Coastal Processes (DP IB Geography)

Sub-aerial processes

• 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

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

• 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

Wave processes

• 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

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