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)
Rockfalls and Debris falls:
The vertical freefall of mass
- Bedrock or regolith falls rapidly downward
- When blocks impact, they fragment and continue moving
- Talus blocks pile up at the base of the slope
Submarine mass movements are often preserved by
burial
What are the three types of submarine mass movements based on degree of disintegration:
- Submarine slumps: semi-coherent blocks break and slip
- Submarine debris flows: broken material moves as a slurry
- Turbidity currents: sediment moves as a turbulent cloud
Gigantic submarine mass movements are documented
- Much larger than land-based mass movements
- Mass movements are tied to tsunamis
Why do Mass Movements Occur?
- Mass movements require that earth materials:
○ Be subjected to topographic (slopes) forces
○ Be weakened or loosened from their attachments - Fragmentation and weathering
○ The upper crust is broken by jointing and faulting
○ Chemical and physical weathering produces regolith - Slopes may be stable or unstable
- Slope stability is a dynamic balance between forces
○ Downslope force - gravitational pull
○ Resisting force - material properties that resist motion - Slope failure occurs when downslope forces prevail
Slope Stability
- Downslope forces = gravity
○ Weight of earth materials
○ Weight of added water
○ Weight of added structures - Resisting forces = material strength
○ Cohesion: chemical bonds, electrical charges, surface tension
○ Friction - Steeper slopes = larger forces
- Loose granular material assumes a slope angle
○ “angle of repose” is due to: particle size and shape and the surface roughness
○ Typical angles of repose: fine sand - 30º, coarse sand - 40º, angular pebbles - 45º
Weak subsurface failure surfaces can initiate motion (layers underneath can cause movement rather than the surface)
- Types of weak failure surfaces include: ○ Saturated sand or clay layers ○ Joints parallel to land surface ○ Weak sedimentary bedding ○ Metamorphic foliation planes
Destabilizing events trigger slope failure
- Triggers are both natural and anthropogenic
- Shocks, vibrations, and liquefaction
- Changes in slope loads, steepness, and support
- Changes in slope strength
There may be no trigger event that cause
mass movement (rare!!)
Shocks, vibrations, and liquefaction
- Ground vibrations decrease material friction
- On an unstable slope, the downslope force takes over
- Vibrations are common (motions of heavy machinery or trains, earthquakes)
- Vibrations can cause saturated sediment to liquefy
Changes in characteristics can destabilize a slope
- Loading: adding weight to the top of a slope
○ Water - as rain or via humans (lawns, septic systems)
○ Materials - buildings, waste materials, fill, etc. - Angle: steepening a slope beyond the angle of repose
○ River incision
○ Excavation for buildings and roads - Removing support - undercutting leads to failure
○ Natural - a river eroding the base of a slope
○ Human-induced - excavating the base of a slope
Changes in slope strength
- Weathering: creates weaker regolith
- Vegetation: stabilizes slopes. Removing vegetation:
○ Greatly slows removal of excess waste
○ Destroys an effective stapling mechanism (roots)
○ Slope failures are common after forest fires destroy vegetation - Water: reduces slope strength in several ways:
○ Adds a great deal of weight
○ Water in pores pushes grains apart, easing disintegration
○ Removing water strengthens a failure surface
How Can We Protect Against Disaster?
- Identifying regions at risk
- Several factors are significant to mass movements:
○ Relief - steeper slopes have more mass movement
○ Climate - more rainfall creates more water problems
Protecting Against Mass Movement
- Geologic mapping can identify areas at risk
○ Highlight past failures
○ Reveal currently unstable slopes: cracked and bulging ground, measurable changes in surveyed land features - GPS can detect slow movements
Preventing Mass Movements
- Revegetation
- Redistributing mass by terraces
- Regrading
- Drainage
- Slowing or eliminating undercutting
- Engineered structures
- Controlled blasting
Revegetation
Adding plants have two effects:
○ Removes water by evapotranspiration
○ Roots help to bind and anchor regolith
Redistributing mass by terraces
- Removes some of the mass loading a slope
- Catches debris
- like making big steps lol
Regrading
reshaping slopes below the angle of repose
Drainage
Dewatering reduces weight; increases strength
Slowing or eliminating undercutting
increases stability
- Removing agent of erosion at the base of a slope
- Reducing the effect of the agent of erosion (i.e riprap: loose rocks used to form a foundation for a breakwater or other structure = artificial shoreline)
Engineered structures
Safety structures built to improve slope stability or to reduce mass movement
○ Retaining walls - barriers that pin the base and trap rock
○ Covers - fencing or coating that drapes over the outcrop
○ Rock staples - rods drilled into the rock to hold loose facing
○ Avalanche sheds - structures that shunt avalanche snow
Controlled blasting
Surgical removal of dangerous rock
