MASS MOVEMENT Flashcards
what is mass movement?
processes whereby masses of rock and soil move downhill under influence of gravity
types of mass movement
landslide, mudslide, rock avalanche
what causes it
arises from rock fragmentation through weathering and is a major geomorphological process on slopes (esp steep slopes) resulting in decline in slope angle over time
frequently a geohazard
ultimate controlling factor of mass movement
gravity
controlling factors of mass movement
nature of material
moisture content of material
slope angle
influenced by human characteristics
rock sitting stationary on top of slope
what would cause rock to move down slope? – trigger factors
increase weight
lubrication
increase slope angle
controlling factors:
explain nature of slope materials
- materials vary according to geology - main division is between consolidated and unconsolidated material
- unconsol material = has angle of repose
- size and shape of particles are main determinants
cons material
-slope steepness can be maintained but instability is still possible -
- slope if undercut (over
steepened)
- vegetation cover is removed (clearing or fire)
deep weathering alters the rock by reducing binding forces and make slopes susceptible to movement/failure
angle of repose
maximum angle at which a slope of loose material lies without movement
slopes steeper than angle of repose are unstable
angle of repose - fine vs course particles
fine - shallower angle of rep
course - larger angle of rep
controlling factors: explain moisture content
saturation of material reduces internal friction and promotes movement
water between all particles keeps them together and allows them to flow
controlling factors: explain slope characteristics (angle)
uncon materials have angle of rep - when this angle is exceeded - failure
combination of other factors
slopes developed on softer/weathered rock may lie at or close to the angle of repose (shale is often relatively soft and weathers into small fragments)
.
Harder and more resistant rocks (sandstone, limestone) may fragment into large blocks which can form steep cliffs e.g. Table Mountain.
bedding planes
- define
- attitude of bedding planes
-
- the attitude of a plane is
- the attitude in relation to actual slope angle may be important in determining the type of movement
trigger factors
vibration - earthquakes and tremors
heavy rainfall - lubrication
human mod of slopes = over-steeping, increase lubrication and vibration
how are mass movements classified?
nature of materials
velocity of movement
nature of movement
mass movements = creep
- very slow movement of soil under the influence of gravity
- promoted by cycles of wet/dry, hot/cold, freeze/thaw – produce heaving
- 1.0 - 10 mm per year
- Deformation is evident in appearance of fences, trees, telephone poles etc.
special case of creep in cold climates
solifluction
explain solifluction
accelerated creep that is restricted to high latitude - tundra
surface layers thaw in summer and saturated soil flows across frozen subsoil (permafrost)
faster rate - 1 -10m per year
mass movements = flow
further classified as earthflow or mudflow – depending material involved
faster than creep 100km/h
flow : mudflow
fine grained material
associated with heavy rainfall
special case of flow under volcanic conditions
- pryroclastic flow/lahar
- mixture of hot volcanic gasses and ash
- mount vesuvius
flow : debris flow
-mixture of finer and coarser grained matieral
mass movements = slide/slump
landslide is the most commonly used term - includes slumps, debris slides and rock slides
mass movement - slump (slides)
involve rotational movement as a single unit along a well defines slippage plane
slumped block usually tilts backwards and is separated from the original face by a curved incision scar – often spoon shaped
rate of move - slower than slide = 50km/h
mass movements = fall
feature of cliffs - associated with consolidated parent materials
fall : rockfall
detachment of individual blocks which free fall instantaneously from a cliff
result = accumulation of
blocks of fallen rock at the
base of the slope
talus/scree
accumulation of blocks of fallen rock at the base of the slope
Human activity ===== Mass movement
humans affect mass movement
-excavation, dam construction
mass movements affect humans
-particularly the high magnitude-low frequency events such as catastrophic landslides.
case of Vaiont, Italy
- concrete dam built – massive which caused instability
- massive debris slide filled the dam and caused spill over into river = flood wave of 70m moved down valley
- almost 3000 dead, whole village consumed
could the Vaiont disaster have been avoided?
- geo investigation would have revealed weakness in surrounding limestones and shales
- scar of ancient slide on valley side above reservoir
- small rockslide in 1960 - a few years prior
landslide hazard assessments
- careful observation and monitoring of risk factors
- assessment of history of mass movement act in area
- avoid construction on vulnerable areas - no oversteepening, manage drainage
- geo engineering solutions
- early warning systems
define mitigation
the action of reducing the severity, seriousness, or painfulness of something.
