Chapter 13: Landslides and Other Mass Movements Flashcards
regolith
soil and loose sediment or debris
mass movement
aka mass wasting
the downslope transport of rock, snow, and ice
landslide
mass movement of rock or regolith down a slope
four features that help classify types of mass movement
- material involved
- velocity of the movement (slow, intermediate, or fast)
- character of the moving mass (coherent or chaotic; wet or dry)
- the environment in which the movement takes place
creep
refers to the slow, gradual downslope movement of regolith on a slope; happens when regolith alternatively expands and contracts in response to freezing and thawing, wetting and drying, or warming and cooling.
solifluction
in areas where permafrost exist, the summer months might allow the top few inches to thaw, but the water cannot seep down so the ground becomes soggy and eventually flows or moves in the direction of the slope
slump
moving rock or regolith does not disintegrate but rather stays somewhat coherent
failure surface
the point or plane where a slump block slides over/on; some are planar, but commonly they curve and resemble a spoon lying concave side up
mudflow
a moving slurry of mud, aka mudslide
debris flow
a slurry consisting of a mixture of mud and larger, pebble to boulder sized fragments
lahars
devastating mudflows that spill down valleys bordering volcanoes and consist of a mixture of volcanic ash and water from the snow and ice that melts in volcanoes heat or from heavy rains
rockslide
a sudden movement of rock and debris down a non-vertical slope (debris slide if it consists of mostly regolith)
snow avalanche
a chaotic jumble of snow surging downslope; can occur when a cornice (a large drift of snow that builds up on the lee side of a windy mountain summit) suddenly gives way and falls onto slopes below and knocks free additional snow
what triggers rock/debris falls?
when a body of rock separates from a cliff face along a joint (a natural crack in rock)
talus
a sloping apron of rocks along the base of the cliff
marine slumps
semi-coherent blocks slip downslope on weak horizons
turbidity currents
sediment disperses in water to yield a turbulent cloud of suspended sediment that rushes downslope as a submarine avalanche or density current.
what elements contribute to mass movement
- fracturing and weathering of the substrate; weakens substrate so it cannot hold up against the pull of gravity
- the development of relief which provides slopes down which masses move;
- an event that sets mass in motion
slope failure
when material starts moving on an unstable slope
what does slope failure depend on?
the balance between two forces:
- downslope force (inhibits sliding)
- resistance force (inhibits sliding)
angle of repose
when granular debris piles up to form the steepest slope it can without collapsing; also dependent on water content
- fine sand 30deg
- coarse sand 45deg
- angular pebbles 45deg
what causes slope failure?
- shocks, vibrations, and liquification
- changing slop loads, steepness, and support
- changing slope strength; can be due to weathering, vegetation cover, and water content
head scarp
a hill side where a slump block has move away(down), forms a scar; at the opposite end of the slump is the “bulging toe”
movement occurs wen ___ > _____
downslope force > resisting force
colluvium
poorly sorted, poorly stratified or unstratified sediment; forms due to mass wasting or other downslope movement.
