Mass Wasting Flashcards
Mass movement (wasting) is:
- Downslope motion of rock, soil, sediment, snow, and ice
- Driven by gravity on a sloping surface
- Characterized by a wide range of rates (fast to slow)
- All slopes are unstable and change continuously
- Often aided by human activity
Mass movements are costly natural hazards
- Crucial component of the rock cycle
- May cause damage to living things and buildings
- Hazards can produce catastrophic losses
Mass movements are important to the rock cycle
- Initial step in sediment transportation
- Significant agent of landscape change
Types of Mass Movement
Classification is based on 4 factors:
○ Type of material (rock, regolith, snow, or ice)
○ The velocity of movement (fast, intermediate, or slow)
○ Nature of the mass (chaotic, coherent, or slurry)
○ Movement environment (subaerial or submarine)
Slow to Fast Mass Movements:
- Creep, solifluction, and rock glaciers
- Slumping
- Lahars and mudflows
- Debris flows
- Rockfalls and slides
Creep
slow downhill movement of regolith (loose shit on the surface)
- Due to seasonal soil expansion and contraction
○ Wetting and drying
○ Freezing and thawing
○ Warming and cooling
- Grains moved:
○ Perpendicular to slope upon expansion
○ Vertically downward by gravity upon contraction
- Evident by tilting of landscape features (i.e trees, telephone poles, retaining walls, foundations, tombstones)
Solifluction
low downhill movement of tundra
- Melted permafrost slowly flows over deeper-frozen soil
- The process generates solifluction lobes (ripples of soil moving downhill)
Slumping
sliding of regolith as coherent blocks - Slippage occurs along spoon-shaped (curved) "failure surface" - Variety of sizes and rates of motion - Slumps have distinctive features: ○ Head scarp - upslope cliff face ○ Toe - material at the base ○ Discrete faulted slices - Slumps are common along seacoasts and river cut banks ○ Blocks that fall into the water are often quickly eroded - Slumps can move slowly ○ Can observe them develop ○ Reduces potential harm
Mudflows, debris flows, and lahars
H2O-rich movement
- Move at a variety of speeds
○ Faster - more water or steeper slope angle
○ Slower - less water or lower slope angle
- Tend to follow river channels down the valley
- Spread out into broad lobes at the base of a slope
- Essentially unlimited competence (capability)
Mudflows
A slurry of water and fine sediment
- Common in tropical settings with abundant rainfall
Debris flow
a mudflow with many large rocks
Lahar (volcano):
special volcanic mud or debris flow
- Volcanic ash (recent or ongoing eruptions)
- Water from heavy rains or melted glacial ice
Rock and Debris Slides:
- Rock slide: a slide consisting of rock only
- Debris slide: a slide comprised mostly of regolith
○ Movement down the failure surface is sudden and deadly
○ Slide debris can move at 300km/h on a cushion of air
The Vaiont Dam disaster, Italian Alps
- Now we evaluate underlying geology for critical structures
- Reservoir added water into the underlying limestone = mountain collapsed
- Limestone over shale in a deep synclinal gorge
- October 9, 1963: 600mil tons of limestone slid into reservoir
- Resulting wave destroyed villages; killed 2.6k people
- The dam still stands, water flooded over dam b/c debris now fills the reservoir
Avalanches
- Snow avalanche - over-steepened snow that detaches
- Tend to reoccur in clearly defined avalanche chutes
- Lethal to people caught in the way
- Wet avalanches (thick wet snow) behave like a viscous slurry
○ Hug the slope, entraining little air
○ Move relatively slowly (usually ~30km/h) - Dry avalanches move cold, powdery snow
○ Move above ground surface on layer of pressurized air
○ Move rapidly (up to ~250km/h)
