Geomorphology Logo| Home | Syllabus | Calendar | Lab | Grades | Geog/Geol Dept | UWSP |


A. Introduction

Downslope movement of material due to gravity

Learning Outcomes - you should be able to:

B. Types of Mass Movement


  • seasonal creep: movement of soil by heave

    • caused by expansion & contraction
      Picture (250x196, 13.6Kb)

    • freeze-thaw

    • wetting & drying

    • operates in upper few feet of soil

  • continuous creep: strain response to stress generated by weight of overburden

    • driven by gravity alone, not heave

    • may affect consolidated rock

    • functions at levels well below the surface

Photo: continuous creep
Photo: NOAA/NGDC, Landslide Hazard Photos Set 1


  • abrupt free fall of loosened blocks or boulders of solid rock
    Diagram: rockfall
    Photo: rockfall

  • bedding, jointing & fracturing of bedrock important

  • speed: very fast

  • failure may be initiated by:

    • weathering

    • water pressure in joints

    • undercutting

  • landforms

    • scree slopes, talus cones

    • material accumulates at angle of repose

Photo: rockfall
1996 Yosemite National Park - 200 ton rock disintegrated upon hitting the ground and created an air blast that flattened approximately 2000 trees.
Photo: E. Harp, USGS. NOAA/NGDC Natural Hazard Photos Landslide Set 2.


Diagram: translational slide

Photo: translational slide
Translational slide, Laguna Beach, CA, 2005.
Photo: J. Bowers, USGS. USGS Landslides Hazards Program, Landslide Photo Collections.
Photo: landslide
Landslide, Glacier Bay. Photo: USGS Landslides Hazards Program, Landslide Photo Collections.

Diagram: rotational slide

Photo: slump in California
Slumps, Pacific Palisades, southern California.
Photo: B. Bradley, University of Colorado. NOAA/NGDC Natural Hazards Photos Landslides Set 1.
Photo: slump North Dakota
Head of a slump, Black Hills, North Dakota.
Photo: B. Bradley, University of Colorado. NOAA/NGDC Natural Hazards Photos Landslides Set 1.


  • deformation of an entire mass that flows downslope as a viscous fluid

  • wet flows: fluid = water

    • debris flows

      • slurry of rock, finer grained sediment and water

      • speed: very fast

      • triggers: volcanic eruptions, intense storms in steep mountainous regions

    • earth flows

      • more fine-grained than debris flows; mud flows

      • speed: slow to fast

      • triggers: intense or prolonged storms; rapid snowmelt

    • solifluction

      • thin soil/sediment layer above impermeable surface (rock, ice)

      • saturated material loses cohesion

      • solifluction lobes

      • speed: slow

      • triggers: melting/thawing

Photo: solifluction lobes
Solifluction lobes, Kyrgyzstan. Photo: copyright Marli Miller, University of Oregon. Earth Science World Image Bank, Photo hhrimm.

  • dry flows; fluid = air

    • rock avalanche

      • may start as rockfall

      • rock disintegrates & flows along a cushion of air

      • speed: fast

      • triggers: weathering & weakening, earthquakes, volcanic eruptions

    • debris avalanche:

      • rock chunks mixed with finer-grained sediment

      • may transform to debris flow if water gets added

      • speed: fast to extremely fast

      • evidence of numerous debris avalanches and debris flows associated with stratovolcanoes

        • steam explosions

        • actual eruptions

        • earthquakes

    • snow avalanche

Photo: debris avalanche
Debris avalanche, Mt. St. Helens, 1983; speed over 150 mph; average thickness 150 ft; maximum thickness 600 ft. Photo: L. Topinka, USGS. USGS Cascades Volcano Observatory, Mt. St. Helens 1980 Debris Avalanche.

Photo: debris flow
Mt. Hood debris flow, White River, 1998. Photo: C. Gardner, USGS. USGS Cascades Volcano Observatory.

Photo: earth flow
Photo: USGS Landslides Hazards Program, Landslide Photo Collections.


Photo: rock avalanche
Rock and snow avalanche, Mt. Hauscaran, Peru, 1970; death toll 66,700; speed 210-280 km/hr. Photo: B. Bradley, University of Colorado. NOAA/NGDC NGDC Natural Hazards Slides Sets, Landslides Set 1.

Photo: debris avalanche
Approximately 300,000 year old debris avalanche, Mt. Shasta. Schuster, R.L. and Highland, L.M. (2001) USGS Open File Report 01-0276.

C. Driving & Resisting Forces


Diagram: normal and shear forcesSlope

Weight of hillslope material

Diagram: stresses acting on a block on a slopeStress calculations

Shear stress: sum of the driving forces

Diagram: slope stability with unconsolidated materialShear strength, S: sum of the resisting forces

F: Factor of Safety

D. Triggers of Mass Wasting

Weathering and weakening of material

Heavy rainfall


Tectonic activity


Human activities

E. Predicting Slope Failure

Identifying old failures

F. Summary

| Next Lecture | Previous Lecture | Calendar | Home |
©Karen A. Lemke:
Last revised December 1, 2013