Download Mass Movement - Ms. Twomey SMC Geography

Mass Movement
Definition:
Is the process by which material moves down a slope under the
influence of gravity
Type of material includes: Rocks, soil, stones, mud, and snow
Causes = Natural processes ( weathering and erosion), Natural
disturbances ( Earthquakes) or Human Activity ( dam collapse, forest
clearance, road building)
What factors influence mass movement???
1. Gradient of slope, the steeper the slope the more likely that
mass movement
2. Slope Material, if solid (compacted or cemented) less likely to
move.
3. Water content: if large amount of water, it acts as a lubricant
4. Vegetation: roots help to bind soil and absorbs much of the
rainfall
5. Tectonic Activity: triggered by volcanoes and earthquakes
Classifying Mass Movement
 Type of mass movement
 Speed of mass movement
 Type of material
Processes of Mass Movement
Soil Creep
Slowest type
Occurs on slopes of less then 5 degrees
Less then 0.5cm per year
Terracettes
Poles tilted
Base of tree turned down slope
Walls broken
Slides
Are sudden and rapid downward movements of masses of rock and soil
under the influence of gravity.
Rock Slides
Occur along bedding planes, rocks slide along bedding planes down
the slope at high speed (assuming slope is steep)
Slumps
Occur when the base of the slope is removed either by man or
natural processes (undercutting). Spoon shaped sliding
Flows
Is a downhill movement of soft wet material (soil, clay, silt and peat)
That has been made fluid by rain or melted snow
Earth flows
Local features, common on hill sides or valley sides especially after
heavy rainfall
Slow movement, produce bulges or lobes in the landscape, and leave
scars on the landscape (no vegetation)
Bog flow / Bog bursts
Affects blanket bogs in upland areas. Peat soil
Lots of rainfall
Slieve Aughty Mountains in 2003
Mudflow
Consists of 30% water
Can occur after heavy rainfall, snow melt or volcanic eruptions in
upland areas
Water increases volume and acts as a lubricant
Streams of mud like slurry pour down valleys
80 km per hour, very powerful
LAHAR
Associated with volcanic eruptions
Nevado del Ruiz (Columbia, 1995)
Hot magma and pyroclastic material melted snow and ice, water
picked up volcanic ash and other material, moved down slope and
killed over 21000 people in village of Armero.
Fall
Movement of a single rock or thousands of rocks, normally no soil
cover
High speed
Very steep slope
Speeds up to 300 km per hour
Rock fall
Can be triggered by freeze thaw, build up to form scree or talus
slopes
Rock Avalanche
Rock fall dislodging other rocks and causing them to move down slope
too.
Human Influences on mass movement
Over grazing…..sheep….Galway and Mayo… 2 million
Soil erosion is common in upland areas
Nephin area in County Mayo
Connemara uplands in Co, Galway
30% has been seriously eroded
Cause : high stock rates ( 5 times higher then UK)
Rivers and lakes have been affected by silt loading
Solution????
Sahel Region
Narrow, semi-arid region between Sahara Desert to the North and
equatorial forests to the south
Natural vegetation = Savannah ( grassland and open woodland)
People: Nomadic, moving to follow rains
Moving between 5km to 10km a year
Desertification
Why???
Climate change (drought)
Human Interference
Cattle = number of cattle = wealth, cultural
Between 1950 and 1970 rainfall was much heavier, so there was more
pasture land
50% increase in the number of cattle
Then drought hit, below average rainfall since below 1968
Result = Overgrazing
Land stripped of its protective cover, vegetation unable to regrow,
soil exposed, nutrients blown away in the wind
Overcropping
Population growth, high birth rates and immigration due to wars and
drought
Farmers began to settle not move, used land more intensively
Crops like rice, maize and cash crops like nuts and cotton were grown
in marginal land
NO FALLOW YEAR
No chance for land to replenish nutrients and yield was decreased
Deforestation
Very important for maintaining soil fertility
Leaves form humus
Roots bind soil together
Wood is major energy source, 90% of heating and cooking and
construction
Soil is exposed
Impact
Lack of vegetation leaves soil exposed, any heavy downpours, water
acts as a lubricant, mudslides and landslides are common