Mass Movements

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

Introduction

• Rock, soils, and other materials move downhill– Sometimes rapidly,

sometimes slowly– Pose threats to people– Modify the landscape

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

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

Cohesion

• Besides shear stress, cohesion also resists motion

• Is tendency of materials to stick together to resist movement

• Cements and roots can increase cohesion

Water Pressure

• Increases chance of failure• Pressure of pores filled with water reduces

frictional resistance

Forms of Mass Movement

• Mass movement – movement of earth materials by gravity– Mass wasting is term also used

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

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

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

Flows

Rockslides

• Mass of bedrock moving rapidly downslope

• One of most dangerous mass movement types

Fall

• Rocks break off and fall to base of slope/cliff– Talus

accumulates at base

U.S. Map of Landslide Risk