Sub-Aerial Processes (AQA A Level Geography)

Coastal Weathering

• Weathering is the breakdown of rock in its place of origin (in-situ)

• There are many types of weathering

Types of Weathering

Mechanical (physical) weathering

Freeze-thaw/frost shattering/ice wedging 

• This happens in places where nighttime temperatures often reach below freezing 

Wetting and drying

• In warmer climates where the water doesn’t freeze, a similar process called wetting and drying occurs which works in the same way, just without the ice

  • This often occurs in the inter-tidal zone; the area is exposed at low tide but covered at high tide

  • Rocks expand when they are wet and contract again when dry 

Salt Crystallisation

• Salt crystallisation occurs because the salt crystals are bigger than the water molecules 

  • This exerts pressure on the rock, causing it to break down

Exfoliation/onion skin weathering

• This occurs in hot climates 

• When rocks warm up during the day, the rock expands, exerting outward pressure

• At night, the rocks cool and contract 

• Over time, this contraction and expansion causes very thin pieces of the rock to flake off 

Biological weathering

• Nesting birds and small burrowing animals like rabbits, can cause rock to breakdown through biological weathering

Chemical weathering

• There are three types of chemical weathering 

  • Carbonation - when rain is slightly acidic and reacts with carbonate rocks like limestone, causing them to dissolve

  • Oxidation - when iron minerals in the rock react with oxygen in the air to cause rusting and breakdown the rock 

  • Solution - when other salt minerals in the rock are dissolved 

Coastal Mass Movement

• Mass movement is:

The downhill movement of material under the influence of gravity

• Throughflow and runoff caused by heavy rain can make cliffs more unstable and increase the likelihood of mass movement

• Mass movement includes landslides, slumping and rockfalls

• The type of movement is determined by:

  • Angle of slope (the steeper the slope the faster the movement)

  • Nature of regolith

  • Amount and type of vegetation

  • Water

  • Type and structure of rock

  • Human activity

  • Climate

• Soil Creep:

  • Common in humid climates with movement of less than 1cm per year 

  • Soil expands when it freezes, gets wet or is heated up in the sun

  • As the soil expands, it lifts at right angles to the slope

  • When the soil shrinks, it falls 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° with speeds between 1 to 15km per year

  • Usually happens after the soil has become saturated with a flow of water across the surface

  • Vegetation is flattened and carried away with the soil

• 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 reasons:

    • Extreme weathering: freeze-thaw action can loosen rocks that become unstable and collapse

    • Rainfall: too much rain will soften the surface leading to the collapse of the slope

    • Earthquakes can dislodge unstable rocks 

    • Hot weather can dry out soil causing it to shrink, allowing 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

  • Because of the way it slumps, it leaves behind a curved indented surface

Forms of mass movement