Mass Movement •  Mass movement is the downslope

transfer of loose rock from hillsides to low lying areas

-very common occurrence -on continents and ocean floor -all slopes are mobile & constantly changing due to gravity

•  Gravity is the force which drives mass-wasting

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

•  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; is 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

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

End of Chapter 11

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