Sub-Aerial Processes (AQA A Level Geography)

• Weathering is the breakdown of rock in its place of origin (in-situ)
• There are many types of weathering

Types of Weathering

Type of  weathering	Process
Mechanical	When rocks break up with no chemical changes
Biological	Rock breakdown due to organic activity
Chemical	Rock breakdown due to a chemical reaction

Types of weathering

Mechanical (physical) weathering

Freeze-thaw/frost shattering/ice wedging 

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

Freeze-thaw / frost shattering / ice wedging

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

Salt crystallisation

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

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 

• 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