MASS MOVEMENT

Question 1

what is mass movement?

Answer 1

processes whereby masses of rock and soil move downhill under influence of gravity

Question 2

types of mass movement

Answer 2

landslide, mudslide, rock avalanche

Question 3

what causes it

Answer 3

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

Question 4

ultimate controlling factor of mass movement

Answer 4

gravity

Question 5

controlling factors of mass movement

Answer 5

nature of material
moisture content of material
slope angle

influenced by human characteristics

Question 6

rock sitting stationary on top of slope

what would cause rock to move down slope? – trigger factors

Answer 6

increase weight
lubrication
increase slope angle

Question 7

controlling factors:

explain nature of slope materials

Answer 7

• 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

Question 8

angle of repose

Answer 8

maximum angle at which a slope of loose material lies without movement

slopes steeper than angle of repose are unstable

Question 9

angle of repose - fine vs course particles

Answer 9

fine - shallower angle of rep

course - larger angle of rep

Question 10

controlling factors: explain moisture content

Answer 10

saturation of material reduces internal friction and promotes movement

water between all particles keeps them together and allows them to flow

Question 11

controlling factors: explain slope characteristics (angle)

Answer 11

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.

Question 12

bedding planes

• define
• attitude of bedding planes

Answer 12

-

• the attitude of a plane is
• the attitude in relation to actual slope angle may be important in determining the type of movement

Question 13

trigger factors

Answer 13

vibration - earthquakes and tremors
heavy rainfall - lubrication
human mod of slopes = over-steeping, increase lubrication and vibration

Question 14

how are mass movements classified?

Answer 14

nature of materials
velocity of movement
nature of movement

Question 15

mass movements = creep

Answer 15

• 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.

Question 16

special case of creep in cold climates

Answer 16

solifluction

Question 17

explain solifluction

Answer 17

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

Question 18

mass movements = flow

Answer 18

further classified as earthflow or mudflow – depending material involved

faster than creep 100km/h

Question 19

flow : mudflow

Answer 19

fine grained material

associated with heavy rainfall

Question 20

special case of flow under volcanic conditions

Answer 20

• pryroclastic flow/lahar
• mixture of hot volcanic gasses and ash
• mount vesuvius

Question 21

flow : debris flow

Answer 21

-mixture of finer and coarser grained matieral

Question 22

mass movements = slide/slump

Answer 22

landslide is the most commonly used term - includes slumps, debris slides and rock slides

Question 23

mass movement - slump (slides)

Answer 23

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

Question 24

mass movements = fall

Answer 24

feature of cliffs - associated with consolidated parent materials

Question 25

fall : rockfall

Answer 25

detachment of individual blocks which free fall instantaneously from a cliff

result = accumulation of
blocks of fallen rock at the
base of the slope

Question 26

talus/scree

Answer 26

accumulation of blocks of fallen rock at the base of the slope

Question 27

Human activity ===== Mass movement

Answer 27

humans affect mass movement
-excavation, dam construction

mass movements affect humans
-particularly the high magnitude-low frequency events such as catastrophic landslides.

Question 28

case of Vaiont, Italy

Answer 28

• 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

Question 29

could the Vaiont disaster have been avoided?

Answer 29

• 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

Question 30

landslide hazard assessments

Answer 30

• 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

Question 31

define mitigation

Answer 31

the action of reducing the severity, seriousness, or painfulness of something.