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Physical Causes And Consequences Of Mass Movement
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Physical Causes And Consequences Of Mass Movement Presentation Transcript
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1. 3.4.2.7 Mass movement Physical causes and consequences of mass movement 1.
Understand the concept of slope as an open system with inputs and outputs. 2. Identify
different types of movement: –fast movements (mudflows, slumping, landslides,
avalanches) –slow movements (solifluction and soil creep). 3. Know the processes
responsible for these movements, together with the hazards that can arise from them.
2. references • Waugh Geography an Integrated Approach pg 46- 49 • Bishop –g 123 127
3. Mass movement • Mass movement is the movement downhill of weathered rock
material (regolith soil, loose stones and rocks) under the influence of gravity. It differs
from mass transport, which is movement caused by wind water or ice. • In mass
movement, soil, loose stone and rock material falls, rolls, slides or flows downhill.
Although water or ice may be involved, gravity is the driving force. Landslides and mud
flows involve the downslope movement of huge quantities of material and can
completely alter a slope’s shape.
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4. Mass movement - Describes all downhill movements of weathered material (regolith)
– soil, loose stones and rocks, in response to gravity - When gravitational forces exceed
forces of resistance slope failure occurs and material moves downhill
5. Slopes Slope movement is common particularly where there are: • steep gradients •
weak rocks • heavy rainfall • basal undercutting ( river or marine erosion or human
actions) WHY?
6. The Slope System
7. A slope is an open system affected by • Biotic • Climatic • Gravitational • Groundwater
• Tectonic inputs. Inputs may vary in time and scale. The amount, rate and type of
movement depends upon the degree of slope failure
8. Classification of Mass Movements Carson and Kirkby (1972) Based on the speed of
movement an the amount of water present Waugh Fig 2.12 pg 46
9. A slope is an open system affected by biotic, climatic, gravitational, groundwater and
tectonic inputs which vary in time and scale. The amount, rate and type of movement
depends upon the degree of slope failure
10. • Mass movement may occur on slopes or it may involve subsidence or sinking of the
ground. • Two forms of mass movement on slopes are landslides and creep.
11. Slow movements a. Soil creep b. Solifluction
12. Soil Creep (Waugh pg 46) • Creep is slow mass movement less than 1cm per year. • It
is a continual process • Occurs mainly in humid climates where there is vegetation cover
• Rock creep refers to the movement of individual boulders on a rock slope or the
movement of boulders on a talus slope. • Soil creep is the slow downslope movement of
soil. • The rates of down slope movement of individual rocks on talus slopes can vary
from 40m per year to as little as a few centimetres per year. The faster movements may
be caused by factors such as the impact of boulders onto the talus surface. The slower
movements are more commonly caused by freezing of water in the spaces between
boulders,
13. Freeze thaw • Wet – dry periods When the regolith • During times of heavy freezes
the ice rainfall moisture increases the volume and crystals increase the weight of the soil
causing volume of the soil by expansion and allowing the regolith to move 9%. . As the
soil downhill under gravity. In expands particles are the dry period the soil will dry out
and then contract. lifted at right angles to the slope = heave. When the ground thaws, the
regolith contracts and
14. Soil creep usually occurs on slopes of about 5o and produces terracettes.
15. Terracettes • Terracettes are evidence of soil creep. • Terracettes are step like features
– often 20 – 50 cm in height which develop as the vegetation is stretched and torn. •
Terracettes appear as a ribbed pattern on the surface of steep slopes indicating that the
surface layer is breaking up into small steps. The details of terracette formation are not
known except that they tend to form on steep slopes with or without vegetation.
