GEOG210N132017 - Mass Wasting final.ppt

Topic 11 - Earth Surface Process:
Mass Wasting
 What is Mass Wasting?
 Causes and Triggers of Mass Wasting
- Role of Water
- Degree of Slope
- Vegetation Cover
- Type of Clay
- Earthquakes

Mass Wasting
 Classification of Mass Wasting Processes:
- Types of Materials
- Types of Motions
- Rate of Motion:
=> Rapid Forms of Mass Wasting:
- Slump
- Rockslide
- Rockfall
- Debris Flow/Mudflow/Lahar
- Earthflow

Mass Wasting
 => Slow Movements:
- Creep (soil Creep)
- Solifluction
- Permafrost
 Mass Wasting and Landforms

What is Mass Wasting?
 One end-product of weathering is called
regolith:
 According to Ehlen (2005),
regolith is the weathered
loose material lying above
the unaltered bedrock
 These loose materials are easier to move by
geomorphic processes or denudation agents
(i.e., mass wasting, running water, wave action,
glacier, wind and others) to produce landforms

Mass Wasting: A Major Hillslope Process

 Mass wasting, also called Mass Movement,
involves the downslope movement of loose
weathered materials (regolith) in bulk or
individually under the influence of gravity
 Though regolith wetness may increase the rate of
movement, it does not require a physical
transportation medium ( like water, glacier,
wind) to move materials
 The rate of movement may be very slow and
imperceptible to very fast moving, like during a
rock avalanche

 Some mass wasting processes (mud flow, lahar,
landslides, rock avalanche, etc.) are sources of
geologic or natural hazards
 When human settlements
are located on or near steep
hillslopes, they become
exposed to this form of
natural hazards
 Over 20 lives are lost yearly
in the 50 states of the U.S.
due to mass wasting

United States Map of Landslide Potentials

 In the United States of America, over 60% of all
mass wasting hazards occur along:
- mountainous hillsides in the Pacific Northwest
triggered by heavy rainfall or earthquakes
- over steepened coastal California cliffs
triggered by strong coastal waves
- Seattle area volcanic mudflows (Lahars)
 Fewer mass wasting hazards occur:
- in the East on Appalachian hillslopes and
- in the Central Plains on steeper river bluffs on
the Mississippi and other major river systems
 Least hazard areas occur on the plain & Florida

 The least mass wasting hazard zones in the
country occur on the low hillslopes in the:
- Central Plains
- Florida and adjacent Atlantic and Gulf
Coastal Plains.
 The global Landslides map indicates that areas
with high topographic slopes are also areas of
high mass wasting risks

Causes of Mass Wasting – Force of Gravity
 The most important driving force of mass
wasting is the force of gravity acting to pull
hillslope regolith downslope
 Total force of gravity (Fg) resolves into 2 forces:
- normal force (Fn) (resisting force or stress)
- parallel force (Fp) (shear force or stress)

 Fp is related to total force of gravity (Fg)
as follows:
Fp = Fg sinø
where Fg = weight in pounds or kilograms
ø = slope angle
 As slope angle changes, Fp will vary from
zero (stable slope) on a horizontal surface
to a maximum Fp (unstable slope) on a
vertical slope

 However, local conditions or factors of the
hillslope may work in synergy to increase the
shear stress due to gravity
 They include the:
- removal of lateral support by human actions
- removal of underlying support by rivers
undercutting hillslope base
- loading of slope with rain water or equipment
- lateral pressure and transient stresses

Other Factors Contributing to Increasing Shear Stress Due To
Gravity
Removal of Lateral Support
Through Undercutting or
Slope Steepening
Erosion by rivers & glaciers,
previous rock falls or slides,
construction works
Removal of Underlying
Support
Undercutting by rivers or waves,
subsurface solution, loss of strength
by extrusion of underlying sediments
Loading of Slope Weight of rain water, vegetation,
accumulated debris, weight of army
tanks or construction equipment
Lateral Pressure Water in cracks, freezing in cracks,
hydration of swelling clay
Transient Stresses Earthquakes, movement of trees in
wind

Causes of Mass Wasting – Resisting Forces
 According to Coulomb (1773), slope failure (like
mass wasting) occurs when shear stress (s) (due
to gravity) is large enough to overcome the
resisting forces of cohesion and frictional
resistance of slope materials
 Hence:
s = c + Sn tanø
Where:
s = shearing stress
c = cohesion
Sn = normal stress
ø = angle of internal friction or shearing
resistance

