In a rockslide, the rocks move more or less as a unit, whereas in a
rock avalanche the rocks move independently like a fluid.
B. In a rockslide, the rocks move independently like a fluid, whereas in
a rock avalanche the rocks move more or less as a unit.
C. A rockslide is faster than a rock avalanche.
D. A rockslide involves more debris than a rock avalanche.
MASS
WASTING
Some MC….
Which of the following statements about mudflows is false?
A.
B.
C.
D.
Mudflows tend to move slower than debris flows.
Mudflows are most common in semi-arid regions.
Mudflows contain large amounts of water.
Mudflows can carry large boulders.
MASS
WASTING
Some MC….
Telephone poles that lean slightly downhill are a likely result
of which of the following processes?
A.
B.
C.
D.
creep
debris slide
earthflow
rock avalanche
MASS
WASTING
Some MC….
A slump is __________.
A.
B.
C.
D.
a rock flow
a rock slide
a flow of unconsolidated material
a slide of unconsolidated material