Subaerial Processes (Edexcel A Level Geography)

Weathering

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

• There are several types of weathering

Types of Weathering

Mechanical (physical) weathering

Freeze-thaw/frost shattering/ice-wedging

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

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

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

• 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

• 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 Landforms

• 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