1 Mass Wasting and landform development 2 Mass Wasting and landform development Mass wasting Def.: the downslope movement of rock and soil under the direct influence of gravity 3 Role of mass wasting The step that follows weathering Transfers debris downslope to stream valleys 4 Controls and triggers of mass wasting Gravity is the controlling force Important triggers include 1) Saturation of the material with water (rainfall) –Diminishes particle cohesion –Water adds weight 5 Controls and triggers of mass wasting 2) Oversteepening of slopes –Oversteepening slopes are unstable –Stable slope angle (angle of repose; 25-45o) is different for various materials 6 Important triggers include: 3) Removal of anchoring vegetation Root systems bind soil 7 “cut and fill” construction 8 Important triggers include: 4) Ground vibrations from earthquakes –May trigger land slides and result in property damage –Can cause liquefaction – water saturated surface materials behave as fluid-like masses that flow 9 Other triggers Freezing – thawing Construction 1 Volcanic eruptions Freeway traffic 10 Landslides without triggers Slope materials weaken over time Random events that are unpredictable 11 Classification a) Type of material involved • Debris • Mud • Earth • Rock 12 Classification b) Type of motion Fall (free-falling pieces) Slide (moves along a surface as a coherent mass) Flow (moves as a chaotic mixture) 13 Classification c) velocity Fast (avalanche, ~200km/hour) Slow (creep, mm or cm/year) 14 Rock avalanche in Alaska triggered by the 1964 earthquake 15 16 Types of mass wasting Rockfall 17 Rock fall, Oregon 18 Rock fall and Talus Slope, Banff National Park, Canada 19 Types of mass wasting Slump (rotational slide) Movement of material as a unit along a rotational surface Occurs along oversteepened slopes 20 Slump and earth flow) Fig. 9.10, p. 252 21 Slump, southwest Montana 22 Incipient Slump, Missouri River, North Dakota 23 Slump, Missouri River, North Dakota 2 24 Types of mass wasting Creep Extremely slow downhill movement Aided by soil moisture, ice wedging, and critters and plants Fig. 9.17, p. 258 25 Creep 26 Some visible effects of creep Fig. 9.18, p. 259 27 Types of mass wasting Earthflow Def.: Downslope movement of clay-rich, water saturated materials Fig. 9.6, p. 249 28 An earthflow 29 Earthflow near San Francisco 30 Types of mass wasting Rockslide (planar slide) Blocks of bedrock slide down a slope Generally very fast and destructive 31 Gros Ventre rockslide Fig. 9.13, p. 254 32 Earthquake Lake, Montana 33 Types of mass wasting Debris flow and/or mudflow Soil and regolith with a large amount of water Often confined to channels 34 Lahar Debris flow composed mostly of volcanic materials, on the flanks of volcanoes 35 Lahar 36 Types of mass wasting Solifluction Def.:Slow, downslope movement of water-saturated materials common to permafrost areas Solifluction lobes, Tien Shan Mountains, Kyrgyztan 37 Permafrost regions 38 Ground subsidence due to thawing permafrost 39 Ground subsidence due to permafrost, Alaska 40 End of Chapter 9 3.