Mass Wasting

Assignments:

  1. film Earth Revealed: Mass wasting movie. Courtesy of Anneberg Media, URL <http://www.learner.org/resources/series78.html>.  Requires Windows media Player.  Sign in and view#*16 Mass Wasting.
  2. Landslide Types and Processes (Verions 1.1 pdf), Lynn Highland, U.S. Geological Survey Fact Sheet 2004-3072, URL: http://pubs.usgs.gov/fs/2004/3072/pdf/fs2004-3072.pdf
  3. Landslide Images (LSH) - What's going on?
Online Resources

Landslide Types and Processes, adapted from July 2004, U.S. Geological Survey Fact Sheet 2004-3072, Version 1.0. URL: http://nationalatlas.gov/articles/geology/a_landslide.html.

Landslide 101, U.S. Department of the Interior, U.S. Geological Survey, Landslide Hazards Program
URL: http://landslides.usgs.gov/learningeducation/ls101.php

Landslide images, U.S. Department of the Interior, U.S. Geological Survey, Landslide Hazards Program
URL: http://landslides.usgs.gov/learningeducation/historical.php

wikiglobe
Terms: Mass wasting (mass movement), landslide, angle of repose, cohesion, shear stress, collluvium, heave, regolith, debris, earth, flow, slide, slump, topple, creep, lahar, debris, avalanche, solifluction, gelifluction, permafrost, active layer, talus

Mass Wasting (mass movement) is the down-slope transfer of rock and debris by gravity. It is an important slope process and is responsible for transporting large quantities of sediment into streams. In addition it's annually responsible for causing millions of dollars worth of damage. Movement can occur as slow as a few millimeter/year (creep) to more that a hundred kilometers/hour (avalanche). Moderate to fast moving mass-wasting phenomena are commonly referred to as landslides. 

What causes material to move down slope?

The force driving mass movement is gravity.  The down-slope component of gravity is determined by the weight of the material and the angle of the slope or plane of failure (fig. 1).  

 
Figure 1. SLOPE FORCES: The force of gravity (Fg) is resolved into two forces, one parallel to the slope (Fp) and one normal to the slope (Fn). The normal force acts to hold material on the slope whereas the parallel force acts to pull material down the slope. Imagine what will happen to the relative magnitude of each of these forces as slope angle is increased or decreased.

Forces that resist movement

1. internal friction, the mechanical resistance between masses.  These factors tend to increase resistance in rock and sediment

    • Rock: roughness along internal surfaces, such as joint or bedding planes
    • Sediment: grain angularity, size, shape, sorting and consolidation

2. Cohesion (shear strength related to how well the material is bonded)

    • Rock: influenced by lithology, degree of fracturing and weathering
    • Sediment: influenced by clay and water content, and cementing agents, such as calcite or iron oxide

In summary when the driving forces (e.g. shear stress) overcome resisting forces (e.g. shear strength) the slope will fail.   The maximum angle that a material will hold without failure is called it's angle of repose. Sand has an angle of repose of around 35° whereas a bedrock cliff may have an angle of repose of nearly 90.

  • Shear stress: the down-slope component of gravity
  • Shear strength: the normal component of gravity + frictional resistance + cohesion

Basic Classification of mass wasting phenomena:

1. Mechanism of movement

  • Slope Related
    • Heave: ratchet-like movement resulting from expansion and contraction
    • Flow: deformation takes place throughout the moving body
    • Slide: movement occurs along planar surfaces (translational) with little internal shearing of the moving mass.
    • Slump: Rotational slide
    • Fall: Material falls through the air
    • Topple: Rotational fall where the top moves faster than the base
    • Lateral Spread: lateral extension resulting from failure of underlying weak zone
  • Surface Related (many may not consider this a mass wasting phenomenon)
    • Subsidence: vertical movement of the surface resulting from removal of underlying support. Common in karst regions and in areas where water or oil is being pump.

Figure 2. Heave.  Ice formation pushes pebbles and sediment upward perpendicular to the slope.  When the ice thaws the sediment contracts vertically downward resulting in a slight displacement downslope.

earthflow Figure 3. Earthflow in a thick regolith-mantled slope in California.

2. Speed

  • fast(km/hr) or slow (mm/year) rates of movement

3. Composition (debris, earth, rock, ice)

  • Debris: regolith where 20-80% is greater than 2mm
  • Earth: regolith where 80%<2mm

4. Water content

Water content can range from dry to saturated.  In contrast to streams where sediment is carried by water, mud and debris flows are as viscous as concrete. 

Types of mass wasting phenomena

1. Slow movement

  • soil or rock creep is accomplished by heave or flow.  Because the upper layers move faster than the layers beneath creep will cause the down-slope tilting of anything, such as a tree (fig. 4), post, or monument, embedded in the soil.
  • solifluction is the shallow flow of a saturated layer over an impermeable, typically frozen zone. The concentration of water along the surface of the impermeable layer facilitates flow. Solifluction or gelifluction (fig. 6) occurs in permafrost or periglacial regions where the saturated active layer, which melts during the summer months, flows over the still frozen permafrost zone.  Affected slope surfaces are covered with small overlapping flows called solifluction lobes (figure 6). In tropical regions a similar phenomenon occurs when soil above an impermeable horizon rich in clay and iron-oxides becomes over-saturated.

