1. 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

2. Topic 11 - Earth Surface Process: 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

3. Topic 11 - Earth Surface Process: Mass Wasting
=> Slow Movements: Creep (soil Creep) Solifluction Permafrost
Mass Wasting and Landforms

4. 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 =>

5. Mass Wasting: A Major Hillslope Process

6. What is Mass Wasting?
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

7. What is Mass Wasting?
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
1995 Earthflow Hazard at La Conchita, CA

8. United States Map of Landslide Potentials

9. What is Mass Wasting?
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

10. What is Mass Wasting?
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

12. 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)

13. Causes of Mass Wasting – Force of Gravity
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

14. Causes of Mass Wasting – Force of Gravity
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

15. 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

16. 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

17. Causes of Mass Wasting – Resisting Forces
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

18. Causes of Mass Wasting – Resisting Forces
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)

19. Causes of Mass Wasting – Resisting Forces
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

20. Causes of Mass Wasting – Resisting Forces
=> inherent frictional properties of slope materials such as: particle size distributions, shape and arrangement number of contact points crushing resistance

21. Causes of Mass Wasting – Resisting Forces
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

22. 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

23. Causes of Mass Wasting – Resisting Forces
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)

24. Causes of Mass Wasting – Resisting Forces
- 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

25. 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)

26. Causes of Mass Wasting – Trigger Factors
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

27. Causes of Mass Wasting – Trigger Factors
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

28. Mass Wasting: Angle of Repose of Slope Materials

29. Causes of Mass Wasting – Trigger Factors
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

30. Causes of Mass Wasting – Trigger Factors
- 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

31. Causes of Mass Wasting – Trigger Factors
- Massive slide can be triggered by earthquakes - Earthquake could cause liquefaction of sand with some water

32. 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

33. Main Types of Mass Wasting
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)

34. Main Types of Mass Wasting
=> 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

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

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

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

38. 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

39. Slow Mass Movement: Creep
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

40. Slow Mass Movement: Creep
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

41. Mass Wasting in Cold Climate: Soil Creep

42. Slow Mass Movement: 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

43. Effects of Soil Creep

44. Soil Creep Effect: Curvature of Tree Trunk

45. Soil Creep Effect: Curvature of Tree Trunk Base

46. Soil Creep Effect: Curvature of Tree Trunk

47. Soil Creep Effect: Tilting Fencepost

48. Nearly Vertical Sedimentary Strata Bent in Downslope Direction

49. 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

50. Slow Mass Movement: Solifluction
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

51. Slow Mass Movement: Solifluction
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

52. Solifluction Lobes and Terraces, Lewis Hill, Newfoundland

53. Solifluction Lobes and Terraces, Colorado Rocky Mountain National Park

54. Solifluction Lobes and Terraces

55. Patterned Ground: Stone Rings

56. 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

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

59. 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

60. Moderate Mass Wasting: Rotational Slide or Slump

61. Moderate Mass Wasting: Rotational Slide or Slump

62. Moderate Mass Wasting: Rotational Slide or Slump
Slump curves
Rotational slide surface

63. Moderate Mass Wasting: Rotational Slide or Slump
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

64. Moderate Mass Wasting: Rotational Slide or Slump
Earthflows frequently form at the base of the slump

65. RAPID MASS WASTING PROCESSES
- Rock Falls - Slides - Flows

66. 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)

67. Rockfall

68. Rockfall

69. Mass Wasting: Landslide

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

71. Landslides

72. Mass Wasting: Earthflow

73. 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

74. Mass Wasting: Rapid Rock Falls

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

76. Mass Wasting: Mudflows

77. 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%

78. Mudflow and Lahar on Mount St. Helen

79. Number of Lahars Caused By Different Volcanic Events

80. 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

81. Pinatubo Mt., Philippines Volcanic Lahars (1991)

82. Mudflow Covering the Garage Door

83. MASS WASTING AND LANDFORMS

84. 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

85. Mass Wasting and Landforms
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)

86. Mass Wasting and Landforms
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

87. Channel Walls of Grand Canyon Expanded By Mass Wasting

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

89. Mass Wasting and Landforms
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

90. Landform of Mass Wasting: Talus Slope

91. Landforms of Mass Wasting: Talus Slope of Screes

92. Talus Slope of Scree

93. Examples of Mass Wasting Hazards

94. Mass Wasting and Landforms
Over a long period of time, steep mountain slopes are gradually reduced to a more gentler and subdued slopes.

95. 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

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

97. REVIEW QUESTIONS

98. 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

99. Review Questions for Mass Wasting
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

100. Review Questions for Mass Wasting
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

101. Review Questions for Mass Wasting
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

102. Review Questions for Mass Wasting
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

103. Review Questions for Mass Wasting
15. This diagram illustrates which mass-wasting process?
A. slump B. rockslide C debris flow D. solifluction

104. Review Questions for Mass Wasting
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

105. Review Questions for Mass Wasting
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

106. Review Questions for Mass Wasting
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

107. Review Questions for Mass Wasting
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

108. Review Questions for Mass Wasting
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

109. Review Questions for Mass Wasting
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

110. Review Questions for Mass Wasting
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:

111. Review Questions for Mass Wasting
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

112. Review Questions for Mass Wasting
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
Because:

113. Review Questions for Mass Wasting
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.
Because: