Weathering & Mass Wasting

Mass Movement

collective term for all downslope movements
- triggered by steep slopes (gravity) as material loses resistance
angle of repose = steepest talus slope angle
- steepness increased with more angular pieces and less with sand
  - other factors effect move, especially water
    - minor shaking can also trigger movement (earthquakes)

Slow Mass Movement

creep - slowest movement usually evident with surface features
- repeated freezing (expand) and thawing
solifluction - common in high altitudes where permafrost prevails
- soil become water-soaked and sloshed downslope
- lacks vegetation and deep roots

landslide - massive movement of earth as a unit
- medium moisture - no lubrication needed; no fluid flow
- fast (instaneous collapse)
  - often noisy & destructive leaving mess at bottom
    - 1925 Gros Ventre Landslide (east of Jackson Hole, WY)
rockslide - blocks of bedrock and rock along joints or cracks
- tend to obliterate the landscape taking all in pathway
- associated with rainy, wet conditions, clay soils, slopes
- Madison River example
rockfall - steep drops with loosened rock (usually in spring)
- Yosemite example
slump - characterized by ampitheater shaped bowl
- common during wet season
- moisture collects at base in clay sedimentary soils
- California’s coastal foothills are prime area
  - due to loose sediment hills
earthflow - almost liquefied movement of regolith
- Earth often moves limited distance and not as liquified as the next mass wasting...
mudflow - most liquefied rapid movement of material (nearly like muddy stream)
- common in California especially with loosely vegetated (fire burned) soils
- also in volcanic regions when ash and water mix to create a deadly slurry
  - Stanislaus River (Bear Valley) example (fairly fast and wet)
    - up to 12 mph and El Nino year with very heavy rain
- "lahars" come from steep, snow & glacier capped peaks
  - rapid melt with eruption burying entire villages
    - Cascade hazard - Mt. Ranier