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. Earth s s External Processes Weathering, mass wasting, and erosion are all called external processes because they occur at or near Earth s s surface Internal processes, such as mountain building and volcanic activity, derive their energy from Earth s s interior 3 1
Mass Wasting: The Work of Gravity 4 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 mass wasting occurs. Steepness of slope Water Adds weight lubricates failure surfaces Vegetation Roots hold soil Absorb water Triggers Earthquakes Thunder 5 Mass Wasting: The Work of Gravity 6 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 mass wasting occurs. Steepness of slope Water Adds weight lubricates failure surfaces Vegetation Roots hold soil Absorb water Triggers Earthquakes Thunder 8 Types of Mass Wasting 9 Figure 3.3 3
Mass Wasting 10 Classified as (Slow down-slope regolith) Falls (Rock falls) Slides Flows (Rock slides) (Debris Flows, Mud flows) Amount of Water Steepness of Slope The Angle of Repose 11 Angle of Repose: The steepest angle at which loose material remains stationary without sliding downslope. Angle of Repose Mass Wasting 12 Classified as (Slow down-slope regolith) Falls (Rock falls) Slides Flows (Rock slides) (Debris Flows, Mud flows) Amount of Water 4
Slow, down-slope sediment. Responsible for building and road damage. 13 http://www.structures.ucsd.edu/taiwaneq/geotechnical.html Mass Wasting 14 Classified as (Slow down-slope regolith) Falls (Rock falls) Slides Flows (Rock slides) (Debris Flows, Mud flows) Amount of Water 1999 Yosemite Rockfall 15 Exfoliation Cracks Talus Cone Curry Village 5
Mass Wasting 16 Classified as (Slow down-slope regolith) Falls (Rock falls) Slides Flows (Rock slides) (Debris Flows, Mud flows) Amount of Water Sliding of Unconsolidated material As an intact unit Curved failure surface(s) Often after rainfall 17 Figure 3.3 Slumping at Devil s s Slide 18 Highway 1 south of SF 6
19 Devil s s Slide What are some of the factors that cause Devil s s Slide to slide? Jeep Trail Cross Section 20 Spring (water seep) Shale, Siltstone & Sandstone Hard Sandstone Mass Wasting 21 Classified as (Slow down-slope regolith) Falls (Rock falls) Slides Flows (Rock slides) (Debris Flows, Mud flows) Amount of Water 7
Slides Rapid down-slope movement Sediment, soil & regolith break loose Mountainous areas Rapid and destructive Often after rainfall 22 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 Mill Creek Landslide 24 Hwy 50 closed 4 weeks 35,000 truck loads of earth material $4.5 million + >$1M/day related costs 8
Cross-Sectional View 25 Gabbro (actually pyroxenite) Burned vegitation Granite (actually granodiorite) Joints From R.H. Syndor, Calif. Geology, May/June 1997 Observations 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. 26 Open cracks observed in 1996 Slide 27 Upper section Slide Lower section Flow Flow Moved 50 feet up-slope 9
Hwy 50 Now Completely Mapped for Landslide Hazards 28 Cleveland Corral Landslide Mill Creek Landslide From T.E. Spitter and D.L. Wagner, Calif. Geol., May/June 1998 Cleveland Corral Slide 29 Near Mill Creek Slide Actively Monitored by USGS Potential for blocking Hwy 50 Potential for damming American River Monitoring System 30 10
Monitoring Movement 31 Slide History 32 11