Introduction to Geology GEO101004 Class 21: Mass Wasting

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Introduction to GeologyGEO-101-004Class 21
Mass Wasting
From http//www.mbari.org/volcanism/Hawaii/HR-Land
slides.htm
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Check Schedule Online

• 3rd exam on 19 April (chapters 12-17)
• Final exam on May 5, 800-1030 am (all chapters
  covered in class)
• In class review sessions before both exams come
  prepared

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• Offshore earthquake in May 1970
• Triggered rock and ice slide on 6,700 m Nevado
  Huascaran
• Rock and ice slide plunged nearly 1 km
• Rushed down the mountainside

• Hurricane force winds ahead of the slide
• 20,000 people in Yungay and Ranrahirca, Peru were
  killed
• The avalanche traveled over 14 km (9 miles)

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Landslide La
Conchita, CA January 2005
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Mass wasting and landform development

• Mass wasting refers to the 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 of mass wasting and running
  water produce stream valleys

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Mass wasting and landform development

• Slopes change through time
• No minimum angle is required for mass wasting to
  occur
• Most rapid and spectacular mass-wasting events
  occur in areas of rugged, geologically young
  mountains
• Mass wasting and erosional processes slowly lower
  the land surface
• Plate tectonic processes are continuously
  producing topography that is once again eroded
• If this were not the case the Earths surface
  would be eventually flat

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Controls and triggers of mass wasting

• Important factors include
• Controls and triggers
• Various processes work to weaken the slope over
  time, making it more susceptible to gravity
• Eventually the slope is weakened to the point
  that an event triggers downslope movement
• What might some of these triggers be?

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Controls and triggers of mass wasting

• Important factors include
• The role of water
• Diminishes particle cohesion (friction)
  particles slide past one another with ease
• Water adds weight

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Controls and triggers of mass wasting

• Important factors include
• Oversteepening of slopes slope angle
• Stable slope angle (angle of repose) is different
  for various materials
• Oversteepened slopes are unstable
• Causes Stream undercutting a valley wall, waves
  pounding against a cliff, silly humans
• Angle of repose the steepest angle at which a
  material is stable 25-45o (depending on grain
  shape)
• Angle of repose also applies to non-granular
  material just takes more time to respond

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The VaiontDam Disaster

• Stupid Human Factor
• Engineers did not consult geologists!
• Built in 1960
• 265 m tall, crossing Vaiont Canyon in the Italian
  Alps
• Bedrock in canyon dipped steeply towards the lake
• Bedrock is highly fractured limestone with
  numerous cavities and clay layer
• As the reservoir filled the rocks became
  saturated and the clays swollen
• The mountain started to moved down into the lake
  at 1 cm, then 10-20 cm per day
• Finally the mountainside failed, filling the
  gorge to 150 m above the original lake level
• The lake exploded over the dam sending a 90 m
  wall of water down the valley, killing 2600 people

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From http//www.geocities.com/geogsoc2000/Vaiont1.
htm
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Controls and triggers of mass wasting

• Important factors
• Removal of anchoring vegetation
• Roots bind soil and regolith
• Plants protect soil from erosional effects of
  raindrops
• Landslide in Menton, France caused by replacing
  Olive trees (deep roots) with Carnations (shallow
  roots)
• Loss of vegetation through wildfires may result
  in debris flows (e.g Colorado, 1994)
• Fires may bake the ground, resulting in
  increased runoff

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Controls and triggers of mass wasting

• Important factors
• Earthquakes as triggers
• May cause expensive property damage
• Can cause liquefaction water saturated surface
  materials behave as fluid-like masses that flow
• 1994 Northridge earthquake caused thousands of
  landslides
• In some places 75 of slope areas where effected
  by landslides

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Controls and triggers of mass wasting

• Landslides without triggers
• Slope materials weaken over time
• Random events that are unpredictable
• Sacred Falls, Hauula, Oahu
• Rocks plunged 150 meters down a cliff into a pool
  below
• 10 people dead
• No trigger perhaps just long term weathering

From http//landslides.usgs.gov/Sacred/cover.html
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Classification of mass wasting processes

