Lecture 24 mass wasting

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Chapter 16 Landslides & Mass Wasting Earth’s Surface is shaped by external processes… Earth’s Surface is shaped by external processes… In sculpting the Earth’s surface, the two most important agents of erosion are : 1) Mass wasting 2) Running water There are a wide variety of manifestations of the downslope movement of materials by gravity, some faster and some slower pp.490-491 original artwork by Gary Hincks All of these processes have destructive effects… Mass Wasting: Downslope, mass movement of Earth materials Driven by: The pervasive background force of …GRAVITY… Contributing factors: Saturation of sediments by water Water fills pore spaces between sediment grains, reduces internal resistance, adds weight Over-steepened slopes Slopes become unstable once they reach the angle of repose = The steepest angle a slope can attain without slumping Removal of vegetation Plants add slope stability by protection against erosion Earthquakes Strong ground vibrations Stability against gravity depends on the strength of a material, which can be represented by its angle of repose… In sediments, this angle depends on grain and sorting Fig 16.20 c W W Norton In sediments, the angle of repose depends on grain size and sorting of materials… Mass Wasting Types of materials: Soil/regolith -or- Rock/bedrock Types of movement: Rock Falls - Free-fall of material Rock/Debris Slides - Coherent slabs slide along fracture surfaces Mudflows - Soil and rock mixes with water and becomes fluidized Earth or Debris Flows - Materials move as a viscous mass Rates movement: Fastest - Rock of falls & avalanches Avalanches “float” on entrapped air Slowest - Creep (cm/year) Talus slopes Types of mass wasting processes arrayed by typical velocity of movement… Fig 16.12 W W Norton Creep Fig 16.12 W W Norton MASS WASTING Soil/Regolith Creep Creep - Slow (cm/year) downhill movement of material - Driven by alternate expansion/contraction of material during freeze/thaw or cycles of wetting/drying Gravitational force acts on rocks/soil to move them downslope… Fig 16.02 b W W Norton Effect of cycles of freeze-thaw on soil/regolith creep… Fig 16.02 a W W Norton Soil/regolith creep… Soil Regolith Soil/Regolith Creep Slow! Assisted by: “Frost heaving” (expansion of ice upon freezing) Evidence of soil/regolith creep… Tell-tell signs of soil/regolith creep… Fig 16.02 c W W Norton Signs of soil/regolith creep… Fig 16.02 d (c) Martin Miller Signs of soil/regolith creep… Fig 16.03 a (c) Martin Miller Solifluction: Soil creep in high latitude, cold climate areas where freeze-thaw is active… Reducing soil creep… In summary: Things to watch for… [...]...Rock Fall/Debris Fall Fig 16.12 W W Norton MASS WASTING Rock/Debris Falls Blocks of bedrock break free, and fall from a steep cliff face Contributing factors: - Steep slopes - Rocks loosened along joint fractures… …by expansion of water on freezing,... all rock falls! Mediation by terracing Avalanches Fig 16.12 W W Norton Before After Peruvian Valley Rock Avalanche, May 1970 Rock/debris slides Fig 16.12 W W Norton Rock Slides… Beds dip downslope MASS WASTING Rock Slide Blocks of bedrock break free, and slide down slope along a fracture surface Often occurs where strata are inclined, with slip occurring along bedding planes of weak units, like shales
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