16. Evidence of creep: The weight of tombstones, the response of trees on slopes to
sunlight and the activities of burrowing animals, insects and tree roots. Where soil creep
does occur it is probably the result of the frost heave whereby individual particles rise
and fall in response to expansion and contraction due to
17. Solifluction = ‘soil flow’ (Waugh page 47) • Solifluction is the slow downhill flow of
saturated soil. • Movement averages between 5cm and 1m per year. • It is a common
process in periglacial environments. • During winter season both the bedrock and regolith
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are frozen. In summer the surface layer thaws but the underlying layer remains frozen
and acts like impermeable rock. • Because surface meltwater cannot infiltrate downwards
and temperatures are too low for effective evaporation any topsoil will soon become
saturated and will flow as an active layer over the frozen subsoil and rock. This process
produces solifluction sheets or lobes – rounded tongue – like features reaching up to 50m
in width and head – a micure of sand and clay formed in valleys and at the foot of sea
cliffs. • The water reduces the effects of cohesion and friction, thus promoting movement.
• The term gelifluction refers to solifluction that takes place on top of frozen ground.
18. Flow movements c. Earth flows d. mudflows
19. Mudflow • When soil or weak rock such as clay or volcanic ash becomes saturated it
may actually begin to flow downhill. Such flows can be slow but most commonly they
are very rapid exceeding 1km/hr. • Occur after periods of intensive rainfall when both
volume and weight are added to the soil giving it a higher water content than an earth
flow. • Mudflows often form a serious volcanic hazard as melted snow and ice from
volcanic summits can combine with ash to form deadly flows called lahars. (Armero
1985).
20. Waugh fig 2.16
21. Chittagong mudslide 2007 • Bangladesh’s 2007 monsoon started with unusually
heavy rain, intensified by a storm from Bay of bengal 9 – 10th June. • Heavy rain caused
mudslides which engulfed slums in foothill areas of Chittagong on the coast in SE
Bangladesh on 11th June • Death toll reported to be at least 128 of whom 59 were
children • More than 150 injured • Experts had warned of increasing likelihood of
landslides due to the Bangladesh governments failure to curb illegal hill cutting in
Chittagong.
22. • Hill cutting creates flat sites for the construction of new houses. • Trees have also
been cleared to build houses on hill tops. This can block the natural rills or gullies that
drain the landscape. Water is then forced to enter the ground through cracks, which
weakens the soil structure and promotes landslides.
23. Rapid movements Slides Rock falls
24. What is the difference between slides and flows? Flows undergo internal
derangement Slides move ‘en masse’ and are not affected by internal derangement.
25. Landslides
26. 10 deadly landslides
27. There are 4 basic types of landslide categorised by the main method of movement
downhill. A landslide may – fall – Slide – rotate – flow. • Falls usually occur from cliff
faces. • The falling mass or rock usually breaks up along joints and or bedding planes.
28. Landslides • Occur when the line of failure occurs along a shear plane which is
roughly parallel with the ground surface. • They can occur in both soil and rock material.
• If the hill slope is not too steep, vegetation may prevent the moving mass from breaking
up. • Almost all the landslides that occur on natural hill slopes are slides. • The shear
plane often forms within a soil horizon or along the contact between weathered material
and the underlying rock.
29. Landslides • These are sudden movements of material downslope under the influence
of gravity. Landslides vary enormously in the amount of material moved and the
velocities. • Some landslides are fluid like a river of mud. • Some have hardly any water
in them at all and are almost entirely angular blocks of rock. • Most landslides are in
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contact with the ground. Movement takes place along a flat or planar slide plane.
Landslides commonly occur along a bedding plane particularly when the underlying bed
is impermeable leading to a high moisture content which will lubricate the slide surface.
Landslides are very rapid and can cause huge damage and loss of life.
30. Las Colinas Landslide, El Salvador, Feb 2003
31. Often occurs where softer rocks like clay or sands overlie more resistant or
impermeable rocks like limestone and granite
32. Rock falls (Waugh pg 48) • These are spontaneous debris movements on slopes that
exceed 40o. • Rockfalls may result from extreme physical or chemical weathering in
mountains, pressure release, storm wave action on sea cliffs or earthqyakes • Rockfalls
are often caused by frost wedging in cracks or by excess water pressure developing in
cracks. A crack forms at the top of a cliff and over a period of months or years it widens
at an increasing rate until failure occurs and the cliff face comes crashing down. •
Rockfalls begin by travelling through the air. The rapid movement usually occurs on the
steepest slopes. Individual rock fragments or whole slabs of rock suddenly become
detached and fall to the base of the slope. They may be detached by gradual process such
as freeze thaw or by sudden and dramatic events such as earthquakes. • The angular
debris collects at the base of the slopes to form scree or talus.
