GLG110 Geologic Disasters and the Environment

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Unit 6 Mass Wasting, Landslides and Subsidence
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I. Mass Wasting

• A. What is Mass Wasting?
• 1. Is defined as the down slope movement of
  rock and regolith near the Earth's surface mainly
  due to the force of gravity.
• 2. Includes the following
• a. Landslides, earthflows, mudflows,
• rockfalls, debris and snow avalanches, and
  subsidence
• 3. Can be initiated by heavy rainfall or
  Earthquakes.
• 4. Sculpt the Earths surface and can cause
  stream valleys.

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Examples of Mass Wasting
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II. Slope Processes

• Slopes are the most common landforms
• Most appear stable, but material on slopes is
  constantly moving
• Slope form depends on
• Underlying rock type
• Climate
• Regional tectonics
• Regional drainage pattern

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Cliff Face
Talus Slope (rock fall debris)
Hard Granite
Soil
Convex Slope
Straight Slope
Concave Slope
Weak Rock
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Talus Slope at Wheeler Peak, Great Basin
National Park, Nevada
Source Tom Bean/DRK Photos
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• Slope - how steep the angle of the hillside is
• Slope stability depends on the driving and
  resisting forces that act on the slope
• As angle of slope increases, driving forces
  increase

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Driving forces move material down the slope (Most
common weight of slope material, vegetation,
and structures on slope)
Resisting forces oppose downward movement (Most
common strength of slope material)

• Driving Forces and Resisting Forces

Friction
Weight of Material
Upward force of supporting materials
Gravity
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III. Slope Stability

• Slope stability is evaluated by computing safety
  factor
• Safety Factor - ratio of resisting forces to
  driving forces
• If S.F. gt 1 the resisting forces are stronger and
  the slope is stable
• If S.F. lt 1 the driving forces are stronger and
  the slope is unstable

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• Slope stability changes over time as conditions
  change causing ratio to change
• Driving and resisting forces are determined by
  interactions between
• Type of earth materials
• Slope angle and topography
• Climate
• Vegetation
• Water
• Time

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IV. Factors affecting Mass Movement A. Role
of Slope Material

• Affects type of mass movement
• Shale or weak volcanic material slopes tend to
  creep, flow, or slump
• Affects falls when strong, resistant rock is
  underlain by weak, easily erodable rock, can
  result in undercutting and subsequent rock fall

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• Affects type and frequency of slides
• Patterns of movement
• Rotational- occurs along curved slip surface,
  produces terrace-like structures
• Translational- planar, along slip planes within
  the slope
• Slip planes include fractures, layers, bedding
  planes, foliation, etc.

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B. Role of Slope Topography

• Angle of Repose - maximum angle at which
  unconsolidated material on slope is stable 30-35

The larger and more uniform the size of fragments
the steeper the angle of repose

• Controlled by
• Coarseness, shape, size and uniformity of size of
  fragments
• Water content

Steep slopes often associated with rockfalls and
debris avalanches
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C. Role of Climate

• Climate - characteristic weather at specific
  place over time
• Includes
• Average temperature
• Amount and timing of precipitation
• Which then infiltrates slope thus affecting
  stability
• Affects vegetation which influences slope
  stability
• Seasonal weather patterns

Common mass movements in arid and semiarid
regions fall, debris flow, shallow slips
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D. Role of Vegetation

• Vegetation is a function of climate, soil type,
  topography, and fire history
• Can increase or decrease the chances of mass
  movement
• Shields soil thus mitigates runoff during heavy
  precipitation
• Roots help fix soil in place
• Adds weight to the slope
• Corner on Route 67 added vegetation to stop mass
  movement
• Peru, no vegetation, mudslides occurred.

In subhumid to humid areas vegetation is
abundant, thick soil develops so mass movements
complex landslides, flows, and creep
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E. Role of Water

• Almost always directly or indirectly involved
  with mass movements
• Water fills pore spaces between sediments,
  reduces internal resistance, adds weight because
  water is heavier than air.

