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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