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Mass Wasting

Presentation Modified from Instructor Resource Center on CD-ROM , Foundations of Science , 4 th Edition, Lutgens & Tarbuck

Mass Wasting 2

 Down -slope movement of , loose material & under direct influence of gravity.

3 Earth ’s External Processes  , mass wasting, and are all called external processes because they occur at or near Earth ’s surface  Internal processes, such as building and volcanic activity, derive their energy from Earth ’s interior

1 4 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

List factors that will affect how easily 5 mass wasting occurs.

 Steepness of slope  Water  Adds weight  “lubricates ” failure surfaces  Vegetation  Roots hold soil  Absorb water  Triggers   Thunder

6 Mass Wasting: The Work of Gravity

 Controls and triggers of mass wasting  Removal of vegetation • Root systems bind soil and together  Earthquakes • Earthquakes and aftershocks can dislodge large volumes of rock and unconsolidated material  Thunder

2 Mass Wasting 7

 Down -slope movement of rock, loose material & soil under direct influence of gravity.

List factors that will affect how easily 8 mass wasting occurs.  Steepness of slope  Water  Adds weight  “lubricates ” failure surfaces  Vegetation  Roots hold soil  Absorb water  Triggers  Earthquakes  Thunder

9 Types of Mass Wasting

Figure 3.3

3 10 Mass Wasting

 Classified as …

Creep (Slow down-slope movement of soil or regolith)

Falls Slumps Slides Flows (Rock falls) (Rock slides) (Debris Flows, Mud flows)

Amount of Water

11 Steepness of Slope – The

Angle of Repose: The steepest angle at which loose material remains stationary without sliding downslope .

Angle of Repose

12 Mass Wasting

 Classified as …

Creep (Slow down-slope movement of soil or regolith)

Falls Slumps Slides Flows (Rock falls) (Rock slides) (Debris Flows, Mud flows)

Amount of Water

4 13 Creep  Slow, down -slope movement of soil or sediment.  Responsible for building and damage.

http://www.structures.ucsd.edu/Taiwaneq/geotechnical.html

14 Mass Wasting

 Classified as …

Creep (Slow down-slope movement of soil or regolith)

Falls Slumps Slides Flows (Rock falls) (Rock slides) (Debris Flows, Mud flows)

Amount of Water

15 1999 Yosemite

Exfoliation Cracks

Talus Cone

Curry Village

5 16 Mass Wasting

 Classified as …

Creep (Slow down-slope movement of soil or regolith)

Falls Slumps Slides Flows (Rock falls) (Rock slides) (Debris Flows, Mud flows)

Amount of Water

Slumps 17

 Sliding of  Unconsolidated material  As an intact unit  Curved failure surface(s)  Often after rainfall

Figure 3.3

18 Slumping at Devil ’s Slide

 Highway 1 south of SF

6 19

Devil ’s Slide

What are some of the factors that cause 20 Devil ’s Slide to slide?

Jeep Trail Cross Section

Spring (water seep) Shale, Siltstone & Sandstone

Hard Sandstone

21 Mass Wasting

 Classified as …

Creep (Slow down-slope movement of soil or regolith)

Falls Slumps Slides Flows (Rock falls) (Rock slides) (Debris Flows, Mud flows)

Amount of Water

7 Slides 22  Rapid down -slope movement  Sediment, soil & regolith break loose  Mountainous areas  Rapid and destructive  Often after rainfall

Figure 3.3

1997 Highway 50 23

 “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

24 Mill Creek Landslide

 Hwy 50 closed 4 weeks  35,000 truck loads of earth material  $4.5 million  + >$1M/day related costs

http://vulcan.wr.usgs.gov/Projects/CalifLandslide/framework.html

8 25 Cross -Sectional View

Burned Gabbro vegitation (actually pyroxenite )

Granite (actually granodiorite )

Joints

From R.H. Syndor, Calif. , May/June 1997

Observations 26  Parent rock has composition similar to gabbro  Pyroxenite  A forest 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.

Open cracks 27 observed in 1996 Upper section “Slide ” Slide Lower section “Flow ”

Flow

Moved 50 feet up -slope

9 Hwy 50 Now Completely Mapped for 28 Landslide Hazards

Cleveland Corral Landslide Mill Creek Landslide

From T.E. Spitter and D.L. Wagner, Calif. Geol., May/June 1998

29 Cleveland Corral Slide

 Near Mill Creek Slide  Actively Monitored by USGS  Potential for blocking Hwy 50  Potential for damming American River

http://vulcan.wr.usgs.gov/Projects/CalifLandslide/framework.html

30 Monitoring System

http://vulcan.wr.usgs.gov/Projects/CalifLandslide/framework.html

10 31 Monitoring Movement

http://vulcan.wr.usgs.gov/Projects/CalifLandslide/framework.html

Slide History 32

http://vulcan.wr.usgs.gov/Projects/CalifLandslide/framework.html

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