Geo Chap 10

Geo Chap 10 - February 21, 2008 Page 1 Geological...

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February 21, 2008 Page 1 Geological Catastrophes: Chapter Ten I. Mass Movement A. Downslope movement of rock or soil as a coherent mass 1. Landslides, earthflows, mudflows, rockfalls, debris/snow avalanches, and subsidence B. Catastrophic mass movements usually triggered by other event (earthquake, volcanic eruption, rainstorm) 1. ~25 fatalities/yr in US + $1 - 2 billion damage 2. Occurs in every state II. Slope Processes A. Slope: steepness of hillside B. Most appear stable but really are dynamic and evolving C. Slope form depends on: 1. Underlying rock type 2. Climate 3. Regional tectonics 4. Regional drainage pattern III. Slope Stability A. Slope stability depends on driving and resisting forces B. Forces pulling slope downward 1. Gravity 2. Water saturation C. Must be matched or exceeded by forces holding slope in place 1. Cohesion 2. Friction D. Driving < Resisting for stability IV. Gravity Driving Force A. Gravity wants equipotential surfaces. B. Mass movements are how gravity wins C. For stability: 1. Force pushing mass against hill must be > or = to force pulling mass downslope D. Angle of repose 1. Maximum angle at which unconsolidated material on slope is stable ~30-35° on Earth …ignoring cohesion V. H 2 O Driving Force A. Too much water decreases stability due to water pressure B. P = ρw g h 1. ρw: density of water 2. h: depth from Earth’s surface to potential slide surface C. P = mwg
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February 21, 2008 Page 2 1. mw: mass/unit area of the water. When mw is multiplied by g, have force per unit area (same units as the other forces acting on the mass that might slide downslope). D. How pore pressure affects stability 1. ν(m g cosθ - P) ≥ m g sinθ 2. Reduces downward force holding mass to slope E. Water filling reservoirs can saturate rocks in toes of slopes elevated pore- water pressures VI. Other Ways H 2 O Destabilizes Slopes A. Erosion 1. Streams or waves can physically erode away (remove) loose material. Removal from base of slope (“toe”) instability B. H 2 O can chemically weaken Earth materials by dissolving
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Geo Chap 10 - February 21, 2008 Page 1 Geological...

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