1. Chapter 12
Mass
Wasting

2. 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

3. Mass wasting and streams sculpt Earth’s surface
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

4. 2) Controls and triggers
MASS
WASTING
2) Controls and triggers
controlling force: gravity
other important factors:
water saturation
over-steepened slopes
vegetation removal
earthquakes
Water fills pore spaces between sediments,
reduces internal resistance, adds weight
Too steep = unstable
angle of repose = steepest stable angle of a slope
Plants add slope stability by
protection against erosion
Strong ground vibrations

5. Angle of repose: Steepest slope that can remain stable
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

6. Downslope force Normal force
Gravitational force which is directed to Earth’s center, it has two components (vectors).
Friction is a force directed opposite to the downslope force.
Downslope force
Normal force
Vector diagram
Angle of dip

7. Downslope force Normal force If down slope force < friction,
rock stays put
Friction
Downslope force
Vector diagram
Normal force
Angle of dip

8. Downslope force Normal force
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.

9. 3) Mass wasting processes
Type of material
Type of motion
Rate of movement
Talus slopes
Soil/regolith -or- rock/bedrock
Fall - free-fall of material
Slide - coherent material slides
along defined surface
Flow - material flows as viscous fluid
Fastest - rock avalanche:
floats (flows) on trapped air
Slowest - creep: inches per year
(or less)

10. 4) Rockslide Blocks of bedrock break loose, slide down slope
MASS
WASTING
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

13. Slow downhill movement of material.
MASS
WASTING
5) Earth creep
Creep
Slow downhill movement of material.
From alternate expansion/contraction of material from
freezing/thawing or wetting/drying

15. 6) Earthflow humid areas hillsides rich in clay/silt slow rates MASS
WASTING
6) Earthflow
humid areas
hillsides
rich in clay/silt
slow rates

16. 7) Debris and mud flow Flow of soil/regolith containing lots of water
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

17. 8) Slump and debris slides
MASS WASTING
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

23. Some MC…. Which of the following statements is false?
MASS
WASTING
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.

24. Some MC….
MASS
WASTING
Which of the following is not an important factor in causing mass movements?
A. amount of water in materials
B. nature of slope materials
C. steepness and instability of slopes
D. temperature of slope materials

25. Some MC….
MASS
WASTING
A hill consisting of loose, dry sand that slopes at the angle of repose and has no vegetation ______.
A. is stable unless oversteepened by excavation
B. may flow if it becomes saturated with water
C. will be more stable if vegetation takes root on the hill
D. all of the above

26. Some MC….
MASS
WASTING
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.

27. Some MC…. Which of the following statements about mudflows is false?
MASS
WASTING
Which of the following statements about mudflows is false?
A. Mudflows tend to move slower than debris flows.
B. Mudflows are most common in semi-arid regions.
C. Mudflows contain large amounts of water.
D. Mudflows can carry large boulders.

28. Some MC….
MASS
WASTING
Telephone poles that lean slightly downhill are a likely result of which of the following processes?
A. creep
B. debris slide
C. earthflow
D. rock avalanche

29. Some MC…. A slump is __________. A. a rock flow B. a rock slide
MASS
WASTING
A slump is __________.
A. a rock flow
B. a rock slide
C. a flow of unconsolidated material
D. a slide of unconsolidated material