Coastal Processes (SL IB Geography)

• Sub-aerial processes occur on the land 
• These processes shape our coastlines

Weathering

• When weathered, rocks disintegrate or break down in situ
• Weathering impacts sediment on the coastline
• Weathering also influences the rate of recession
• The different types of weathering are:
  • Mechanical (physical) – rocks fracture and break into pieces
  • Chemical – chemical reactions cause rocks to degrade 
  • Biological – animals and plants cause rocks to weaken

The types of weathering and causes

The types of weathering and causes

Mechanical weathering

• Freeze-thaw weathering occurs when water finds its way into the cracks in the rock
  • Water freezes and expands (roughly 10%), increasing the pressure on the surrounding rock
  • Water then thaws and moves further down into the gaps and cracks
  • As this process repeats, the rock will eventually give way and break
  • This typically occurs during colder months

The process of freeze-thaw weathering

The process of freeze-thaw weathering

• Salt Crystallization
  • Salt crystals can enter cracks in the rock through deposition or evaporation
  • Water enters cracks at high tide and evaporates at low tide
  • This leaves salt crystals behind, which build up over time
  • The increase in salt can put pressure on the surrounding rock, causing it to break
    

• Wetting and drying
  • This process typically affects clay-type rocks
  • Rocks get wet during high tide and expand
  • As they dry, they contract
  • As this process repeats, the rock can fragment and break

Chemical weathering

• Oxidation – Oxygen can react with other chemicals, e.g. calcium to form iron oxide. This causes the rock to degrade
• Hydration – rocks absorb minerals, which can create crystals. Just like salt crystals, they cause the rock to break
• Carbonation – as carbon dioxide reacts with seawater, it produces carbonic acid, which damages the rock
• Solution – some minerals can dissolve when they come into contact with water, causing the rock to break down. This is commonly seen with acid rain

Biological weathering 

• Animals, vegetation (and even humans) can cause rocks to break down
• Tree roots can grow from small cracks in the rock and exert pressure. This causes the rock to break 
• Some creatures bore into rocks, causing weaknesses
• Some plants and animals can produce acids that wear away the rock

Mass movement 

• Mass movement moves material downwards due to the force of gravity
• There are many different types of mass movement
• Factors determining the amount of mass movement include:
  • Sediment type or size
  • Amount of water
  • Presence of vegetation
  • Speed of movement (how steep the slopes are)
• Soil creep
  • This is a very slow process
  • This occurs on gentle slopes
  • The material expands and contracts when in contact with water or ice
  • This expansion moves the material downwards
• Earth flows and mudflows
  • Soil moves downhill when it has become saturated with water
  • Earth flows act like a liquid 
  • This leaves behind a depression at the top and a lobe shape at the bottom
  • Mudflows occur when the material is extremely wet

• Landslides
  • A large chunk of material moves down a slope
  • It splits into smaller pieces of material when it reaches the bottom of a slope
  • This is very common when slopes are steep
• Slumping
  • Rotational slumping or slip is very common at the coastline
  • The material moves as a single piece downwards, leaving behind a curved gap 
  • It occurs when weaker rocks or rocks made of unconsolidated materials become saturated with water
• Rockfall
  • Rocks can fall downwards at steeper slopes
  • This is typically caused by freeze-thaw weathering
  • It leaves scree behind at the base

Types of mass movement

Types of mass movement

• Wave processes transfer, erode and deposit material at the coast
• Wave processes are responsible for shaping the coastline

Littoral drift

• Littoral drift is also known as longshore drift
• This is the main process in which waves transport and deposit sediment
• Waves hit the coastline at an angle due to prevailing wind forces
• Swash carries material up the shore and backwash brings the material back towards the sea at a right angle (gravity)
• Material moves along the beach through the wave processes of:
• Traction – large rocks roll along the seafloor 
• Saltation – smaller rocks or pebbles bounce on the seafloor
• Suspension – small particles of sediment float in the water
• Solution – the water carries dissolved material
• Longshore (littoral) drift creates depositional landforms along the coast, such as spits

The process of littoral drift

The process of littoral drift

Hydraulic action

• Destructive waves with high energy can cause the rock to break from the impact
• Destructive waves can also force air into the cracks and faults in the rock
• Changes in air pressure cause rocks to crack
• This can cause pieces of rock to break off as the cycle repeats

Abrasion

• Waves carry all types and sizes of sediment 
• Destructive, high-energy waves throw this sediment against the rock at the coastline
• As this process repeats, chunks of rock break away
• It mirrors sandpaper, where smaller pieces of sediment wear away the rock
• Softer sedimentary rock is more affected by abrasion