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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