Chapter 15 Mass Wasting - GPC：15章质量消耗预测.pptVIP

Chapter 15Mass Wasting Mass wasting The downslope movement of rock, regolith, soil, etc. under the direct influence of gravity Does not require a transporting medium (water, air, ice) Plays a role in developing landforms (surface features) Weathering, by itself, doesn’t produce significant landforms Landforms are developed as weathering products are removed from their original place Weathering weakens breaks rock apart Mass wasting transports the debris downslope Generally to a stream, which carries material away This material may then be deposited anywhere downstream Ultimate destination – the ocean For mass wasting to occur, need slopes that is, differences in elevation Most rapid mass wasting occurs in rugged, geologically-young mountains as mtn building subsides, mass wasting erosion lowers the land leads to a gentler terrain Causes of mass wasting Water Slope Vegetation Other “triggers” Water’s role a major “trigger” heavy rains or snowmelt saturate surface materials pores in sediment fill w/water, reducing cohesion between particles particles can then slide apart easily (ex.: use slightly wet sand to make a sand castle. Add more water to the sand, what happens?) Clay A “dry” clay is fairly rigid Wet clay is very “slick” Water also adds mass (“weight”), and helps to start movement of material downslope Effects of slope If slope is too steep, material will move (oversteepening) Examples: Stream undercutting a valley wall Waves pounding against a cliff Human activity (that is, stupidity) Angle of repose A characteristic of unconsolidated, granular materials (sand size or larger) The steepest angle where a pile of the material is stable Generally 25 – 40o from horizontal Larger, more angular fragments form the steepest slopes Vegetation Root systems bind soil materials together Plants also shield underlying materials from erosional effects of raindrops More important when the vegetation removed Build a house on a scenic hillside, remove the natural veget