Mass Wasting Flashcards
what is mass wasting
the downslope movement of rock, regolith and soil under the influence of gravity
what are two roles of mass wasting
- geologic processes that often follows weathering
- combined effects of mass wasting and running water produce stream valleys
how do weathering and mass wasting compliment each other
weathering weakens the material and allows gravity to easier act of the material
what process does mass wasting often follow
weathering
describe the evolution of mass wasting on slopes over time
in areas of rugged, geologically YOUNG mountains there are the MOST RAPID events
over time mountain sides lower through mass wasting and erosion
the older landscapes are LESS dramatic downslopes than young ones so they have LESS dramatic movements downslope
what is the controlling force of mass wasting
gravity
what are triggers of mass wasting
- water
- over steepened slopes
- loss of vegetation
- earthquakes
how does water act as a trigger for mass movements
saturation of the material with water molecules reduces the particle cohesion which decreases the materials resistance to gravity
AND
saturated soils have more weight which makes the soils also more susceptible to gravity because it changes the balance on the slope
how do steepening slopes act as a trigger for mass movements
slopes have a set slope angle that maintains the balance of the slope (not too steep but not too shallow), over steepened slopes lose that balance,
(they have a steeper slope angle which makes it unstable and more susceptible to failure)
what is a natural process for steeper slopes
- undercutting by streams or wave action (erodes the slope and increases steepness)
-
how does removal of vegetation act as a trigger for mass movements
removing vegetation from a slope removes the root systems that bind soil and regolith which were acting as protection for the slope from rainfall (increases saturation)
how does undercutting by streams/rivers cause steeper slopes
the process removes the slope’s base which both causes an increase in slope angle AND increases the gravitational force acting parallel to the slope
what are some human processes that cause steeper slopes
- cutting into its side too deeply
- excavation for road cuts and hillside building sites
- dumping or filling areas to build houses on top of hills
how do earthquakes trigger mass movements
the shaking can dislodge large volumes of rock and unconsolidated material = the sediments lose their strong cohesion and induce failure
how to classify mass movements
- type of material involved (debris, mud, earth or rock)
- type of motion
(fall, slide, flow) - rate of movement (fast or slow)
what type of movement is this
materials fall through the air from talus slopes
fall
what type of movement is this
material moves along a surface as a coherent mass
slide
what type of movement is this
materials viscously deforms - becomes a jumble of material
flow
what is a talus
a slope that is built of angular rock fragments that fall to the base of the cliff
are talus rock falls fast or slow
SLOW
what is a slump
the downward sliding mass of rock OR unconsolidated material as a unit along a CURVED surface
what are distinguishing features of slumps
- crescent shaped scarp at the head
- raised toe at the end
- curved slump plane/surface with step-like blocks for each section of the slump detaches
where are slumps common
along river valleys and eroding shorelines
what kinds of slopes do slumps occur
those that are oversteepened and overloaded
slump
what is indicative of a slump
the step-like pattern of the different slump slides
what is a rockslide
blocks of bedrock that slide down a slope very quickly
are slumps rotational or translational
rotational
where are rockslides common
where rocks and joints parallel the slope which gets undercut
what is a common example of rock slide
frank slide
what is a major cause of slides
frost wedging
rock slide
Soil slide
rock slide
rock fall
debris flow
very wet mixture of soil and regolith confined to channels that flow down the slope
where are debris flows found
in high relief areas (mountainous like BC) that have high saturation of soils
what is an example of debris flow
lahars
what are Lahars
debris flows composed mostly of wet VOLCANIC materials that rush down the flanks of volcanoes
debris vs earthflow
earth flows happen in lower reliefs than debris flows AND are made up of soil and regolith NOT debris
where are earthflows found
on hillsides in humid regions
how do earthflows form
when water saturates the soil and regolith, the weight eventually has the material break away from the hillside (leaving scar behind)
what makes up earthflows
commonly involves materials rich in clay and silt
earth flow
earthflow
what are two examples of slow moving mass movements
creeps and lateral spread
what is a creep
the gradual downhill movement of soil and regolith
what triggers a creep
the alternative freeze/thaw and expansion/contraction of the surface material
what is a dead giveaway for a creep
tilted trees, fences, utility poles and soil ripples
what is the fastest mass movement
debris flow
What is this mass movement?
