Transcript File

Chapter 10: Mass Movement
Mass movement
• Loose, weathered material (regolith)
• Moves downslope under the influence of gravity
• Movement can be either:
 slow – soil creep
 or rapid – landslides, bog bursts, mudflows, avalanches
Chapter 10: Mass Movement
Factors affecting mass movement
• Slope (gradient) of the land surface
• Water content
• Human activity
• Plants and trees
• Tectonic activity
Chapter 10: Mass Movement
Mass movement types
• Soil creep
• Slump-rotational slide
• Earthflow
• Rock fall
• Debris avalanche
• Mudflow
• Bog burst
• Avalanche
• Landslide
Chapter 10: Mass Movement
Processes of mass movement
1. Soil creep
• 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
Chapter 10: Mass Movement
2. Slump-rotational slide
• 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
Chapter 10: Mass Movement
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
Chapter 10: Mass Movement
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
Chapter 10: Mass Movement
5. 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
Chapter 10: Mass Movement
6. 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
Chapter 10: Mass Movement
7. Bogburst
• 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
Chapter 10: Mass Movement
Avalanche
• 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
Chapter 10: Mass Movement
Major causes of avalanches may include the following:
1. Weather – they are most likely to occur after a heavy snowstorm
2. Snowfall – recent snowfall exerts increased pressure on existing
snow deposits.
3. Temperature – warmer temperatures experienced over several
days may cause upper layers of snow to melt and weaken
4. Wind direction – snow may be packed unevenly on the leeward side
of a mountain due to prevailing wind direction
5. Slope angle – most avalanches occur on slopes of between 30 and
45 degrees
6. Slope orientation – most avalanches occur on slopes facing north,
north-east and east
7. Human activities – deforestation and tourist activity may lead to
the occurrence of avalanches
Chapter 10: Mass Movement
9. Landslide
• 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
Chapter 10: Mass Movement
Landslide case study
• February 2010 Maierato, Calabria, southern Italy
• Entire hillside moved downslope
• Occurred following a period of heavy rain
• Over 2300 residents were evacuated from their homes
Chapter 10: Mass Movement
Human activities can impact on the operation of
surface processes
• Impact of overgrazing
• Impact of overcropping
• Impact of deforestation
Chapter 10: Mass Movement
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
Chapter 10: Mass Movement
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