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2.4.1 Mass Movement in the Tropics (Factors affecting MM Processes…
2.4.1 Mass Movement in the Tropics
Slope
An inclined unit on Earth's surface
Usually visualized as a profile from the hillcrest to valley bottom
Geometry of a slope is measured by gradient, length and width
A slope shows a vertical sequence of soils, weathered rock and bedrock along its profile
Slopes give a
convex concave appearance
Crest → Scarp/ Free face → Debris/ Constant Slope → Pediment/ Waning Slope
Debris slope receives rock fragments from materials above and
reflects the local climate
Factors affecting MM Processes
Lithology
Favourable to Mass Movements
Presence of Clay
: soils rich with clay are unstable as clay readily absorbs water and are easily saturated thus resulting in more mobile soils. Common on slopes of clay bedrock and locations where
chemical decay
(eg: of feldspar) is widespread and production of clay is vigorous
Presence of lines of weaknesses
: Allows water to enter these joints/bedding planes and ↓ shear strength, causing slope failures. Bedding plans also serve as a slide plane along which material can be displaced
Dip of the side plane
: Slope would be more vulnerable to movement where slide plane is dipping due to greater gravitational pull as compared to a stable steep slope whose slide plan is against the slope
Human Activities
Slope modification
by humans steepens slopes and causes instability in particles at rest
Slides occur where
road constructions
are cut into regolith or rock, creating an artificial slope that exceeds the angle of repose
Deforestation: ↑ slope movement due to lesser vegetation
Farming: Grazing by animals removes the protective cover and overlying soil
Eg: Landslides are common along the coastal cliffs of California where roads are carved into deformed sedimentary rocks
Vegetation
Plays a dual role
:green_cross:
Can ↓ shear strength and ↓ soil stability
Increasing soil moisture
Vegetation canopy intercepts rainfall and roots discourages surface runoff this allowing rain to infiltrate slowly, ↑ soil moisture
Foliage protects soil from compacting due to rain-splash effect, providing more pore spaces for water to fill
Roots and organic matter that traps moisture ↑ shear stress and ↓ shear strength when soil is saturated
Reducing cohesiveness of slope material
Tree roots discontinues and wedge blocks apart, causing the detachment and fall of boulders and ↓ cohesiveness and strength
:check:
Can ↑ shear strength and ↑ soil stability
Extracts moisture from soil
↓ soil moisture and ↓ pore water pressure
Soil particles are binded together
Roots provide a strong interlocking network that holds unconsolidated rocks and sediments together, ↑ cohesiveness of slope material
Environmental Factors
Earthquakes due to tectonic activity
EQs release alot of energy through violent shaking which can cause a buildup of water pressure in the pore spaces of sediment leading to
liquefaction
, causing an ↑ in pore water pressure, ↓ shear strength
EQs and their intial
shocks and aftershocks
can loosen fragments of rocks on steep slopes, overcoming cohesion of the slope and set the regoliths in motion
Eg: In 1976, an EQ in Guatemala set off > 10,000 mass movements.
In 1929, an EQ in northwestern South Island New Zealand triggered at least 1850 landslides. Most landslides occurred on well-bedded and jointed mudstones and fine sandstones
Undercutting Action of a stream along its bank
Undercutting results in a slope > angle of repose causing mass movement and flooding
Heavy and persistent rains
Grounds become saturated and unstable after long
durations
of
high intensity
rainfall, so mass movements occur
Eg: A large debris flow in the Gros Ventre River basin of western Wyoming in 1925 was triggered due to prolonged rains and melting snow. Water saturated the porous sandstone, causing 37 million m3 of rock, regolith and vegetation to move rapidly downslope.
Volcanic Eruptions
Large volcanoes may consist of stratified lava flows, rubble and pyroclastic layers that form steep slopes
Lahar flows occur as the abundance of loose pyroclastic material on the flanks of a steep volcano is unstable
During explosive eruptions, the materials become saturated by rain or melting snow warmed by volcanic heat
Eg: In 1985, lahar flow raced down the slopes of the ice-capped Andean volcano named Nevado del Ruiz at >150km/hr down the Lagunillas River valley and completely destroying Armero, Colombia 50km away
Role of Water
Plays a dual role
A dual role in slopes
Dry Grains
:
Forms a pile with a slope angle determined by angle of repose
Angle is partly controlled by frictional contact between grains
Slightly Wet Unconsolidated Materials
Very high angle of repose
as surface tension between the water and the grains tends to hold the grains in place so mass movement is tough
When water does not completely fill in the pore spaces between the sol grains, it forms a thin film around each grain which crates surface tension between grains so
shear strength
↑
Rocks are Saturated with Water
Angle of repose is very small
and materials flow like fluids
Water fills up pores of rocks and ↑ pore water pressure, eliminating the grain to grain frictional contact, decreasing its internal cohesiveness and
↓ shear strength
, resulting in slope failure and very easy to trigger mass movement
Classified into either
endogenetic
(↓ strength/ internal resistance)
and
exogenetic
(↑ in stress/ external forces)
. All 5 are exo & endo apart from
geology which is endo only
Topography
Slope Gradient
Falls require a vertical/ near vertical slope to occur
Determines rate of mass movement due to gravity as the agent
Humid Climates
:evergreen_tree:
Transport-limited slopes: Gentle and promotes vegetation growth
Active weathering so regolith build up > rate of erosion, thus thick regolith formed
Accumulation of weathered material covers up free face and is removed episodically by
mass movement
Streams have a highly dissolved solution loads but considerable amount of solid loads too
Arid Climates
:hot_pepper:
Weathering-limited slopes: Steep, almost vertical below the crest
Rate of erosion where debris is removed > rate of weathering where debris is produced, so rocks are transported down fast
Thin regolith where fresh rocks are exposed and vegetation is sparse
Streams have clastic material that are suspended and bedload
Any point on a slope is a balance between the
erosive force
that removes material from the surface and moves downslope and
resisting force
that opposes its removal
Erosive force depends on
movement of material en masse (mass movement)
and
running water
Resisting force depends on
slope material
Slope Stability
A slope is stable if
sum of the applied shear stress does not exceed the sum of the shear strength of the slope material
Shear Stress
: Force which causes movement of the body parallel to slope , due to gravity which is influenced by gradient of slope. Steeper slope, ↑ SS, greater potential for materials to move
Shear Strength
: Internal resistance of the body to movement. Influenced by frictional resistance, cohesion between particles and the binding action of plant roots
Stability (F) = Resisting force (strength) / Driving force (stress)
[safety ratio]
F > 1 : strength > stress and rock would not move
As F approaches 1 : approaching
geomorphic threshold
F < 1 : strength < stress and mass movement occurs
Angle of Repose
Steepest slope angle that loose fragments will remain stable as they pile up
Slopes are stable if angle of slope
smaller than
angle of repose
Varies depending on
size
,
shape
,
sorting of fragments
and
moisture between the grains
Angle of repose is commonly 30-35° for dry sand. Larger angular rocks form a steeper pile thus ↑ angle of repose
Mass Movement
Movement of regolith and masses of rock downslope under the pull of gravity
only
[no erosional agents involved]
Agent is
Gravity
which pulls on a mass till the critical shear-failure point is reached and the material either
falls
,
slides
,
flows
or
creeps
MM processes show great variety of
scale
,
speed
,
material
and
resulting landforms
An important part of
denudation
Different from erosion as it requires saturation of soil