Reference Book: Fundamentals of Physical Geology, Sreepat Jain. The slides were prepared keeping focus on FY BSc Geology students.

1. Mass Wasting
Mr. Chinmay U. Dongare
Assistant Professor (Temp.)
Department of Geology, The Maharaja Sayajirao University of Baroda,
Vadodara-02
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2. Mass Wasting
• Gravity-initiated down slope movement of Regolith (loose
particles of soil and rock) without the aid of a transporting
medium (such as water, ice, or wind) is called Mass Wasting.
• The term is often used interchangeably with Mass Movement
and comes within the gamut of erosional processes, between
weathering and stream transport.
• The loose particles of soil and rock (Regolith) are picked up by
the transporting agent and then moved to a site of deposition
(such as an ocean basin or a stream bed).
• For the regolith to move on its own (the basic criterion for a
mass wasting process), it must be on a slope allowing gravity
to affect and cause motion.
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3. Factors responsible mass wasting
• The slope at which loose and unconsolidated material sits at rest is
called the Angle of Repose and it is typically between 25 and 40.
• Besides slope, another factor that plays a key role in mass wasting
events is water.
• Water lubricates the material and adds weight, thereby resulting in
increased instability and greater motion.
• Besides these, geology and relief, precipitation, slope modification,
undercutting, shocks, volcanic eruptions, submarine slope failures,
frost heaving, gelifluction, and rock glaciers.
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4. Classification of mass wasting
• Many classifications of types have been proposed.
However, it is now universally agreed that for such
classification to be successful, it should be based on
the following criteria:
(a) the type of material in motion (particle size, degree
of coherence)
(b) the nature of motion (falling, toppling, sliding,
flowing, etc.)
(c) the speed of motion.
(d) Water content
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5. MSUB_CUD
•Broadly, the mass wasting processes can be divided
into two categories—Flow and Slide

6. • This might involve either one or few
rocks.
• Generally, the rocks detach from
part of a steep slope, drop, and
bounce as they move very rapidly
down slope.
• They are quite dangerous as they
can occur without warning, and
more so as the rocks are traveling at
high velocity. Talus, an accumulation
of fallen material, is quite a
characteristic feature of Rockfalls.
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Rock Fall

7. • Rock avalanches are often caused
by heavy rain, melting snow, or
quite often by earthquakes.
• Here steep slopes are involved.
• As a fast-moving fall touches the
base of the mountain, it breaks
into thousands of fragments that
continue tumbling down slope at
high velocity, this is Rock
avalanche.
• Among all mass movements, it is
one of the most destructive.
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Rock Avalanche

8. • This occurs when a tabular
mass of rock glides down a
slope, which is usually
underlain by more of the
same rock, with planes of
weakness parallel to the
slope.
• These planes of weakness
include either a bedding
plane or a joint surface.
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Rock Slide

9. • Debris fall is the collapse of
weathered rock material and/or soil
from a steep slope or cliff.
• Debris Avalanches are very high
velocity flows of large volume
mixtures of rock and regolith that
results from the complete collapse of
a mountainous slope.
• They move down slope and travel for
considerable distances along relatively
gentle slopes.
• They are often triggered by
earthquakes and volcanic eruptions.
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Debris Fall and Debris Avalanches

10. • When blocks of rock, or masses of
unconsolidated material slide
down a slope it is called Debris
slide.
• These are very destructive mass
movement types and are generally
triggered by heavy rain, melting
snow, or earthquakes.
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Debris Slide

11. • It is high velocity mixture of sediment
and water with a consistency of a wet
concrete.
• It differs from a Debris flow in that fine-
grained (sand, silt, and clay) material is
predominant.
• Mudflows usually result from heavy rains
in areas where there is abundance of
unconsolidated sediment that can be
picked up by streams.
• Mudflows can travel for long distances
over gently sloping stream beds.
• Because of their high velocity and long
distance of travel they are potentially
very dangerous.
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Mudflows

12. • These form in humid areas on
hillsides following heavy rain or
melting snow, in fine-grained
materials (clay and silt).
• They also occur at the toe of
slumps when associated with
heavy rains and remain active for
long periods of time.
• They generally tend to be narrow
tongue-like features that begin at a
scarp or on a small cliff.
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Earthflow

13. • Earthflows are slower, seldom perceptible to the eye except
to observations extending several hours or days.
• Earthflows are not confined to channels as mudflows are.
• There is lower water content in Earthflow than that in
mudflows.
• Earthflows are not characteristic of dry regions, as are
mudflows, but are common in humid areas.
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Earthflow V/S Mudflow

14. • Slumps are sliding mass of soil or
other loose material along a curved,
rotational surface.
• The upper surface of each slump
block remains relatively undisturbed,
as do the individual blocks.
• Slumps leave arcuate scars or
depressions on the hill slope.
• Heavy rains or earthquakes usually
trigger slumps.
• At the bottom (or toe) of the slump,
Earth flow, or flow of soil, occurs.
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Slumps

15. • Debris flow results from heavy
rains. This causes saturation of the
soil and regolith with water,
followed by its flow.
• Debris flow sometimes starts with
slumps and then flows downhill
forming lobes with an irregular
surface consisting of ridges and
furrows.
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Debris Flow

16. • It occurs in areas underlain by permafrost
within the periglacial environment.
• A periglacial environment is defined as any
place where the geomorphic processes
related to the freezing of water occurs.
• Solifluction produces distinctive lobes on hill
slopes
• These occur in areas where the soil remains
saturated with water for long periods of
time and the sediment flows over an
impermeable material.
• The flow rates are in the order of a cm per
year.
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Solifluction

17. • It is an unusually slow continuous
movement of regolith and soil downhill.
Creep occurs on almost all slopes, but
the rates vary.
• Evidence for creep is often seen in bent
trees, offsets in roads and fences, and
inclined utility poles.
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Earth Creep

18. Causes and Consequences of Mass Wasting
• Human activity is one of the causes of mass movement.
• Increased human activity would increase the driving forces of
mass movement.
• The density of the human activity, such as infrastracture, plus
the pull of gravity would increase the likelihood of a landslide
or a mudslide, or weaken a part of the slope.
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19. • Deforestation: trees bind the soil together, therefore the deforestration
increase the rate of slope movement.
• building on steep on gradient: adds more downward force(gravity) to the
slope
• road construction: upsets the equilibirium
• traffic vibration: destables slope
• loading slopes waste materials: adds more gravity force to the slope
• extraction: loosen soil
• intensifiction of agriculture: loosen soil and removes protective
vegetation
• quarrying at foot of slopes: upsets the equilibrium
• grazing of animals: loosen soil and removes protective vegetation
• plowing: removes protective vegetation
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20. Case Study
Mudflow in Aberfan, Wales on Oct. 21st, 1966
The mudflow occured in Aberfan, a small mining
village in Wales after 7 days of heavy rain.
Cause: The waste materials from mining were
piled up on the sides of mountain. The piles of
coals were washed away by the heavy rainfall,
and slided down to the town.
The piles of coals were slided down to an
elementary school and residences located at the
foot of the mountain. 144 people died from the
disaster, including 116 children.
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21. Case Study
Landslide in Leyte Island, Philippines, in 2006.
Whole village was covered with mud, soil,
and debris. About 45 hectares of land was
buried under 20 to 45 meter thick of mud.
92 people were killed and 1000 were
missing by the landslide.
More than 10000 people became homeless.
Cause: heavy rain, earthquake of Magnitude
2.6, deforestation, and lack of serious
replanting of forests
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22. Kedarnath Landslide
16th January 2013
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