landslides and other mass movements

Question 1

mass movement

Answer 1

earth’s surface is mostly unstable
downslope motion of roil, soil, sediment, snow,ice
dirven by gravity
wide range of rates

Question 2

all slopes are unstable and change continuously

Answer 2

mass movement is important to the rock cycle
initial step in sediment transportation. agent of landscape change

Question 3

4 factors to classify mass movement

Answer 3

type of material
velocity of movement
nature of the mass
movement environment

Question 4

creep

Answer 4

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

Question 5

solifluction

Answer 5

low downhill movement of tundra
related to freeze-thaw activity
melted permafrost flows over deeper frozen soil
generates solifluction lobes on hillside

Question 6

slumping

Answer 6

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

Question 7

mudflows, debris flows, lahars

Answer 7

water rich movement
fast: more water or steeper slope angle
slow: less water or lower slope angle
follow river channels down valley

Question 8

descirbe mudflows and lahars

Answer 8

mudflow: slurry of water and fine sediment, tropical setting with abundant rainfall
lahar: volcanic mud or debris flow (mix of volcanic ash and water)

Question 9

rock and debris slides

Answer 9

sudden movement downslope
rock slide: rock only
debris slide: mostly regolith (unconsolidated rocky material covering bedrock)
sudden and deadly movement down failure surface

Question 10

avalanche

Answer 10

turbulent clouds of debris and air
snow avalanche: oversteepened snow that detaches
tend to reoccur in clealry defined avalanche chutes

Question 11

wet vs dry avalanches

Answer 11

wet: move slowly, hug slope, little air, viscous slurry
dry: cold powdery snow, layer of pressurized air, move rapidly (25o kmh)

Question 12

rockfalls and debris falls

Answer 12

vertical freefall of mass
when blocks impact, they fragment and continue moving
talus: accumulation of angular rock fragments by mechanical weathering of rock mass

Question 13

submarine mass movement

Answer 13

slumps: semicoherent blocks break and slip
debris flows: broken material moves as slurry
turbidity currents: sediment moves as turbulent cloud
often preserved by burial

Question 14

gigantic submarine mass

Answer 14

larger than land-based mass movements
important process for shaping land
tied to horrible tsunamis

Question 15

why do mass movements occur?

Answer 15

• 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

Question 16

slope stability is a dynamic balance between two forces

Answer 16

downslope force: gravitational pull
resisting force: material properties resist motion (cohesion and friction)
slope failure occurs when downslope forces prevail

Question 17

angle of repose

Answer 17

smaller particle sizes= smaller angle of repose

Question 18

weak failure surfaces

Answer 18

saturated sand or clay layers
joints parallel to land surface
weak sedimentary bedding
metamorphic foliation planes

Question 19

triggering event

Answer 19

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

Question 20

changes in slope strength

Answer 20

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

Question 21

preventing mass movements

Answer 21

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

Question 22

engineered structures to reduce mass movement

Answer 22

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