1
Mass Wasting
Presentation Modified from Instructor Resource Center on CD-ROM , Foundations of Earth Science , 4 th Edition, Lutgens & Tarbuck
Mass Wasting 2
Down -slope movement of rock, loose material & soil under direct influence of gravity.
3 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
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 Earthquakes Thunder
6 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
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
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 road 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 Rockfall
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 Landslide 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. Geology, May/June 1997
Observations 26 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.
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
11