Marine Processes (AQA A Level Geography)

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

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