Powerpoint Presentation Physical Geology, 10e

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Mass WastingChapter 9
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9.1 Mass Wasting

• Mass wasting is downhill movement of masses of
  bedrock, rock debris or soil, under the pull of
  gravity
• Landslides are much more costly over time in the
  U.S., in terms of both lives and dollars, than
  all other geologic and weather hazards combined
• Mass wasting is, with proper planning, perhaps
  the most easily avoidable of the major geologic
  hazards

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Classification of Mass Wasting

• Types of mass wasting are classified based on
• Rate of movement
• Wide range from lt 1cm/year to gt100 km/hour
• Type of material
• Did moving mass start out as solid bedrock or as
  debris (unconsolidated material at Earths
  surface)
• Type of movement
• Flow, slide, or fall

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Classification of Mass Wasting

• Types of movement
• Flow
• Descending mass moves downhill as a viscous fluid
• Slide
• Descending mass remains relatively intact, and
  descends along well-defined surfaces
• Translational slide - movement along plane
  parallel to motion
• Rotational slide (slump) - movement along a
  curved surface
• Fall
• Material free-falls or bounces down a cliff

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Factors Controlling Mass Wasting

• Factors making mass wasting likely
• Steep slopes
• Shear forces maximized by gravity
• Large relief
• (large elevation change from top of
• mountains/hills to valley floor)
• Thick layer(s) of loose rock,
• debris, soil
• Presence of water
• Lubricates moving rocks/debris/soil
• Lack of vegetation
• No roots to hold rock/soil in place
• Seismic (earthquake) activity

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9.2 Types of Mass Wasting

• Creep
• Very slow downslope movement of soil or
  unconsolidated debris
• Major contributing factors include water in soil
  and daily freeze-thaw cycles
• Can be costly to maintain homes, etc., on
  creeping ground as foundations, walls, pipes and
  driveways crack and shift downslope over time

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Types of Mass Wasting

• Debris flow - mass wasting in which motion takes
  place throughout the moving mass (flow)
• Earthflow - debris moves downslope, slowly or
  rapidly, as a viscous fluid
• Commonly occurs on steep hills, with thick debris
  cover, after heavy rains
• Solifluction of saturated debris is an example
• Mudflow - flowing mixture of debris and water,
  usually down a channel
• Most likely to occur on steep unvegetated slopes
  with thick debris cover
• Heavy rains on the slopes of stratocone volcanoes
  with fresh ash layers are triggers
• Debris avalanches are very rapid and turbulent
• Can reach speeds of several hundred km/hr

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Types of Mass Wasting

• Rockfall - when a block of bedrock breaks free
  and falls or bounces down a cliff
• Commonly an apron of fallen rock fragments
  (talus) accumulates at cliff base
• Rockslide - the rapid sliding of a mass of
  bedrock along an inclined surface of weakness
• Rock avalanche - a very rapidly moving, turbulent
  mass of broken-up bedrock
• Debris slide - a coherent mass of debris moving
  along a well-defined surface
• Debris fall - a free-falling mass of debris

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Preventing Landslides

• Preventing mass wasting of debris
• Construct retaining wall with drains
• Dont oversteepen slopes during construction
• Preventing rockfalls and rockslides on highways
• Remove all rock that is prone to sliding
• Stitch together outcrop
• Important to know the susceptibility of land to
  mass wasting before building any road or
  structure