Week 9, Mass Movements and Avalanches Flashcards
Gravity is always toward the ____ of the earth, not ____ to surface.
Gravity is always toward the centre of the earth, not perpendicular to surface.
For an object to move parallel to a slope we need 2 criteria:
Must overcome
Friction (shake or kick boulder) AND inertia (rain)
How water increases instability
- adding weight to soil or porous rock
- expanding and weakening clay minerals
- decrease rock cohesion, also called cement dissolution
- subsurface erosion
- increasing pore pressure due to burial weight, destabilizes soil
- raising water table
How can we trigger a mass movement event?
- heavy rain
- earthquakes
- thawing of frozen ground
- surface disruption by humans
1 kg boulder experiences __N of force.
1 kg boulder experiences 9.8N of force.
Mass Movement Classification
Downward: Falls and subsidence
Down and outward: Slides and flows
Mass Movement Classification: Falls
Individual blocks detach along fractures
Mass Movement Classification: Subsidence
Collapse into void, dominantly vertical downward movement, moves as separate blocks.
Mass Movement Classification: Slides
Slide of blocks on surface
Mass Movement Classification: Flows
No sliding surface, more as fluids. (Includes creep)
2 types of slides
rotational and translational
Rotational Slide
Decrease in the driving mass, increase in the resisting mass. Does not typically slide for long distances.
Translational Slide
Mass slides on surfaces of weakness, can slide for long distances.
Block Slides: Material remains intact
Debris Slides: Material deforms as it slides
Lateral Slides: Underlying material breaks and flows
Creep
Slowest but most common failure.
Caused by successive expansion/contraction due to freezing/thawing, wetting/drying of clay minerals heating/cooling.
Expansion due to heating it is perpendicular to the surface while contraction is due to gravity.
Complex Events
Mass movements that combine falls, slides, and flows along their path.