Mass Movement (DP IB Geography): Revision Note

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