 Thus, the second group of causes of mass
wasting is the resisting forces aimed at
preventing slope materials to be pulled
downslope by gravity
 The resisting forces include:
=> Normal force or stress (Fn or Sn )
=> Shear strength of slope materials

 Normal force (Fn ) is related to Fg as
follows:
Fn = Fg cosø
where Fn is an indication of frictional resistance
 Hence, Fn (frictional resistance of slope
materials) is at a maximum on a horizontal
surface (stable slope) and zero on a vertical
slope (unstable slope)

 On the other hand, the shear strength of slope
materials are enhanced by the:
=> cohesion of slope materials by the:
- chemical bonding of rock & soil particles
- presence of cementing materials of regolith
- capillary cohesion in partially moist soils
(e.g. silt and clay soils) causes film of
water to be drawn over particles with the
resulting negative pore-water pressure
producing more adhesion

=> inherent frictional properties of
slope materials such as:
- particle size distributions, shape and
arrangement
- number of contact points
- crushing resistance

 However, local conditions and factors on
hillslopes work to reduce the resisting forces
(i.e., shear strength) of slope materials
 They include:
- weathering effects
- changes in pore-water pressure
- changes of structure
- organic effects, among others

Factors Contributing to Reduce Resisting Forces of slope
Material (Slope Strength)
Weathering Effects Disintegration of granular rocks,
hydration of clay minerals,
dissolution of cementing
minerals in rock or soil
Changes in Pore-water
Pressure
Saturation, softening of material
Changes in Structure Creation of fissures in shale and
clays, remoulding of sand and
sensitive clays
Organic Effects Burrowing of animals, decay of
tree roots

 In conclusion, slope failures resulting in mass
wasting occur when the resisting forces (shear
strength) of slope materials are less than its shear
stress due to gravity
 Based on slope stability, hillslopes often exist in
one of three states:
- stable slope: shear strength > shear stress (or
safety factor > 1.3)
( i.e. shear strength divided by shear stress)
- actively unstable slope: shear strength <
shear stress (or safety factor < 1)

- conditionally stable slope: failure occurs when
shear strength changes temporarily (or safety
factor 1-1.3)
 Mass wasting tends to occur under the state of
actively unstable or conditionally stable slopes or
when safety factor is generally less than 1.3

Causes of Mass Wasting – Trigger Factors
 Based on field data, the four most
important triggers of mass wasting are:
- Role of water
- Degree of slope
- Type of clay
- Vegetation cover and
- Earthquakes (play different roles
in creating downslope movements)

 Role of Water in Mass Wasting:
- Heavy rains/rapid snow melt saturate
weathered regolith and help to trigger
mass wasting
- Saturation reduces internal resistance
of materials and making the materials
to move easily
- Water adds weight to the materials
causing it to slide or flow downslope

 Role of Degree of Slope:
- Over steepened slopes tend to collapse
or move easily by gravity
- Unconsolidated particles of sand or
coarser materials tend to assume a
stable slope called the angle of repose
- The angle of repose is the steepest angle
at which material remains stable

Mass Wasting: Angle of Repose of Slope
Materials

 Role of Degree of Slope:
- If the degree of slope is increased, the
slope materials come under increasing
force of gravity and causing slope
instability and verse versa
 Role of Vegetal Cover:
- Plant roots hold the soil in place
- They protect soil against erosion and
contribute to slope stability

- When anchoring vegetation is removed
by forest fire or farming or
construction work, surface materials
frequently move downslope, especially
in semi-arid areas
 Role of Earthquakes:
- earthquake and its after-shocks can
dislodge enormous volume of rock and
debris and important in sudden release
of rocks to cause rock avalanche

 - Massive slide can be triggered by
earthquakes
- Earthquake could cause liquefaction of
sand with some water

Main Types of Mass Wasting
 Several methods of mass wasting classification.
The simplest is based on rate of movement that
groups mass wasting into two, Slow - Moderate and
Rapid Mass Wasting and further subdivided based
of type of movement:
 Slow/Moderate Mass Wasting:
- Creep
- Solifluction
- Rotational Slides or Slump (Moderate)
 Rapid Mass Movement:
- Falls or Avalanche
- Slides
- Flows

