Chapter 9 Mass Wasting

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Chapter 9 Mass Wasting
GEOL 101 Introductory Geology
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Mass Wasting

• Mass wasting downslope movement of rock,
  regolith, and soil under the direct influence of
  gravity
• Role of mass wasting
• Geologic process that often follows weathering
• Combined effects if mass wasting and running
  water produce stream valleys

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Mass Wasting Classification

• Type of material
• Debris
• Mud
• Earth
• Rock
• Snow
• Type of movement
• Fall (free-falling pieces)
• Slide (material moves as a coherent mass)
• Flow (material moves as a viscous fluid)
• Rate of the movement
• Rapid (Fast)
• Slow

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Type of Movement
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Rate of Movement
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Controlling factors
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Gravity Forces
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Oversteepened Slopes
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Common Types of Mass Wasting

• Creep
• Debris flow
• Earthflow
• Mudflow
• Debris Avalanche
• Rockslide and Rock Fall

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Slow Forms of Mass Wasting

• Creep
• Gradual movement of soil regolith downhill
• Aided by the alternate expansion and contraction
  of the surface material
• Solifluction
• Flow of water-saturated debris over impermeable
  material, often permafrost
• Can occur on gentle slopes

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Creep
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Creep
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Creep Mechanism
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Permafrost Regions in the Northern Hemisphere
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Permafrost Mechanism
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SolifluctionPermafrost in Fairbank, AK
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Permafrost DamageFairbanks, AK
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Rapid Forms of Mass Wasting

• Earthflow
• Form on hillsides in humid regions
• Water saturates the soil
• Involve materials rich in clay and silt
• Mudflow
• More water involved

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Rapid Forms of Mass Wasting
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Earthflow
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EarthflowSonoma County, CA
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Mudflow Damagenorthern, CA
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Lahar (Debris Flow)1980 Mt. St. Helens eruption
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Rock AvalanceGlacier National Park, MT
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Rapid Forms of Mass Wasting

• Slump
• Movement of a mass of rock or unconsolidated
  material as a unit along a curved surface
• Occurs along oversteepened slopes
• Rockslide
• Blocks of bedrock slide down a slope
• Generally very fast and destructive

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Slump (with Earthflow near toe)
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Slumpsouthwest, MT
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Gro Ventre Rockslide
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Vaiont Canyon, ItalyLandslide
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Landslide ScarDawson, Yukon
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Slide Surface Orientation
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Rock FallWallawa Mountains, OR
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Controls and Triggers

• Gravity is the controlling force
• Important triggers include
• Saturation of the material with water
• Diminishes particle cohesion
• Water adds weight
• Oversteepening of slopes
• Stable slope angle (angle of repose) is different
  for various materials
• Oversteepened slopes are unstable
• Removal of anchoring vegetation
• Ground vibrations from earthquakes
• May cause expensive property damage
• Liquefaction water saturated surface materials
  behave as fluid-like masses that flow

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Undercutting by Waves
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Earthquake Shaking(Liquefaction)
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Earthquake Triggered LandslidesLake Tahoe, CA
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Mass Wasting Prevention

• Slope Monitoring
• Engineering
• Planting vegetation
• Cut-and-fill
• Lower water table
• Terrace slopes
• Retaining walls
• Rock bolts
• Avalanche Shed

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Slope Monitoring
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Mass Wasting Prevention
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Mass Wasting Prevention
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Mass Wasting Prevention
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Mass Wasting Prevention