2.B4b

• most erosion occurs when:

  • waves are at their largest as they have most energy
  • waves hit cliffs at 90 degree angle
  • there is a high, spring tide as waves are propelled further up the beach
  • there is heavy, prolonged rain, saturating the cliff and weakening the rock
  • eroded material is carried away, as there's no protection at the cliff base
• 4 types of erosion:

  • hydraulic action
  • abrasion
  • attrition
  • corrosion (solution)
• hydraulic action: air is trapped under high pressure as waves advance. As waves retreat, air explodes outwards. Overtime, this causes the joint to widen and rocks to fracture
• how is hydraulic action influenced by wave type and lithology?

  lithology: the more joints in the rock = more vulnerable

  wave type: destructive, high energy waves cause more damage --> more in the winter
• abrasion: wave breaks at the cliff base, throwing sediment at it, chipping away at the rock
• how is abrasion influenced by lithology and wave type?

  lithology: loose sediment must be available e.g. shingle or pebbles, softer sedimentary rocks = more vulnerable

  wave type: destructive, plunging waves --> more in the winter
• attrition: as rocks are moved around by the water they knock into each other gradually becoming smaller and rounder
• how is attrition affected by wave type and lithology?

  wave type: destructive, high energy waves cause more attrition --> more in the winter

  lithology: softer rocks are very rapidly reduced in size by attrition
• corrosion (solution): The weak acid in seawater and some types of seaweed react with particular rock minerals, causing erosion and weakening.
• how is corrosion affected by wave type and lithology?

  lithology: mainly affects alkaline rocks such as limestone = more vulnerable

  wave type: acidic waves cause more corrosion
• wave cut notch: found at the base of cliffs due to long-term wave attack by hydraulic action and abrasion --> the wave attack zone where the cliff is undercut
• formation of a wave-cut platform:

  1. hydraulic action and abrasion create a wave-cut notch
  2. the notch increases in size, creating an overhang
  3. the unsupported overhang collapses due to gravity and weathering at the top of the cliff --> mass movement
  4. backwash transports material away from the cliff, creating the wave-cut platform
• cave, arch, stack, stump sequence:

  1. joints and faults in the cliff are eroded by hydraulic action, abrasion and salt crystallisation
  2. overtime, joints widen, forming a cave that is enlarged by hydraulic action and abrasion
  3. when 2 caves meet on either side of a headland, or a single cave is eroded through a headland, an arch is formed. The gap is further enlarged by erosion and weathering - becoming wider at the base
  4. the unsupported roof collapses due to gravity, forming a stack
  5. the stack is surrounded at all sides so is eroded and weathered and may only appear at low tide
• erosional landforms case study: Jurrasic coast in Dorset

  • natural arch: durdle door - made of limestone exploited by erosion of weaknesses between vertical layers of rock - wealden beds behind more easily eroded
  • stack: Old Harry made of chalk