22.214.171.124 Mass movement
Physical causes and
1. Understand the concept of slope as an open system with inputs
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.
• 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.
- 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
Slope movement is common particularly where
• steep gradients
• weak rocks
• heavy rainfall
• basal undercutting ( river or marine erosion or
A slope is an open system affected by
• Tectonic inputs.
Inputs may vary in time and scale.
The amount, rate and type of movement
depends upon the degree of slope failure
Classification of Mass Movements
Carson and Kirkby (1972)
the speed of
Waugh Fig 2.12 pg
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
• Mass movement may occur on slopes or it
may involve subsidence or sinking of the
• Two forms of mass movement on slopes
are landslides and creep.
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,
• Wet – dry periods When the regolith
• During times of heavy freezes the ice
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
Soil creep usually occurs on
slopes of about 5o and
• Terracettes are evidence of soil creep.
• Terracettes are step like features – often 20 – 50 cm
in height which develop as the vegetation is stretched
• 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
Evidence of creep:
The weight of
response of trees on
slopes to sunlight and
the activities of
insects and tree roots.
Where soil creep does
occur it is probably the
result of the frost
individual particles rise
and fall in response to
contraction due to
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 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
• The term gelifluction refers to solifluction that takes place on
top of frozen ground.
• 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
• 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).
Chittagong mudslide 2007
• Bangladesh’s 2007 monsoon started with unusually
heavy rain, intensified by a storm from Bay of bengal 9 –
• 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
• 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.
• 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
There are 4 basic types of landslide
categorised by the main method of
A landslide may
• Falls usually occur from cliff faces.
• The falling mass or rock usually breaks
up along joints and or bedding planes.
• 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.
• These are sudden movements of material downslope
under the influence of gravity. Landslides vary
enormously in the amount of material moved and the
• 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 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.
Often occurs where softer rocks like clay or sands overlie
more resistant or impermeable rocks like limestone and
(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.
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
• 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
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
Factors that promote landslides
• Natural Hazards and Disasters fig 64 pg
55 Holmes and Warn
Factors that influence soil
• Rock structure
• Human activity
• 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.
• 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.
• Landslides are more likely on steep hillslopes. The steeper the
slope the more unstable it is likely to be.
• Slope stability may be reduced where ground water seeps to the
• 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.
• 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
• 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 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
• 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 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.
• 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
Talus - an accumulation of fallen material
found at the base of cliffs
May 2005“Slope instability is entirely due to
human activity.” Using examples, assess this
statement. [20 marks]
• 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
Explain how the stability of slopes can be
affected by natural and human factors. [20