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Mass Movements. Introduction Explain why slopes fail under force of gravity Explain factors influencing slope failure Describe types of mass movements.

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Presentation on theme: "Mass Movements. Introduction Explain why slopes fail under force of gravity Explain factors influencing slope failure Describe types of mass movements."— Presentation transcript:

1 Mass Movements

2 Introduction Explain why slopes fail under force of gravity Explain factors influencing slope failure Describe types of mass movements and why they usually occur Discuss importance of mass movement processes in shaping landscapes and associated hazards

3 Introduction Rock, soils, and other materials move downhill – Sometimes rapidly, sometimes slowly – Pose threats to people – Modify the landscape

4 Slope Stability and Mass Movement Gravity is main force driving materials downslope Slopes have threshold of steepness – Angle of repose is steepest angle at which a given material will remain stable – `sand 33-35 degrees, boulders as steep as 45 Frictional strength of material resist movement – Rough surfaces greater frictional strength – Increased weight of material increases resistance

5 Force of Gravity Two components – Shear stress –pushes block parallel to slope – Other component pushes block into slope Stress is perpendicular to slope surface The greater the weight of block the more resistance to movement

6 Cohesion Besides shear stress, cohesion also resists motion Is tendency of materials to stick together to resist movement Cements and roots can increase cohesion

7 Water Pressure Increases chance of failure Pressure of pores filled with water reduces frictional resistance

8 Forms of Mass Movement Mass movement – movement of earth materials by gravity – Mass wasting is term also used

9 Creep and Solifuction Slow almost imperceptible mass movement – Revealed by tilted trees, fence posts – Soil particles rise with freeze or wetting, settle lower on slope when warmed or dry – Flow of soil occurring mostly cold high latitudes

10 Slides Blocks of rocks moving on distinct plane – Landslide is general term – Slump is movement on a curved plane – Rockslides – large blocks break loose, move rapidly downslope Important factors in slides – Groundwater pressure – Oversteepening slopes

11 Flows Fluid movement of loose earth materials – Earth flows are slow moving flows of mostly fine grained or clay-rich soil/sediment – Debris flows – fast-moving mixture of sediment and water Mudflows are debris flows of mostly muddy sediment Debris flows involve water

12 Flows

13 Rockslides Mass of bedrock moving rapidly downslope One of most dangerous mass movement types

14 Fall Rocks break off and fall to base of slope/cliff – Talus accumulates at base

15 U.S. Map of Landslide Risk


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