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Mass Wasting
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2Mass Wasting
� Down-slope movement of rock, loose material & soil under direct influence of gravity.
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Earth�s External Processes� Weathering, mass wasting, and erosion are all
called external processes because they occur at or near Earth�s surface
� Internal processes, such as mountain building and volcanic activity, derive their energy from Earth�s interior
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Mass Wasting: The Work of Gravity
� Mass wasting is the downslope movement of rock and soil due to gravity
� Controls and triggers of mass wasting� Water�Reduces the internal resistance of materials
and adds weight to a slope� Oversteepening of slopes
5List factors that will affect how easily mass wasting occurs.
� Steepness of slope� Water
� Adds weight� “lubricates” failure surfaces
� Vegetation� Roots hold soil� Absorb water
� Triggers� Earthquakes� Thunder
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Mass Wasting: The Work of Gravity
� Controls and triggers of mass wasting� Removal of vegetation# Root systems bind soil and regolith together
� Earthquakes# Earthquakes and aftershocks can dislodge large volumes of
rock and unconsolidated material
� Thunder
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7Mass Wasting
� Down-slope movement of rock, loose material & soil under direct influence of gravity.
8List factors that will affect how easily mass wasting occurs.
� Steepness of slope� Water
� Adds weight� “lubricates” failure surfaces
� Vegetation� Roots hold soil� Absorb water
� Triggers� Earthquakes� Thunder
9Types of Mass Wasting
Figure 3.3
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10Good Mass Wasting Resource
� http://www.tulane.edu/~sanelson/geol204/masswastproc.htm
11Mass Wasting
� Classified as…
Amount of Water
Falls(Rock falls)
Slides(Rock slides)
Flows(Debris Flows,
Mud flows)
Slumps
Creep(Slow down-slope movement of soil or regolith)
12Steepness of Slope – The Angle of Repose
Angle of Repose
Angle of Repose: The steepest angle at which loose material remains stationary without sliding downslope.
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13Mass Wasting
� Classified as…
Amount of Water
Falls(Rock falls)
Slides(Rock slides)
Flows(Debris Flows,
Mud flows)
Slumps
Creep(Slow down-slope movement of soil or regolith)
14Creep
� Slow, down-slope movement of soil or sediment.
� Responsible for building and road damage.
http://www.structures.ucsd.edu/Taiwaneq/geotechnical.html
15Mass Wasting
� Classified as…
Amount of Water
Falls(Rock falls)
Slides(Rock slides)
Flows(Debris Flows,
Mud flows)
Slumps
Creep(Slow down-slope movement of soil or regolith)
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161999 Yosemite
Rockfall
Talus Cone
Curry Village
Exfoliation Cracks
17Mass Wasting
� Classified as…
Amount of Water
Falls(Rock falls)
Slides(Rock slides)
Flows(Debris Flows,
Mud flows)
Slumps
Creep(Slow down-slope movement of soil or regolith)
18Slumps
� Sliding of� Unconsolidated material� As an intact unit� Curved failure surface(s)� Often after
rainfall
Figure 3.3
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19Slumping at Devil’s Slide
� Highway 1 south of SF
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Devil’s Slide
21What are some of the factors that cause Devil’s Slide to slide?
Cross Section
JeepTrail
Spring(water seep)
Hard Sandstone
Shale,Siltstone &Sandstone
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22Mass Wasting
� Classified as…
Amount of Water
Falls(Rock falls)
Slides(Rock slides)
Flows(Debris Flows,
Mud flows)
Slumps
Creep(Slow down-slope movement of soil or regolith)
23Slides� Rapid down-slope movement
� Sediment, soil & regolith break loose� Mountainous areas� Rapid and destructive� Often after
rainfall
Figure 3.3
241997 Highway 50 Landslide
� “Mill Creek Slide”� January 24, 1997� Closed Hwy 50� Destroyed 3 cabins� Dammed South
Fork of American River for 5 hours
� Many cabins destroyed in flooding after Am. River breeched dam
http://vulcan.wr.usgs.gov/Projects/CalifLandslide/framework.html
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25Mill Creek Landslide
� Hwy 50 closed 4 weeks
� 35,000 truck loads of earth material
� $4.5 million� + >$1M/day
relatedcosts
http://vulcan.wr.usgs.gov/Projects/CalifLandslide/framework.html
26Cross-Sectional View
Joints
Granite(actually granodiorite)
Gabbro(actually pyroxenite)
Burned vegitation
From R.H. Syndor, Calif. Geology, May/June 1997
27Observations� Parent rock has composition
similar to gabbro� Pyroxenite
� A forest fire burned this area in 1992.
� There are distinct sets of joints in the rock.
� Early snow fell in Dec. 1996.� Unusual tropical rains fell
after the snow.� Jan. 1997 received a record 19 inches of rain.
Explain how each of these observations contributed to the Mill Creek landslide.
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Upper section “Slide”Lower section “Flow”Slide
Flow
Open cracksobserved in 1996
Moved 50 feetup-slope
29Hwy 50 Now Completely Mapped for Landslide Hazards
Mill Creek Landslide
Cleveland Corral Landslide
From T.E. Spitter and D.L. Wagner, Calif. Geol., May/June 1998
30Cleveland Corral Slide
� Near Mill Creek Slide� Actively Monitored by USGS� Potential for
blockingHwy 50
� Potential fordamming American River
http://vulcan.wr.usgs.gov/Projects/CalifLandslide/framework.html
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31Monitoring System
http://vulcan.wr.usgs.gov/Projects/CalifLandslide/framework.html
32Monitoring Movement
http://vulcan.wr.usgs.gov/Projects/CalifLandslide/framework.html
33Slide History
http://vulcan.wr.usgs.gov/Projects/CalifLandslide/framework.html