Cee 1030 1st Edition Lecture 15 Current Lecture Mass Wasting Mass wasting movement of rock regolith and or soil downslope under the direct influence of gravity Landform development Weathering weakens and breaks rock apart Mass wasting transfers debris down slope via gravity not talking about the part of the process that is weathering strictly looking at things caused by gravity Stable and unstable slopes when loose sediments such as sand or gravel are piled particles will move downslope until the pile s slope becomes stable sediment piled too steeply is unstable will collapse Angle of repose angle of repose the steepest angle at which a pile of unconsolidated sediments remain stable the angle of repose varies according to size and angularity of the sediments particles how much water is mixed with the particles the friction of the surface beneath the pile Role of particle size and shape angle of repose is greater for sand than marble because of particle size difference Role of water dry particle no water angle of repose determined by friction of grain to grain contract small amount of water increases cohesion of particles increases angle of repose large amount of water eliminates particle cohesion decreases friction and adds weight decreases angle of repose Role of underlying surface angle of repose is affected by friction of particles against surface beneath sediment pile These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute low friction surface smaller angle of repose high friction surface greater angle of repose Oversteepened slopes oversteepened slope landscape with slope greater than the material s angle of repose Examples stream undercutting a valley wall waves eroding base of a cliff oversteepened slopes are unstable sooner or later the oversteepening will be eliminated by mass wasting and the slope restored to stability Effects of vegetation removal deforestation increases rates of mass wasting by decreasing the angle of repose root systems bind soil and regolith together leaves on plants shield soil surface from erosional effects of rain Earthquakes as triggers ground vibrations from earthquakes can cause liquefaction liquefaction when shake water saturated surface sediments can behave as a fluid and flow Landslides without triggers problem not all mass wasting events are triggered by earthquakes slope materials weaken over time and random events are unpredictable Submarine landslides mass wasting is common and widespread on the ocean floor landslides often occur along passive continental margins and flanks of submarine volcanoes triggered by buildup of unstable sediments or by forces such as storm waves or earthquakes submarine landslides can trigger tsunamis Mass wasting processes basis of classification 1 type of material rock debris earth mud 2 type of motion fall slide flow 3 rate of movement rapid vs slow Material of mass wasting type of material involved rock descending material began as bedrock debris descending material began as regolith with more coarse particles than fines earth descending material began as regolith with more fine particles than coarse Movement of mass wasting Type of motion involved fall free fall of sediment slide sediment moves along a surface flow material moves as a viscous fluid Velocity of mass wasting fast mass wasting events tend to happen in geologically young areas ten to wear down the peaks and fill in the valleys slow mass wasting events as a landscape ages less dramatic downslope movements occur Falls fall rapid down slope movement of rocks or regolith that have broken away from sloped outcrops particles lose contact with surface as they free fall through air or bounce and roll downslope slide slow to rapid downslope movement of a relatively intact rock or regolith that has detached from a sloped outcrop moves along a well defined rupture surface translational slide material moves downslope along a flat surface rational slide slump material moves downslope along a curved rupture surface rotation as it moves and causing surface features to tilt Flows flow continues downslope movement of water saturated material rock and regolith that moves like a viscous liquid different types of flows differentiated by speed motion slow flow type creep motion fast flow type flow motion very fast flow type avalanche debris avalanche very rapid flow of water saturated rock regolith vegetation or even ice downslope Earth flows earth flow moderate to rapid movement of water saturated clay rich fine grained sediment downslope commonly on hillsides in humid areas during heavy precipitation or snowmelt earthflows typically from a tongue or teardrop shaped mass that moves slowly but persistently Debris flows mud flows debris flow moderate to rapid movement of water saturated mostly coarse grained regolith downslope Lahars lahar a type of debris flow that occurs on volcano slopes when unstable layers of ash and other debris become water saturated and move rapidly down slope Creep creep slow movement of water saturated rock and or regolith downslope often driven by repeated expansion and contraction of surface sediments cause by repeated cycles of freezethaw or wet dry key indicators of creep bent tree trunks titled fences breaks in retaining walls tension cracks in roads Solifluction soil creep solifluction a type of creep in which water saturated soils flow slowly downhill typically in areas where water cannot escape from the saturated surface layer by infiltration to deeper layers