september 29 course website: today: chapter 8 mass movements
TRANSCRIPT
September 29September 29
Course Website: http://glg110.asu.eduCourse Website: http://glg110.asu.edu
Today:Today:
Chapter 8Chapter 8
Mass MovementsMass Movements
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OverviewOverview
• Mass Wasting Definitions• Slope Processes• Slope Stability• Types and Examples of Mass
Movements• Human Activity and Landslides• Minimizing the Hazard
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What examples of mass What examples of mass movements can you think of?movements can you think of?
• Land slides • Mud slides• Avalanche• Slump and creep
• Where in the US do we hear about landslides?
GLG 110 Fall 2003GLG 110 Fall 2003 Landslide hazard map, USA
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DefinitionsDefinitions
• Mass Wasting = comprehensive term for any type of downslope movement of Earth materials– More precisely = downslope movement
of rock or soil all together
• Includes:– Landslides, earthflows, mudflows,
rockfalls, debris and snow avalanches, and subsidence
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Mass WastingMass Wasting
• Downslope movement of rock, regolith, soilDownslope movement of rock, regolith, soil• Force of gravity dominatesForce of gravity dominates• Often initiated by heavy rainfall or earthquakeOften initiated by heavy rainfall or earthquake• Distinct from erosional processesDistinct from erosional processes• Follows weatheringFollows weathering• Mass-wasting + running water = stream Mass-wasting + running water = stream
valleysvalleys
Mass wasting & erosion Mass wasting & erosion sculpt Earth’s surfacesculpt Earth’s surface
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Slope ProcessesSlope Processes
• Slopes are the most common landforms
• Most appear stable but really are dynamic and evolving– Material on slopes is constantly moving
• Slope form depends on:– Underlying rock type– Climate– Regional tectonics– Regional drainage pattern
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Slope Processes
Concave Slope
Cliff Face
Straight Slope
Hard Granite
Weak Rock
Soil
Talus Slope(rock fall debris)
Convex Slope
Note – the slopes on this figure in your text are mislabeled!
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Slope StabilitySlope Stability
•Slope = how steep the inclination 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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Slope StabilitySlope Stability
Image: University of MinnesotaGravity
Weight of Material
Friction
Upward force of
supporting materials
Driving ForcesDriving Forces and Resisting ForcesResisting Forces
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)
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Slope StabilitySlope Stability
• Slope stability is evaluated by computing safety factor
• Safety Factor = ratio of resisting forces to driving forces
– If S.F. > 1 the resisting forces are stronger and the slope is stable
– If S.F. < 1 the driving forces are stronger and the slope is unstable
Resisting Forces
Driving ForcesS.F. =
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Slope StabilitySlope Stability• 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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Role of Slope MaterialRole 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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Role of Slope MaterialRole of Slope Material
• 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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Role of Slope & TopographyRole 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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Role of ClimateRole 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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Role of VegetationRole 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
In subhumid to humid areas vegetation is abundant, thick soil develops so mass movements = complex landslides, flows, and creep
Image: Geolith Consultants
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Role of WaterRole of Water• Almost always directly or indirectly involved
with mass movements• Water fills pore spaces between sediments,
reduces internal resistance, adds weight
• Affects:– Decreasing stability when
slope becomes saturated – Slumps or slides can occur
years after deep water infiltration
– Can erode base or toe of slope decreasing stability
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Role of WaterRole of Water
• 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
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Role of TimeRole 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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Types of Mass MovementsTypes 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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Carson and Kirby, 1972 in Ritter, et al., 2002
Classification of Mass Movementsbased on water content and
speed of movement
Types of Mass Movements
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Carson and Kirby, 1972 in Ritter, et al., 2002
Types of Mass Movements
Slide/Fall
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Types of Mass MovementsTypes of Mass Movements
• Slide = downslope movement of coherent block of Earth material
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Types of Mass
Movements
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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Types of Mass Types of Mass MovementsMovements
• Fall = free fall of Earth material
Image: USGS/Lloyd DeForrest
Yosemite National Park, California
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Rock falls and rock slidesRock falls and rock slides
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Rock fallsRock fallsFalls are the free-fall of
pieces of rock from a mountain or cliff face. On mountains, ice wedging is the main contributor. As water from snowmelt finds its way into the cracks and joints of the rock face, it may refreeze and being to expand. This expansion widens the cracks in the rock. Over time, the cracks have been widened enough so that they are a point of structural weakness.
