Download chapter 11 Mass Movement

Mass Movement
•  Mass movement is the movement of
regolith (blanket of decayed rock
debris and soil) and masses of rock
downhill
-very common occurrence
-on continents and ocean floor
-all slopes are mobile & constantly
changing due to gravity
•  Gravity is the force which drives masswasting
Resistance to Movement
•  Cohesive strength is the resistance of
an object to move downhill - an
accumulation of forces
– Friction
– Cohesion, stickiness of particles
– Other forces holding a particle in place
(plant roots, cementation)
The Role of Water
•  Water may act to increase or decrease
cohesive strength
– Dry regolith or soil has little or no
cohesion; angle of repose of sand 30°
– Damp regolith or soil is sticky
– Saturated regolith or soil flows easily
Factors increase chances of
downslope movement
1)  Saturation of material with water
2)  Vibration from earthquakes
3)  Alternating expansion/contraction
4)  Undercutting slopes by streams, waves
5)  Man made modification of slopes ex.
Undercutting of slopes, tree removal
Types of Mass Movement
•  Includes all types of slope failures
•  Classified by
– Type of motion (slippage planes or not)
– Type of material (rock thru clay+/- water)
– Rate of movement (slow vs. fast)
Creep
•  Extremely slow movement of soil and
regolith - 1 to 10 mm/yr
•  Evidence: Bulges, wavelike swells in soil
–  Bending of strata downslope
–  Tilted poles, deformed roads/fences, damaged
•  Combination of factors cause creep
– Heaving of soil - expansion & contraction
is the primary cause
•  Wet-dry cycles (Houston)
•  Freeze-thaw cycles
Debris Flows
•  Mixtures of water, mud and rock
– Can begin on steep hill as soil slumps
– Liquified soil flows downhill
•  Up to 50 km/hr; as slow as freshly mixed
concrete; as fast as rapid current of water
– Water lubricates mass of soil and rock;
responsible for high velocity
– Large boulders, building etc. may be
carried by viscous fluid
Mudflows
•  Debris flows consisting of silt and clay
sized particles
– Usually associated with heavy rain
– High water content - up to 30%
– Usually follow stream valleys
– Also capable of floating houses, large
boulders
Landslides
•  Mass movement along well defined
slippage or fracture plane
•  Landslide block moves as a single or
group of units called slumps
•  Rock type, orientation and water
content influence events
•  Matter of seconds or weeks/months
Slump Block
•  Spoon shaped slippage plane
•  Bedding or surfaces are tilted
backward toward the scar
•  Usually have multiple slippage planes
•  Traits: scar, tilting of bedding, jumbled
poorly drained small hills from
previous slides
Figure 11.10 a,b. Diagram of a slump block
landslide
1963 Vaiont Landslide Disaster
•  Worst dam disaster in history; N. Italy
–  Killed ~ 2600 people
•  240,000,000 m3 of rock involved in
landslide
•  Filling of dam caused change in subsurface
hydrogeology
•  Water lubricated bedding planes; weak
limestone interbedded with clay layers
steeply inclined toward reservoir
Fig. 11.3. Vaiont
dam disaster
Rockslide
•  Rapid movement of large blocks of
rock
•  Slippage plane usually associated with:
– Bedding plane
– Joint plane
– Structural weakness
•  Blocks generally degrade as they move
Rock Falls & Avalanches
•  Range from single free-falling rock to
a mass of rock particles
•  Lack water to lubricate flow
•  Flows generally do not move far
•  No slippage plane
Subaqueous Mass Movement
•  Active in areas of rapid sediment
deposition & steep slopes ex. Deltas,
convergent margins; cont. slopes
•  Also occur on flanks of volcanic
islands & seamounts
•  May spread out over large areas
•  Flow several hundred kilometers
Basic elements of slope systems
End of Chapter 11