2. Moderate to fast moving mass-wasting events (Landslides)

  • Earthflows are slow pasty flows in soil or regolith(Fig. 3)
  • Slumps form when regolith fails along an arcuate rupture.  The various elements of a slump include the headscarp, the head, which is rotated counter clockwise into the slope, the body of the slide that moves along the rupture, and the toe.  The toe is an earthflow that extends beyond the slide rupture onto the adjoining lower slope.
  • Soil, rock and debris slides occur along an inclined glide plane, usually a soil horizon, bedding plane, unconformity, or joint surface.
  • Rock fall is the vertical release of debris down a steep slope (figs. 5 and 7).  Rock falls are comon where slopes have been oversteepened by fluvial or glacial activity. Talus (fig. 7) is the slope of accumulated rocky debris that forms at cliff base.
  • Avalanches are extremely fast flows that are often initiated by a fall or slide that breaks up and accelerates.  Collisions between particles keeps the flow in motion.
  • Mud and debris flows are saturated flows typically confined to valleys. Debris flows affecting the Los Angeles Basin travel down canyons during floods.  They are capable of carrying large boulders and even vehicles.
    • Lahars (volcanic mudflows) are a special variety of mud flow composed of saturated volcanic ash.  Saturation can be caused by preciptation during storms (e.g. Pinatubo, 1992) or by the melting of glaciers (e.g. Mt. St. Helens, 1980) commonly capping stratovolcanoes.

creep

Figure 4. Surface creep is evidenced by the down-slope bending of tree trunk. (Click to enlarge)
 

rockfall

Figure 5. Rockfall of massive sandstone in Arches National Park. (Click to enlarge)
Figure 6. Solifluction (gelifluction) lobes covering an Alaskan slope in the Tanana River Basin.  The saturated active layer flows over the permafrost zone.  solifluction
Figure 7.  Rock fall from the jagged peaks of the Picos De Europa, Northern Spain.  The debris slope is called talus, which can take the form of a fan or apron. talus

Slope Stability

A slope can be considered to be in one of three stability states. A number of conditions, such as those listed below, may set the stage for destabilization (preconditioning and preparatory factors) or initiate failure (triggers). Sustaining factors determine the behavior of the slope. slope stability
Figure   7. Stability states of slopes and destabilizing factors.  Modified from Croszier(2004)

Factors that reduce slope stability

  1. Removal of lateral  or underlying support: Over-steepening, undercutting, solution (Boulder, Colorado landslide )
  2. Weakening of slope materials by weathering (fracturing, solution, alteration to clay, etc.)
  3. Structure: inclined joints, cleavage, or bedding plane that dip in the same direction as the slope
  4. Removal or change in vegetative cover: Preventing slope failure (Anaheim)
  5. Addition of mass: Overloading a slope
  6. Wildfires: influence water content and vegetative cover (Wildfires and debrisflows in Southern CA)
  7. Water content: can increase normal stress or decrease it depending on the degree of saturation. (Aberfan disaster, South Wales)  An increase in porewater or seepage pressure will often cause a slope to fail.
  8. Vibration: earthquakes, traffic, etc. (1970 Ancash earthquake) - decreases internal strength

Factors 1-6 often set the state for failure.  Factors 7 and 8 are the most common triggers.

How to recognize an active slope

  1. Scarps
  2. Tension cracks
  3. Tilted trees
  4. Lack of vegetation
  5. Poor soil development
  6. Hummocky or lobate topography
  7. Blocked drainage
Activity

Go to Kelso Landslide site and view the Japanese video clip. URL: http://www.nwgeoscience.com/kelso/.  See if you can answer the following questions:

  1. What kind of mass-wasting event is represented.
  2. Can you determine the rate of movement?  If so what is it?
  3. Do you notices any preconditioning factors that would lead the the slope's instability?
  4. What evidence is there that the slope is unstable.

 

See also Features that may Indicate Catastrophic Landslide Movement, Anaheim Landslide Site, URL:http://anaheim-landslide.com/features.htm

Additional sites of interest

Surface subsidence

Causes

Additional sites to explore:

Bibliography


Bloom, Arthur. 1998, Geomorphology, A systematic analysis of Late Cenozoic landforms, (3rd edition): Prentice Hall, Upper Saddle River, N.J., 482 p.

Chandler, R.J., 1977, The Application of soil mechanics methods to the study of slopes: in Hails, J.R., (ed.), Applied Geomorphology, Elsevier, Amsterdam, p. 157-181.

Chorley, R.J., Schumm, S.A., Sugden, D.E., 1984, Geomorphology: Methuen and Co. Ltd., London, 605 p.

*Clark, M.J., and Small, R.J., 1982, Slopes and weathering: Cambridge University Press, Cambridge, England, 112 p.

*Dalrymple, J.B., et al, 1968, An hypothetical nine unit landsurface model

Easterbrook, Donald J., 1993, Surface Processes and Landforms: Macmillan Pub. Co., 520 p.Howard, A.D., 1967, Drainage analysis in geologic interpretation: a summation: The Amer. Assoc. of Petr. Geol., v. 51, n. 11, p. 2246-2259.

Mayer, Larry, 1990, Introduction to Quantitative Geomorphology: Prentice Hall, Englewood Cliffs, NJ, 380 p.

McCullagh, Patrick, 198, Slopes, in FitzGerald, B.P. (ed.), Modern Concepts in GeomorphologyRitter, D.F., Kochel, C.R., and Miller, J.R., Process Geomorphology (3rd Edition): Wm.C. Brown Publishers, Dubuque, IA, 544 p.

Crozier, Michael, 2004, Slope Evolution, in Goudie, A.S., ed., Encyclopedia of Geomorphology, Volumn 2, Routledge, New York, NY, pp. 963-970.

Summerfield, M.A., 1991, Global Geomorphology. John Wiley and Sons, New York, NY, 536 p. 

Lindley Hanson/Department of Geological Sciences/Salem State College/Geomorphology/GeoIndex/QkRef