Subaerial Processes (Edexcel A Level Geography)

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

Types of Weathering

Name	Process
Mechanical (Physical)	When rocks break down, with no chemical changes
Biological	Rock breaks down due to organic activity
Chemical	Rock breaks down due to a chemical reaction - rock changes

Types of weathering

Mechanical (physical) weathering

Freeze-thaw/frost shattering/ice-wedging

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

Freeze-thaw/ frost shattering

Wetting and drying

• In warmer climates where the water does not 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 becomes exposed at low tide but covered at high tide
• Rocks expand when they are wet and then contract when dry 

Salt crystallisation

• Salt crystallisation occurs because salt crystals are bigger than water molecules 
• This exerts pressure on the rock, causing the rock to break down

Salt crystallisation

Exfoliation/onion skin weathering

• This happens in hot climates 
• When rocks warm up during the day the rock expands 
• At night the rock cools and contracts 
• Over time this causes very thin layers of rock to flake off 

Biological weathering

Biological weathering

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

Chemical weathering

• There are three types of chemical weathering: 
• Carbonation - as rain is slightly acidic, it reacts with carbonate rocks such as limestone, causing them to dissolve
• Oxidation - when iron minerals in the rock react with oxygen in the air to cause rust 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 also make cliffs more unstable and increase the likelihood of mass movement
• It 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 climes with the 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 and 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

• Mass movement creates distinctive landforms including:
  • Rotational scars
  • Talus scree slopes
  • Terraced cliff profiles

Rotational scar

• A rotational scar is:
  • Curved
  • Un-weathered 
  • Un-vegetated
  • Forms as a result of rotational slumping
• The section that has detached is at the base of the cliff often with vegetation attached

Talus scree slope

• A talus scree slope is a fan-shaped mound of material
• Made of block-fall debris which has accumulated at the foot of a cliff
• Often, they have a concave profile

Terraced cliff profile

• The profile of the cliff is stepped 
• This is the result of the lithology or fractures in the rock