 A three dimensional grouping, by adding type of
material moved, yields a larger list of mass wasting
 Thus, a list including three factors, (i) rate of
movement, (ii) type of movement and (iii) type of
materials moved, looks like the following:
=> Slow/Moderate Mass Wasting:
- Creeps:
• Soil creep
• Earth Creep
• Talus Creep
- Solufluction
- Rotational Slide (Slump) (moderate rate)

=> Rapid Mass Wasting:
- Falls:
• Rockfalls
• Debris fall
• Rock avalanche and
• Debris Avalanche:
- Slides:
• Debris Slides
• Rockslides
• Rotational Slides or Slump
- Flows:
• Earthflows
• Debris Flow or Mudflows or Lahar

Types of Mass Wasting Based on
Rate of Movement and Amount of Water

Slow-Moderate Mass Wasting
 Slow - Moderate Mass Movement:
- Creep
- Solifluction
- Rotational Slides or Slump (Moderate)

SLOW/MODERATE
MASS WASTING
PROCESSES
- Creep
- Solifluction
- Slump

Slow Mass Movement: Creep
 It is a slow imperceptible downhill
movement that includes:
- soil creep (Heave)(finer materials)
- rock creep
- talus creep (coarser materials)
 movement consists of numerous minute
discrete movements of slope materials
under the influence of gravity

 rate of movement is slow (0.1 mm to 10
m/yr) depending on:
- slope angle
- susceptibility of the materials
- intensity of the processes
- water content
 movement may extend up to about 20 cm
below the surface but rate of movement
decreases with depth

 movement is aided by heaving of the ground
(expansion and contraction) caused by:
- freezing and thawing
- wetting and drying
- or, other volumetric
changes
 mechanisms of soil creep include:
- differential expansion-contraction
- displacement of particles by organisms
- downhill release of particles by weathering

Mass Wasting in Cold Climate: Soil Creep

 Evidences of creep include:
- tilting of surface objects like fence
posts, tombstones, retaining walls
- curvature of trees and
- bending of rock strata downslope
- formation of terracettes (i.e., step-like
ridges along the hillside

Soil Creep Effect: Curvature of Tree Trunk

Soil Creep Effect: Curvature of Tree Trunk Base

Soil Creep Effect: Tilting Fencepost

Nearly Vertical Sedimentary Strata Bent in
Downslope Direction

Slow Mass Movement: Solifluction
 Solifluction is a type of earth flow found in
periglacial regions underlain by
permafrost
During the summer the surface layer of
permafrost melts creating a water-
saturated layer that becomes mobile
 The underlying frozen ground acts as a
sliding plane along which the mass of soil
can slowly move down slope over

 It moves as imbricate tongues, lobes or
sheets
 Movement is most rapid in the center and
slower near the lateral margins
 It moves at a rate of about 1-10 cm/week
 In the Yukon Plateau, it is as a major
gradation process called equiplanation

 According to Eakin(1916), Russell
(1933), Peltier (1950), solifluction could
result in the high-altitude planation of
mountain ranges into flattened summit
areas in a process called altiplanation

Solifluction Lobes and Terraces, Lewis Hill,
Newfoundland

Solifluction Lobes and Terraces, Colorado
Rocky Mountain National Park

Solifluction Lobes and Terraces

Solifluction: The Permafrost Problem
 In the polar regions of the world, the ground
remains frozen throughout the winter season
and the upper top layers thaws in summer
 This poses a serious
challenge to
construction engineers
 For example: Heating
melts the permafrost
causing land subsidence/
building collapse

Above-Ground Alaska Petroleum Oil Pipeline
Why is the pipeline above
ground when transporting warm
petroleum oil product?

Moderate Mass Wasting - Rotational Slide or Slump
 Slump or rotational slide involves a downward
sliding of a mass of regolith moving as a single
unit along a curved surface of rupture

Moderate Mass Wasting: Rotational Slide or Slump

Slump curves
Rotational slide surface

 Slumping leaves behind a crescent-shaped or
cliff scarp created at the head and the block’s
upper surface becomes tilted backward
 The slump block moves downslope
 Slump occurs when slope is over steepened
 This may happen when anchoring materials at
the base is removed making materials above to
become unstable and reacts to the pull of gravity

