landslides and other mass movements Flashcards
mass movement
earth’s surface is mostly unstable
downslope motion of roil, soil, sediment, snow,ice
dirven by gravity
wide range of rates
all slopes are unstable and change continuously
mass movement is important to the rock cycle
initial step in sediment transportation. agent of landscape change
4 factors to classify mass movement
type of material
velocity of movement
nature of the mass
movement environment
creep
slow downhill movement or regolith
due to soil expansion and contraction (seasonal)
grains move perp to slope upon expansion, vertically downward by gravity upon contraction
evident from tilting of landscape features
solifluction
low downhill movement of tundra
related to freeze-thaw activity
melted permafrost flows over deeper frozen soil
generates solifluction lobes on hillside
slumping
sliding of regolith as coherent blocks
slippage occurs along spoon-shaped failure surface
variety of sizes and rates of motion
scarp: upslope cliff face
toe: material at base
discrete faulted slices
common along seacoasts and river cut banks
mudflows, debris flows, lahars
water rich movement
fast: more water or steeper slope angle
slow: less water or lower slope angle
follow river channels down valley
descirbe mudflows and lahars
mudflow: slurry of water and fine sediment, tropical setting with abundant rainfall
lahar: volcanic mud or debris flow (mix of volcanic ash and water)
rock and debris slides
sudden movement downslope
rock slide: rock only
debris slide: mostly regolith (unconsolidated rocky material covering bedrock)
sudden and deadly movement down failure surface
avalanche
turbulent clouds of debris and air
snow avalanche: oversteepened snow that detaches
tend to reoccur in clealry defined avalanche chutes
wet vs dry avalanches
wet: move slowly, hug slope, little air, viscous slurry
dry: cold powdery snow, layer of pressurized air, move rapidly (25o kmh)
rockfalls and debris falls
vertical freefall of mass
when blocks impact, they fragment and continue moving
talus: accumulation of angular rock fragments by mechanical weathering of rock mass
submarine mass movement
slumps: semicoherent blocks break and slip
debris flows: broken material moves as slurry
turbidity currents: sediment moves as turbulent cloud
often preserved by burial
gigantic submarine mass
larger than land-based mass movements
important process for shaping land
tied to horrible tsunamis
why do mass movements occur?
- earth materials subject to topographic slope forces
- materials weakened or loosened from attachments
- upper crust broken by jointing and faulting
- surface material weaker than solid crustal rock
slope stability is a dynamic balance between two forces
downslope force: gravitational pull
resisting force: material properties resist motion (cohesion and friction)
slope failure occurs when downslope forces prevail
angle of repose
smaller particle sizes= smaller angle of repose
weak failure surfaces
saturated sand or clay layers
joints parallel to land surface
weak sedimentary bedding
metamorphic foliation planes
triggering event
can be natural or due to human activity
mass movement can also occur without a triggering event
adding weight to top of slope and steepening slope beyond angle of repose can destabilize a slope
changes in slope strength
weathering = weaker regolith
vegetation = stabilizes slopes
water = reduction of slope strength due to added weight, lubrication between grain contacts, water in pores pushes grains apart
preventing mass movements
revegetation: removes water by evapotranspiration, roots bind and anchor regolith
terracing: redistributing mass
regrading: reshaping slope below angle of repose
drainage: dewatering reduces weight and increases strength
slowing or eliminating undercutting: by removing agent of erosion or simply its effect
engineered structures to reduce mass movement
retaining walls: pin base and trap rock
covers: fencing or coating that drapes over outcrop
rock staples: hold loose facing
avalanche sheds
controlled blasting: removal or dangerous rock