Creep
Creep
what promotes a lateral spread
a dense clay impermeable bedrock layer
what forms lateral spreads
the impermeable bedrock layer does not allow water to penetrate it, so it collects on the layers above = saturates that layer and eventually that leads to failure
where are lateral spreads common
in areas with PERMAFROST where the active layer gets statured in summer and frozen in winter
what is a dead giveaway of lateral spreads
vegetation on the earth that move as a series of lobes
lateral spread
what is subsidence
a form of subsurface ground failure where the surface collapses on itself
where is subsidence most common
in KARST topography
what causes subsidence
the withdrawal of fluids (water, oil, gas, steam) OR removal of soil materials (dissolution, mining) below the surface
are the causes of subsidence only human activities
NO can be natural as well
subsidence
how does California show subsidence
they have a very high level of irrigation to support the activities there so they pull a load of water from the water table = lowering the table and forcing the soil to compact in the absence of water
what is an example of subsidence
sinkholes
sinkholes form because of
the dissolution of carbonate rocks, limestones and dolomite AS WELL AS fluctuations in water tables
what is a problem caused by sinkholes
the giant holes trigger people to use them as giant waste dumping sites HOWEVER these holes are often right over the water table and any leakage of the garbage will go right into the water table
what are two rotational mass movements
rock and soil SLUMPS
what are two translational mass movements
rock slide (c) and soil slide (d)
what are the slowest flows
rock creep and soil creep (f)
what are examples of fast flows
earth flows and debris flows
what is the fastest flow
debris avalanche
what is a snow avalanche
the rapid downslope movement of snow and ice
how can snow avalanches become MORE deadly
behave like a rock slide where large layers of ice slide down the slope and break up as they go
what are some prevention methods against snow avalanches
- well designed explosions (or triggering controlled avalanches)
- engineering structures to retain or divert avalanches
snow avalanche
submarine landslides
these are underwater slumps and debris flows
submarine landslides
what is an integral part of the growth of passive continental margins
submarine landslides
what triggers submarine landslides
the rapid buildup of unstable sediments OR by storms and earthquakes
where are submarine landslides especially active
near deltas (massive sediment deposits at the mouths of rivers)
what was the Grand Banks case study about
a magnitude 7.4 earthquake hit the south of Newfoundland along the Grand Banks
it ultimately triggered a tsunami which was recorded along the east coast of US and as far south as the Caribbean and Portugal
caused 1 million dollars in damages and cost 27 deaths
why was this the most severe damage regarding the Grand Banks case study
because that stretch along the island was most exposed to the sea and thus the tsunami BUT ALSO had the best topography to enhance and strength the tsunami
what was ridiculous about the La Conchita landslide
the small costal community right on the coast of the ocean was built ON PAST landslide deposits SO THE AREA HAS HAD THEM BEFORE
In the 2005 landside, a debris flow was triggered by heavy rainfall
what was a giveaway that the slope in La Conchita was NOT stable
the daylighting beds - bedding planes that were almost parallel to the slope that have become mostly VISIBLE to the outside of the slope
what was the Vaiont Dam case study about
In Italy a landslide above the dam was triggered by weak carbonate rocks and clay layer and the steep slope surface already showing signs of creep
the fast moving earth filled the dam and displaced tons of water
the displaced water flowed down the dam to the settlement below in up to 300 foot waves
what is NOT a good prevention of mass movements
cut and fill
- cutting into a slope to fill the indent with earth to build on
what is beneficial to have on slopes to prevent mass wasting
surface drainage to PREVENT water from being absorbed into the soil and waterlogging it
what is the benches approach to mass wasting prevention
changing the slope by pushing the top BACK and NOT filling it (like in cut and fill)
what is the ABSOLUTE BEST method for mass wasting prevention
- have a good retaining wall WITH DRAINAGE to prevent failures
- have a cut and fill for the house pad or base
- have piles dug BELOW the fill so they reach stable rock below
