Mass Movement - Leaving Certificate Geography

Report
5th year Geography
MASS MOVEMENT
LEARNING OUTCOMES
 Causes
of Mass Movement
 Different types of Mass Movement
 Case Studies of Mass Movement
 Impact of Human Activities on Mass
Movement
MASS MOVEMENT
The movement of material downhill under the
influence of gravity.
 The loose material is called regolith.

TRIGGERS OF MASS MOVEMENT

Natural

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Torrential rain
Heavy snow
Earthquakes
Volcanoes

Human


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Road building across hills
and mountains
Waste being piled on
unstable slopes
Quarry blasting
Deforestation
Skiing
FACTORS AFFECTING MASS MOVEMENT
Gravity
 Slope
 Water
 Vegetation
 Human Activities
 Type of Material

CLASSIFYING MASS MOVEMENTS

Can be grouped according to;
Speed
Water
Content
TYPES OF MASS MOVEMENT

Very Slow
 Soil
Creep
 Solifluction

Fast and Wet
 Mudflows
 Bogbursts
 Lahars

Fast and Dry
 Rock
Falls
 Avalanches
 Landslides
 Slumps
CHAPTER 10: MASS MOVEMENT
Soil creep
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Very slow movement
Underlying rock becomes heavily
saturated following periods of heavy
rainfall
Water can no longer pass through the
rock
Soil particles expand and later
contract when they dry out
Causes soil particles to creep slowly
downhill under the influence of gravity
SOIL CREEP
2.
Slump-rotational slide

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Loosely consolidated rock layers
moves down slope
Influence of gravity
Moderate speed
May be an isolated occurrence or
they may occur in large areas
Often associated with areas which have been
affected by human activities
E.g. during the construction of roads where the
land has been too steepened and has been left
unstable
Also linked to coasts and river banks where, due to
erosion, slopes have been undercut
Also associated with landscapes following heavy
rain
3. Earthflow
 Water saturated material (fine sand, silt and clay)
flows downslope
 Influence of gravity
 Moderate speed
 Generally occur on hilly slopes
 Associated with heavy rain
 Speed at which they travel varies from
several cm per year to hundreds of metres per day
 More common in humid areas
 Tend to have a lower water content than mudflows
4. Rockfall
 Rocks fall downslope
 Influence of gravity
 Rapid speed
 Occur following periods of heavy rain and frost
weathering
 Strong earthquakes may also cause rock falls to
occur
 Caused by either a biological or a climatic event
changing rock stability
ROCK FALL
5.

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Debris avalanche
Movement of rocks, soil and debris
mixed
with ice or water or both
Rapid movement
Material transported liquefies
Rapid speed
Can move further than the foot of a
slope due to their rapid speed
6.
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Mudflow
Saturated regolith flows rapidly downslope
Influence of gravity
Associated with steep slopes and deep soils
Fast speed
Soil becomes waterlogged and saturated if
there is an impermeable rock layer beneath
Can be of speeds in excess of 100 km per
hour
Can occur as a result of volcanic activity –
known as lahars
Volcano with snow on its top erupts, the
melting snow and ice cause a mudflow
CHAPTER 10: MASS MOVEMENT
Mudflow case study

15 June 1991, Mount Pinatubo, Philippines

Volcanic eruption deposited more than 5 cubic
kilometers of volcanic ash and rock fragments
on the volcano slopes

Heavy rains washed this material down into the
surrounding lowlands in giant, fast-moving
lahars

The next four rainy season’s lahars carried
about half of the deposits off the volcano,
causing even more destruction in the lowlands
Bogburst


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An area of bog moves downslope
Influence of gravity
Slow to fast speed
Tend to occur mostly during autumn and winter due to high
precipitation levels
Periods of very heavy rain after long periods of very dry
weather seem to be associated with the occurrence of bog
bursts
Human activity is also linked to the occurrence of bog
bursts, e.g. peat cutting, draining of land for agriculture and
the construction of wind farms
CHAPTER 10: MASS MOVEMENT
Bogburst case study
 Derrybrien, Slieve Aughty Mountains, Co.
Galway
 16 October 2003: bog had been affected
by a dry spell during the summer and
when heavy rain fell, the peat became
saturated
 Resulted in local roads being closed
 Lough Cutra river system was polluted
 Thousands of fish killed when some 70
acres of bogland flowed down river
 Triggers- Human Activity & Heavy Rain
Avalanche
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Very rapid movement of snow downslope
May occur due to either natural events or
human activity
Fast speed
Usually occur on mountain slopes
Occur when the weight of the snow is too much
for the slope to hold
AVALANCHE
Major causes of avalanches may include the following:
1.
2.
3.
4.
5.
6.
7.
Weather – they are most likely to occur after a heavy snowstorm
Snowfall – recent snowfall exerts increased pressure on existing
snow deposits
Temperature – warmer temperatures experienced over several
days may cause upper layers of snow to melt and weaken
Wind direction – snow may be packed unevenly on the leeward
side of a mountain due to prevailing wind direction
Slope angle – most avalanches occur on slopes of between 30
and 45 degrees
Slope orientation – most avalanches occur on slopes facing north,
north-east and east
Human activities – deforestation and tourist activity may lead to
the occurrence of avalanches
9.
Landslide
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Slope stability changes from stable to unstable
Causes may be loss of vegetation, erosion of
slope, weakening of slope, earthquake activity,
volcanic eruptions and human activities
Fast speed
DAMAGE CAUSED DURING BRAZILIAN
LANDSLIDES (JAN 2011)
Human activities can impact on the
operation of surface processes
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Impact of overgrazing
Impact of overcropping
Impact of deforestation
Sahel case study

Areas that are most affected by
desertification are regions/areas that are
on the edges of deserts

Sahel Region lies to the Southern end of the
Sahara Desert in Africa

Covers an area of approximately 1000 km
in total

Stretches from the Atlantic Ocean coast to
the Red Sea coast and is located at the
southern end of the Sahara desert

Desert is growing at a rapid rate

Experienced an increase in population in
recent years
Sahel case study (continued)
 High birth rate and immigration rates are also high
 High population growth increases the demand for
food
 Increased demand for food and for land to cultivate
 Growth of farming and cultivation has led to soil
becoming exhausted and exposed to erosion
 Increase in the number of animals required for food
has also resulted in overgrazing
 Cattle are also seen as a sign of wealth in the
region.
 Energy needs are satisfied by cutting down trees
CHAPTER 10: MASS MOVEMENT
Sahel case study (continued)
 Vegetation, along with the trees, bind the soil together
 When removed, the soil becomes weak and dry
 When extra land is needed, a practice of ‘slash and burn’
has been used and has increased the barrenness of the
region
 The Sahel Region has experienced long periods of drought
 Also experiences spots of unpredictable heavy rainfall
 This type of rain can damage crops
 When land experiences desertification, fertility is reduced
and this has a direct effect on food supply; it may lead to
famines
VIDEO CLIP
Saharan
Millions
Desertification
of years ago the Sahara
was a green and pleasant land, but
over the centuries desertification
changed its climate.
EXAM QUESTIONS
Examine how human processes can impact on
the process of mass movement. (30m)
 Describe and explain any one process of mass
movement. (30m)


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