• Generally each event is classified by
• Type of material involved
• If soil or regolith dominate, terms such as
  debris, mud, or earth are used in the description
• If a mass of bedrock breaks loose, the term rock
  may be used in the description

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Classification of mass wasting processes

• Generally each event is classified by
• Type of motion
• Fall (free-falling pieces)
• May be promoted by root action and/or the
  freeze-thaw cycle
• Primary way in which talus slopes are built

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Classification of mass wasting processes

• Generally each event is classified by
• Type of motion
• Slide (material moves along a surface as a
  coherent mass)
• Surface may be a joint, fault, or bedding that is
  parallel to slope
• In the case of a slump, the material may move
  along a curved rupture surface

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Classification of mass wasting processes

• Generally each event is classified by
• Type of motion
• Flow (material moves as a chaotic mixture)

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Classification of mass wasting processes

• Generally each event is classified by
• Type of motion

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Classification of mass wasting processes

• Generally each event is classified by
• The velocity of the movement
• Fast
• In the Yungay, Peru event rock and debris moved
  downslope at gt125 mph
• This is termed a rock avalanche
• The debris may literally float on air
• Slow
• Much mass wasting is imperceptibly slow mm/year

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Types of mass wasting

• Slump
• Movement of a mass of rock or unconsolidated
  material as a unit along a curved, or spoon
  shaped surface
• May comprise multiple blocks
• Material does not travel very far
• Common in cohesive materials such as clay
• Occurs along oversteepened slopes
• May be triggered by removal of anchoring material

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Types of mass wasting

• Slump

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Types of mass wasting

• Rockslide
• Blocks of bedrock slide down a slope
• If the material is largely unconsolidated the
  term debris slide is used
• Generally very fast and destructive
• Often triggered by rain or meltwater lubricating
  the underlying surface
• Most common in spring when heavy rains and
  melting snow are most prevalent

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Figure 15.14
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Types of mass wasting

• Debris flow (mudflow)
• Consists of soil and regolith with a large amount
  of water
• Most common in semiarid mountainous regions and
  volcanic slopes
• Often confined to channels
• Consistency may be of wet concrete
• Capable of moving trees, cars, and houses with
  ease

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Types of mass wasting

• Debris flow (mudflow)
• Debris flows spread out into a alluvial fan when
  they reach the end of a canyon
• Fans have built up into high areas nice views
  and easy to build on unfortunately they have
  become popular sites for development

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Types of mass wasting

• Debris flow (mudflow)

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A lahar from the 1980 Mt. St. Helens eruption
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Types of mass wasting

• Earthflow
• Form on hillsides in humid regions
• Water saturates the soil
• Commonly involve materials rich in clay and silt
• Slow and persistent movement
• Sometimes associated with large slumps

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Types of mass wasting

• Slow movements
• Creep
• Gradual movement of soil and regolith downhill
• Aided by the alternate expansion and contraction
  of the surface material by the freeze-thaw
  cycle or by saturation

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Some visible effects of creep
Figure 15.19
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Which statement best describes slumping, a mass
wasting process?
15.05

• A block or blocks of unconsolidated regolith
  slide downhill along a curved slip surface.
• Blocks of hard bedrock rapidly slide downhill
  along fracture surfaces.
• The soil and regolith move downhill very slowly.
• A mass of soil or regolith becomes saturated with
  water and suddenly flows downhill to the base of
  the slope.

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Creep vs. Slump
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Types of mass wasting

• Slow movements
• Permafrost melting
• Permafrost is permanently frozen ground
• Removing the insulating vegetation mat or
  building roads and buildings can cause the
  permafrost to melt

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Types of mass wasting
From http//www.alyeska-pipe.com/
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Types of mass wasting

• Slow movements
• Solifluction
• Promoted by a dense clay hardpan or impermeable
  bedrock layer
• Common in regions underlain by permafrost
• Can occur on gentle slopes

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Question of the Day
You are looking at buying a house what are some
of the telltale signs that you should look for to
avoid buying a house that might be subject to
mass wasting?