 Earthflows frequently form at the base of the
slump

RAPID MASS WASTING
PROCESSES
- Rock Falls
- Slides
- Flows

Types of Mass Movements
 Rapid Mass Wasting:
- Rockfalls and Debris Avalanche
- Slides:
 Debris Slides
 Rockslides
 Rotational Slides or Slump
- Flows:
 Earthflows
 Debris Flow or Mudflows
 Lahars (volcanic mudflows)

Earthquake Triggered Landslide (2001),
Santa Tecla, El Salvador

Earthflow Caused by Infiltrating Septic Tank and
Lawn Irrigation Water, Palos Verdes Hills, CA
Sedimentary Earth Materials with buildings slipped downslope
slowly (70 feet in 3 years) as infiltrating waste water lubricated the
slippery clay layer underneath – A Major Human Factor

Mass Wasting: Rapid Rock Falls

Earthflow that Destroyed Some Houses
at La Conchita, CA in 1995

Rapid Mass Movement: Lahars
(Hot Volcanic Mudflows)
Lahars are volcanic mudflows formed by:
-debris avalanches mixing with snow and
ice melt water
-pyroclastic materials mixing with
rainwater or with surges produced by dam
failures or with natural river water
Lahars with 20-60% sediment are
turbulent or smooth flowing if sediment
content rises to over 80%

Mudflow and Lahar on Mount St. Helen

Number of Lahars Caused By Different
Volcanic Events

Lahars (Volcanic Mudflows)
Flow velocity may range from 1meter
per second to over 40meters per second
Rate of down valley movement of
lahars depends on:
-valley width and slope
-flow volume
-grain size composition
Lahars cause people, cities &
structures to be buried

Pinatubo Mt., Philippines
Volcanic Lahars (1991)

Mudflow Covering the Garage Door

Mass Wasting and Landforms
 Weathering weakens and breaks massive
country rocks into smaller fragments (clay, silt,
sand, pebbles to large rock boulders)
 Mass wasting moves these hillslope fragments
(regolith) downslope by gravity alone
 But when mass wasting combines with actions of
running water (rivers), wind, glacier, ocean
wave and groundwater, different landforms are
produced

 For example, when a stream cuts down its
channel floor alone without the help of mass
wasting, it produces narrow channels with
vertical walls in sandstone
formation in semi-arid to
arid environments
 (Example: Zion Narrows
of the Virgin River,
Southern Utah)

 Expanded Grand Canyon Valley Walls:
weathering and mass wasting working together
has greatly expand the Colorado River channel
width even in an Arid/Semi-Arid Region where
mass wasting is
highly limited
 Humid areas with
more mass wasting
activities tend to
have wider or more
open river valleys

Channel Walls of Grand Canyon Expanded
By Mass Wasting

 Typical hillslope elements are defined and
dominated by different types of mass wasting
and slope wash processes
Source: Richard J. Huggett (2011)

 According to Gilbert (1909), convex slope
segment of the hillslope is the result of soil creep
 The cliff section underneath it called the fall face
segment is dominated by rapid mass wasting
called rock falls
 The straight mid-slope
below the cliff section
is called the talus slope
built with rock falls
or screes often with
high repose slope

Landform of Mass Wasting: Talus Slope

Landforms of Mass Wasting: Talus Slope of Screes

Examples of Mass Wasting Hazards

 Over a long period of time, steep mountain
slopes are gradually reduced to a more gentler
and subdued slopes.

 Mass wasting is an important earth surface
geomorphic process responsible for the long
term evolution of hillslopes
Source: Richard J. Huggett (2011)

Review Questions for Mass Wasting
1. In the evolution of many landforms mass-wasting
is the step that precedes weathering.
A. True B. False
2. Saturating the pore spaces of weathered debris with water
will usually decrease the likelihood of downslope
movement.
A. True B. False
3. Which statement regarding debris flows is NOT true?
A. debris flows may be caused by heavy rains
B. In hilly areas debris flows follow canyons and
stream valleya
C. debris flows create talus slopes
D. debris flow can move huge boulders and trucks

4. When and where is solifluction common?
A. rainy season in the tropics
B. dry season in subtropical deserts like the Sahara
C. summer monsoon season in India
D. summer in northern Alaska
5. Slump describes the very slow, downhill movement of
soil.
A. True B. False
6. This mass-wasting process is most frequently associated
with hillsides in humid regions during times of heavy
rains or snowmelt.
A. earthflow B. rockslide
C. unloading D. solifluction