33. Slumps or rotational slides • Rotate around a shear (=slide) plane that resembles an
arc of a circle rather than being parallel with the ground surface or are concave in shape. •
Slumps often occur in excavations or roadside cuttings. • They are probably the most
common form of visible mass movement in the UK. They can be seen along the coast in
Norfolk and Lulworth. • They usually occur in weak rock (e.g. clay or sands that overlie
more resistant or impermeable rock) or in soil that has become saturated and, in response
to gravity, simply collapses
34. What factors affect slope processes • Ross fig 2.49 The slope system is an open
system becaue there are inputs from outside (e.g. heat and precipitation) and outputs (e.g.
water and weathered rock) into other systems. • Passive causes are those features of an
area that make frequent landslides there likely e.g. rock type, bedding, landforms, water,
climate, human influence
35. Factors that promote landslides • Natural Hazards and Disasters fig 64 pg 55 Holmes
and Warn
36. Localised physical factors that determine landslides: Gradient Topography. Landuse
Geology Permeablility Climate
37. Factors that influence soil development: • Rock structure • Lithology • soil • Climate •
Vegetation • Human activity
38. Rock type • Igneous and metamorphic rocks are strong and capable of supporting
near – vertical slopes, whereas sands and gravels can only support very gentle slopes.
Landslides are more likely to occur where rocks or clays are weak. Bedding • Rock slabs
may become detached along bedding planes or joints leading to rockfalls and landslides.
Landslides are more likely where the bedding of the rock tilts towards the slope. Slopes
with reverse dipping beds are more likely to be stable. Landforms • Landslides are more
likely on steep hillslopes. The steeper the slope the more unstable it is likely to be. Water
• Slope stability may be reduced where ground water seeps to the surface
39. Climate • The climate of an area will affect the type of weathering that operates on a
slope and will govern the nature and presence or absence of water and vegetation. •
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Heavy rain and meltwater add volume and weight to the soil • Heavy rain increases the
erosive power of a river at the base of a slope which might make the slope less stable •
Frost may also be important in causing rockfalls
40. Tectonic activity • Some of the steepest slopes are found in tectonically active areas
which result from the gradual uplift following plate collision. Earthquakes may also
trigger slope failure Weathering • Weathering affects the upper slopes particularly any
bare rock outcrops. Mechanical weathering particularly frost shattering will lead to a
more jagged , angular, bare rock surface whereas chemical weathering will produce more
rounded slopes
41. Vegetation • If a slope is forested or covered in bushes and grass it is less likely to be
active. This is because it will protect a slope from the direct effects of rainfall and help
bind together particles of rock and soil. Basal excavation • Basal excavation can take the
form of a river undercutting a slope or the sea cutting a notch in a cliffline. Human
activity such as road construction can have the same effect. Basal excavation can lead to
a steepening of a slope so making it unstable. Time • The length of time that a slope has
been exposed to weathering. Newly formed landscapes that are steep and unvegetated are
actively weathered and eroded until they assume a shape that is in balance with their
environment
42. terms Talus - an accumulation of fallen material found at the base of cliffs
43. Ross pg 56-62 Waugh 46 – 55 Bishop 123- 135
44. May 2005“Slope instability is entirely due to human activity.” Using examples,
assess this statement. [20 marks]
45. May 06 • Diagrams A and B show two types of mass movement. • (i) Define the term
mass movement. [2 marks] • (ii) Identify the type of mass movement taking place in each
of the diagrams • A and B. [2 marks] • (iii) Explain the processes of mass movement
taking place in diagram A and describe their effects on the natural and human landscape.
[4+2 marks] • (iv) Discuss the relative contribution of natural and human factors to mass
movements of the type shown in diagram B. [10 marks
46. May 07 Explain how the stability of slopes can be affected by natural and human
factors. [20 marks]
47. May 08
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