Mass Movement (DP IB Geography)
Revision Note
Written by: Bridgette Barrett
Reviewed by: Jenna Quinn
Mass Movement
Mass movement is the downhill movement of material under the influence of gravity
Throughflow and runoff caused by heavy rain can also make slopes more unstable and increase the likelihood of mass movement
Mass movement includes landslides, slumping and rockfalls
The type of movement is influenced by:
Angle of slope (steeper is faster)
Nature of regolith
Amount and type of vegetation
Water
Type and structure of rock
Human activity
Climate
Soil Creep:
Speed is below 1cm per year
Common in humid climates
When soil expands, individual particles are lifted up at right angles to the slope
Soil also expands when it freezes, gets wet or is heated up in the sun
When the soil shrinks again, the particles fall straight back down
Soil creep takes a long time because the soil moves only a millimetre to a few centimetres at a time
Flow:
Occurs on slopes between 5° and 15°
Usually after the soil has become saturated with a flow of water across the surface
Vegetation can be flattened and carried away with the soil
Speeds range from 1km to 15km per year
Slide:
A movement of material 'en masse', which remains together until hitting the bottom of a slope
Fall:
Slopes are steep and movement is rapid
Caused by a number of factors:
Extreme weathering – freeze-thaw action can loosen rocks that become unstable and collapse
Rainfall – too much rain will soften the surface, leading to collapse of the slope
Earthquakes can dislodge unstable rocks
Hot weather can dry out soil, causing it to shrink and allow rocks to fall
Slump:
Usually found on weaker rock types (i.e. clay), that become saturated and heavy
This is common at the coast and is also known as rotational slip
It involves a large area of land moving down the slope in one piece
Due to the nature of the slip, it leaves behind a curved surface
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