2B.6b Flashcards
What does MM depend on
- the angle of the slope/cliff
- the rock’s lithology (consolidates slides, unconsolidated slumps) and geology
- the vegetation cover on the cliff face
- the saturation of the ground/ previous weather patterns
State three types of MM
Block falls, rotational slump and landslides
Define mass movement
Mass movement is the downslope movement of material (rock and soil) under the force of gravity
When does mass movement occur.
occurs when the downslope gravitational force exceeds the resisting forces of friction and internal rock cohesion.
What is blockfall
This occurs on slopes >40’, where a rock fragment breaks away and either drops vertically (so it isn’t in contact with the cliff) or bounces downslope.
What causes the rock fragment to break away in block falls
- By mechanical weathering
freeze-thaw
salt crystal growth
which break the cohesive bonds in the rock - By marine erosion
Hydraulic action
Abrasion
Undercutting cliff by creating a wave-cut notch
(Notch removed supporting material that supplied the resistive force holding up the rock)
What is significant about blockfall
Very rapid (takes a few seconds to occur)
What are landslides
A landslide is the downslope movement of discrete blocks of rock down a flat/linear slip plane, maintaining contact with the cliff surface throughout.
What is rotational slump
Occur when the soil is saturated with water, causing a rotation movement of soft
materials (such as clay and sand) forming rotational scars and terraced cliff profiles
State three characteristics of rotational slump
- Rotational slumping involves rock failure and movement along a curved rock plane.
- The slumping material usually moves intact as a single mass, without any internal deformation of material.
- It’s slower than blockfall, often occurring in ‘slow motion’, and may take minutes, hours, days, or even years (for huge masses) to occur.
Describe the process of rotational slump
Bedding planes are seaward – so you get more mass movement anyway
Cracks develop in dry weather (soil / sediment dry out)
Rainwater runs down through the cracks and into the sand underneath
Heavy rain saturates the permeable layers on top – top-loading
It has to move along the bedding plane (high pore-water pressure) – creates internet pressure
Toe erosion by marine processes (undercutting)
The whole cliff starts to rotate
How do landslides occur
- The discrete blocks are released by mechanical weathering of well jointed rocks, (e.g. carboniferous limestone).
-Gravity then pulls the loosened block down the relatively flat slip plane of the joint or bedding plane, to the cliff foot.
How else are discrete blocks released in landslides
Landslides can also be caused by marine erosion of a cliff foot undercutting blocks weakened by jointing. The removal of support allows gravity to release the block, resulting in sliding.
What are strengths in slopes
Lubricant layers
Tree / vegetation roots
Consolidation of material
Porosity of soil
Stresses on slopes
Angle of slope
Rainwater / saturation
Vibration, e.g. car
Earthquakes
Sea/cliff erosion