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Types of Mass Types of Mass MovementsMovements
• Debris Avalanche = very rapid downslope movement of soil, rock, & organic debris
In <2 minutes 40 million cubic yards of rock covered the town of Frank, Alberta, Canada, in 1903
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Types of Mass MovementsTypes of Mass Movements• 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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Carson and Kirby, 1972 in Ritter, et al., 2002
Types of Mass Movements
Flow
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Image: USGS
Types of Mass MovementTypes of Mass Movement
• Lahar = mudflow produced when large volume of volcanic ash and ejecta becomes saturated with water
Armero, Nevado Del Ruiz, 1985
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• Flow = downslope movement of unconsolidated Earth material saturated with water– particles within also
move with respect to each other
• Mudflows (or debris flows) are rivers of rock, earth, and other debris saturated with water.
Types of Mass MovementsTypes of Mass Movements• Debris Flow
= >50% of particles coarser than sand
• Mudflow = >50% of particles finer than sand
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Types of Mass Types of Mass MovementMovement
• Rock Glacier = mass or rock and ice frozen together and flowing downslope
Image: NOAA
Mt. Sopris, Colorado
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Carson and Kirby, 1972 in Ritter, et al., 2002
Types of Mass Movements
Creep/Heave
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Types of Mass Movements
• Creep & Heave = slow, ~imperceptible downslope movement of unconsolidated Earth material– Often related to seasonal
effects and/or precipitation rates
• Slow FlowImage: NOAA
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Carson and Kirby, 1972 in Ritter, et al., 2002
Types of Mass Movements
Slump/ Subsidence
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Types of Mass Types of Mass MovementsMovements
• Slump = rock or soil moving downslope along curved slip plane producing slump blocks
Black Hills, N.D.
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Types of Mass MovementsTypes of Mass Movements• Subsidence =
sinking of mass of Earth material below level of surrounding material– can occur on
slopes or flat ground
Image: NOAA
San Joaquin Valley, California
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Types of Mass MovementTypes of Mass Movement
• 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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Carson and Kirby, 1972 in Ritter, et al., 2002
Types of Mass Movements
Complex Landslide
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Types of Mass Types of Mass MovementsMovements
• Landslides occur when masses of rock, earth, or debris move down a slope. They may be very small or very large, and can move at slow to very high speeds. However slow movement is also seen in the gradual downhill creep of soil on gently sloping land.
La Conchita, California, 1995
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La Conchita Complex Landslide
Image: USGS
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Aerial photos of complex landslide at La Conchita, California
Do you see the landslide scarp?
La Conchita Complex Landslide
1927 1967
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Aerial photos of complex landslide at La Conchita, California
Do you see the landslide scarp?
La Conchita Complex Landslide
1927 1967
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• Note how quickly the vegetation is returning and how it helps conceal the slide scar
La Conchita Complex Landslide
1998
Image: Jeffrey J. Hemphill
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Types of Mass Movement: Types of Mass Movement: Soil creepSoil creep
A very, very slow form of mass wasting. The effect is the way the grass covered slope seems to ooze downhill forming little bulges in the soil. This heaving of the soil occurs in regions subjected to freeze-thaw conditions.
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Mass movements are caused by various Mass movements are caused by various conditions:conditions:
• Volcanic activity many times causes huge mudflows when the icy cover of a volcano melts and mixes with the soil to form mud as the magma in the volcano stirs preceding an eruption.
• Mudslides can also develop when water rapidly accumulates in the ground, such as during heavy rainfall or rapid snow melt, changing the earth into a flowing river of mud or "slurry.".
• Earthquake shocks cause sections of mountains and hills to break off and slide down.
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Mass movements are caused by various Mass movements are caused by various conditions: (cont.)conditions: (cont.)
• Human modification of the land or weathering and erosion help loosen large chunks of earth and start them sliding downhill.
• Vibrations from machinery, traffic, weight loading from accumulation of snow; stockpiling of rock or ore; from waste piles and from buildings and other structures.
• However, the force behind mass movements is the gravitational pull of the earth on soil, rocks, and mud.
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Effects of human activitiesEffects of human activities
The rock slide that covered the town of Frank, Alberta, Canada on April 29, 1903 was in part caused by human activities. The sedimentary rock layers in Turtle Mountain dipped away from the valley containing the mining town of Frank. However, joints in the limestone layers did dip toward the town. The mountain was also composed of some structurally weak limestone, shale, siltstone, and coal layers that were deformed by the weight of more massive limestone located above. The mining of coal at the Turtle Mountain's base reduced the support to overlying materials. Together these factors lead to the sudden movement of 33 million cubic meters of rock in approximately two minutes.
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Project:Project:
• You are a concerned land owner in Malibu, CA. You own a nice little beach house near the beach. Developers have been building mansions on the hillside behind your property. Write a letter to the building commissioner with your concerns for your property. List the reasoning for your concerns. Explain how the hillside development could effect you. Give an example of what could eventually happen (use a real example from your text). Should be at least 5 sentences in the format of a letter.