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
Types of mass movement