2 Mass WastingThe downslope transfer of material through the direct action of gravityComponent of erosion and transport of sedimentFollows weathering, which weakens and breaks the rock
3 Controls on Mass Wasting Requires weathering of rock or unconsolidated materialPrimary driving force is GRAVITYForce mechanics: For the rock to move downslope; force (A) has to be greater than the resistance force (R) holding it up, e.g. friction.Slope angle influences force (A); the greater the angle the greater the forceRA
4 Secondary ControlsFactors that either weaken cohesion forces or increase downslope forceSaturation of the materialOver-steepening of the slopeSlope ModificationGround vibrationsExpansion/contraction cycles of soil/regolith
5 Saturation of Material Addition of waterLubricates the material (decreases cohesion)Adds weight (increases downslope force)Increases pore pressure (increases downslope force and decreases cohesion)
6 Over-steepening of the Slope Can be human-induced or by natural processes – increases the downslope force.Stream undercutting a valley wall (headward erosion, bank erosion, etc.).Waves cutting cliffs on a shoreline.Construction of roads, buildings, homes etc.
7 Angle of ReposeThe steepest angle material can create yet remain stableFunction ofParticle sizeParticle shapeMoisture Content
8 Slope Modification Removal of Vegetation Building of structures Roots of plants and trees hold regolith togetherPlants and trees remove water from the soilRemoval decrease cohesive forceBuilding of structuresDepending upon the structure and the location, either a decrease in cohesive force &/or increase downslope force due to added weight will occur
9 Ground Vibrations Earthquakes – decrease cohesion forces Initiates movement (it is easier for a particle to move than it is to begin to move)Liquefaction induced slidesHuman induced – blasting for construction, large equipment, etc.
10 Expansion/Contraction Cycles Movement of material due toWetting and drying cyclesFreeze-thaw cycles
11 Daily AssignmentRockslideSlumpDebris flowIdentify five concepts/characteristics and place them appropriately on the Venn Diagram.See next slide for list of concepts/characteristics
12 Concepts/characteristics BedrockDownslope movementUnconsolidated materialGros Ventre is an exampleFast movementMechanical weathering is involvedSlow movementDebris flow can be associated withMotion-slide along a defined planeVibrations can triggerMotion – flow, no defined planeType of mass wastingGravity inducedComponent of erosion and transportWater plays a roleCommon after volcanoes – laharCurved slip surface
13 Classification of Mass Wasting Type of MaterialBedrock - RockUnconsolidated material - DebrisSoilRegolithSedimentRate of movementmm/yrkm/hrType of MotionFall – free-fall of detached particles, slope steep enough that material falls to baseSlide – material remains cohesive and moves along a well-defined surfaceFlow – material moves downslope as a viscous flow (most are saturated with water)
14 Creep Slow downslope movement – mm/yr Process so slow one cannot observe it in actionExpansion/contraction cycles play a key roleEnhanced by burrowing organisms, periods of prolonged rains or snow,
16 Solifaction Special type of creep Occurs in regions underlain by permafrost (permanently frozen, water-bearing ground)During warm periods top portion thaws and becomes saturatedSaturated zone flows over frozen layers
17 Debris Flow Mixture of rocks, mud, & water Moves as a viscous fluid Common after heavy rainsRapid movement – up to 50 km/hr, the more water present the faster the rate of movementCommon in semi-arid regions and along volcanoes (lahars)
22 Land-Rock Slide Movement of material along a defined slippage plane Sudden and rapid movementLarge blocks of rock detach along bedding planes, joints, fractures, etc.Occur on steep slopesCan be triggered by rain falls or ground vibrationFastest and most destructive type of mass wasting