7. This term is used to describe material that slides
downslope as a unit along a curved surface.
A. debris flow B. slump
C. lahar D. solifluction
8. One of the primary causes of this mass wasting process is
the alternate expansion and contraction of surface
material caused by freezing and thawing or wetting and
drying.
A. rockslide B. solifluction
C. creep D. debris flow
9. Which of these mass-wasting processes is slowest?
A. slump B. rockslide
C. debris flow D. solifluction

10. How do freezing, thawing, wetting, and drying
contribute to creep?
A. soil becomes much weaker when dry and
frozen
B. gravity exerts a much stronger force ehen soil is
wet and thawed
C. eventually these processes trigger sudden slides
D. the soil expands and contracts, lifting and
dropping particles a slight distance downslope
11. The steepest angle that a pile of dry unconsolidated
particles can sustain before moving downslope is its
angle of __________.
A. repose B. talus C. stability
D. retention

12. The transfer of rock material downslope under the direct
influence of gravity is referred to as __________.
A. weathering B. mass wasting
C erosion D. deformation
13. A debris flow composed mostly of volcanic materials is
called a (an) __________.
A. earthflow B. solifluction lobe
C. lahar D. slump
14. When __________ occurs, a crescent-shaped scarp
(cliff) is created at its head.
A. rockfall B. slump C. creep
D. debris flow

15. This diagram illustrates which mass-wasting
process?
A. slump B. rockslide
C debris flow D. solifluction

16. Curvature of tree trunk close to the base, as
shown in this photo, is a strong evidence of
this type of mass wasting: _________.
A. solifluction B. lahar
C. soil creep D. debris flow

17. Cliff profiles contain the following
distinctive segments:
A. a fall face segment
B. a talus straight slope segment
C. a concave slope
D. A and B
18. The inherent frictional properties of slope
materials are related to:
A. Particle size of slope materials
B. Particle shape
C. Crushing resistance
D. Particle arrangement
E. All of the above answers

19. A slope in which the shear strength of slope
materials is greater than shear stress or with
a safety factor greater than 1.3 is said to be:
A. A stable slope
B. Actively unstable slope
C. Conditionally stable slope
D. A fall face slope
E. None

20. Which of the following statements is not
correct about mass wasting?
A. It is a downslope movement of slope
materials in response to gravitational
stress
B. It does not require any physical medium
such as water, glacier or wind to
accomplish downslope movement of
materials
C. Occurs when shear strength of slope
materials is less than the shear stress

D. Soil creep, solifluction and rock falls
are good examples of mass wasting
processes
E. Actions of running water, wind, and
glacier are required to accomplish
mass wasting processes
21. High-altitude planation of mountain ranges
into flattened summits by solifluction
processes is called:
A. Peneplanation B. Etcplanation
C. Pediplanation D. Altiplanation

22. The rate of movement of soil creep is a
function of:
A. Slope angle
B. Susceptibility of slope materials
C. Water content
D. All of the above answers
23. A landform resulting from the free fall of rock
materials is the:
A. rock glacier B. peneplain
C. talus slope D. alluvial fan
E. Mudflow

24. Soil creep results from:
A. the slow movement of soil organisms
B. chemical reactions between the regolith
and bedrock materials
C. changes in the soil volume
D. Disturbance of soil on a slope
E. the drying out of fine particles on a slope
25. Slumping is different from other forms of
earthflow or mass wasting in that:

A. only gentle slopes are required
B. it involves some backward rotation
C. more water is needed than in other
forms of flow
D. lubricating water is unnecessary
E. it occurs only on very steep slopes
26. Which of the following is the most fluid type
of mass movement?
A. earthflow B. mudflow
C. Solifluction D. slump

27. The normal force (Fn) is an indication of
frictional resistance
A. True B. False
28. The following equation Fn = FgCosØ shows
that Fn is at a zero on a horizontal surface and
at a maximum on a vertical surface or slope
A. True B. False
29 In dry slopes, pore-water pressure is zero or
negative causing adhesion of slope materials
A. True B. False
30. The above-ground Alaskan pipeline
transporting warm petroleum oil product

cannot be transported through underground
pipeline in this region because:
A. It is a forbidden practice in the region by
the local people
B. the ground is permanently frozen
C. the warm oil will melt the permafrost
and induce land subsidence and the
collapse of the pipeline
D. the petroleum oil product must be kept at
the same temperature throughout the
transporting period
E. A and D.

GEOG210N132017 - Mass Wasting final.ppt

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