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