• Affects
• Decreasing stability when slope becomes saturated
  
• Slumps or slides can occur years after deep water
  infiltration
• Can erode base of slope decreasing stability
• Change angle of repose
• Example building a sand castle

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• Water Erosion
• Stream or wave erosion may remove material
  creating steeper slope thus reducing safety
  factor
• May reactivate old landslides
• Liquefaction
• Some clays behave as liquid and flow when
  disturbed
• Does not always require earthquake, can occur
  through toe erosion, which is erosion on the base
  of a landform.

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Water's Role In Mass Movement Before Slide
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Water's Role In Mass Movement After Slide
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F. Role of Time

• Forces on slopes often change with time
• Driving and resisting forces can change
  seasonally as water content changes
• Chemical weathering introduces elements into
  soils which change properties

A slope that is becoming less stable with time
may exhibit increasing creep until failure occurs
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V. Types of Mass Movements

• Important variables in classifying downslope
  movement of Earth materials are
• Types of mass movements
• slide, slump, fall, flow, subsidence, or complex
• Slope material
• Amount of water
• Rate of movement

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• Slide -downslope movement of Earth material

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Blackhawk Landslide, California

• Started as slide
• Rode on blanket of air
• Was pulverized when hit the base of slope
• Created debris blanket 10-30 m thick

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Geology of Sheep Mountain Slide
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Geology of Sheep Mountain Slide
Source Breck P. Kent
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Yosemite National Park, California

• Fall -free fall of Earth material
• Rock fall- piece of rock on a steep slope becomes
  dislodge and falls down the slope.
• Debris falls- involves a mixture of soil,
  regolith, vegetation and rocks

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• Debris Avalanche - very rapid downslope movement
  of soil, rock, organic debris

In lt2 minutes 40 million cubic yards of rock
covered the town of Frank, Alberta, Canada, in
1903
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• Avalanche
• Rapid downslope movement of snow and ice
  sometimes with rock, soil, and trees
• Often begins with slab weighing millions of tons,
  falling from an overloaded slope
• Can travel as much as 62 mph
• Tend to travel down chutes where previous
  avalanches have flown

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Before the Yungay Avalanche
Source Lloyd S. Cluff
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After the Yungay Avalanche
Source Lloyd S. Cluff
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• Flow - downslope movement of unconsolidated Earth
  material saturated with water
• particles within also move with respect to each
  other

• Debris Flow - gt50 of particles coarser than sand
• Velocities between 1 meter/yr to 100 meters/hr
• Mudflow - gt50 of particles finer than sand
• Consistency between soup like and wet concrete
• Velocity up to 50 mph

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• Lahar - mudflow produced when large volume of
  volcanic ash and ejecta becomes saturated with
  water

Armero, Nevado Del Ruiz, 1985
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Mt. Sopris, Colorado

• Rock Glacier- mass or rock and ice frozen
  together and flowing downslope

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• Creep Heave - slow, imperceptible downslope
  movement of unconsolidated Earth material
• Often related to seasonal effects and/or
  precipitation rates
• Slow Flow

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Source Peter L. Kresan
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Source John S. Shelton
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• Slump - rock or soil moving downslope along
  curved slip plane producing slump blocks

Black Hills, N.D.
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• Subsidence - sinking of mass of Earth material
  below level of surrounding material
• can occur on slopes or flat ground

San Joaquin Valley, California
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• Subsidence can occur as a result of
• Withdrawal of groundwater
• Withdrawal of oil and gas
• Dissolution of limestone
• Mining (coal, ore, salt)
• See Lake Peigneur example in book
• Subsidence can cause earth fissures and sinkholes

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La Conchita, California, 1995

• Landslide - complex combinations of sliding and
  flow

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La Conchita Complex Landslide
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Turtle Mountain Landslide
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Turtle Mountain Landslide
Source Betty Crowell/Faraway Places