sub aerial processes and marine processes

Coastal Weathering

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

• There are many types of weathering

Types of Weathering

physical weathering :

wetting and drying, exfoliation, salt crystilisation

chemical weathering:

oxidation, hydration, carbonation, solution

biological weathering:

weathering by vegetation, weathering by animals, weathering by humans

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 

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 

fall: fragments of rock break away from the clif face due to weathering and cause scree to form at the bottom of the cliff

saturated soil flows down a slope causing lobe to form at the end of the slope

slump:saturated soil slumps along a curved surface

Marine Erosion

• Destructive waves erode the coastline 

• Less resistant (softer) rock, such as sandstone, erodes faster than more resistant (harder) rock, such as limestone, but all rocks will erode over time 

• Hydraulic action is the name for a group of erosion processes that occur due to the power of the waves, with no involvement of rocks at all 

  • Sea water forces air into cracks and joints in the rock

  • The pressure from the waves on this air causes the cracks and joints to expand 

  • When larger, loose blocks of rocks are eroded in this way, it is called wave quarrying 

  • If the waves are very large, cavitation will occur 

  • High-pressure air bubbles in waves get trapped in the small cracks in the rock

    • The air bubbles implode, generating shock waves

    • The shockwaves weaken the rock.

• Abrasion occurs when breaking waves that are carrying sediment scrape against the rock surface

  • This is also known as corrasion 

  • It is dependent on how much sediment is available in a particular area of the coastal system 

• The size of sediment particles changes as sediment held in the waves rubs together 

  • This is called attrition

• Acids in the seawater can also wear away the rock, this is solution (corrosion)

Marine Transportation

• Material in the sea arrives from many sources:

  • Eroded from cliffs

  • Transported by longshore drift along the coastline

  • Constructive waves bringing material inland from offshore 

  • Carried to the coastline by a river

• Once in the water, the material is moved in different ways:

  • Traction

  • Saltation

  • Suspension

  • Solution 

Longshore (littoral) drift

• Longshore (littoral) drift is the main process of deposition and transportation along the coast 

• Influenced by the prevailing wind, waves approach the beach at an angle

• As the waves break, the swash carries material up the beach at the same angle

• As the swash dies away, the backwash carries the material down the beach at right angles (90°) 

• The process repeats, transporting material along the beach in a zig-zag movement

• Offshore currents can contribute to the movement of sediment along or up the beach

The process of longshore (littoral) drift

Marine Deposition

• Deposition occurs when waves lose energy 

• Waves lose energy when: 

  • The wind slows or changes direction 

  • The wave experiences friction, usually with the sea floor as the depth decreases 

  • When the coastline changes direction such as at an estuary or headland

• It is a gradual rather than immediate process 

• High energy coastlines deposit large rocks and shingle but generally maintain enough energy to carry smaller sediment particles 

  • This is why these types of coastlines tend to have rocky beaches, such as Brighton on the south coast 

• Low energy coastlines deposit smaller sediment due to low wave velocity, creating sandy beaches