How do the sea's erosional processes carve the distinctive landforms of a rocky coast?
Explain the marine and subaerial processes of coastal erosion and how they produce landforms such as cliffs, wave-cut platforms, caves, arches and stacks
A focused answer to the H2 Geography outcome on coastal erosion. The processes of hydraulic action, abrasion, attrition and solution, the role of subaerial weathering, and the formation of cliffs, platforms and the cave-arch-stack sequence.
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What this dot point is asking
SEAB wants you to explain the processes that erode a coast, both marine and subaerial, and to show how they produce the classic landforms of a rocky coast: cliffs, wave-cut platforms, and the cave-arch-stack-stump sequence. The central insight is that erosional landforms record where wave energy is concentrated and where rock is weakest.
The answer
Marine erosion processes
- Hydraulic action: waves compress air trapped in joints and cracks; the sudden release weakens and breaks the rock. The sheer force of water (wave quarrying) also dislodges blocks.
- Abrasion (corrasion): waves hurl sand and pebbles at the cliff, wearing it away like sandpaper. This is often the most effective erosional process.
- Attrition: rock fragments collide and grind against each other, becoming smaller and rounder. This does not erode the coast directly but produces finer sediment.
- Solution (corrosion): mildly acidic seawater dissolves soluble rocks such as limestone.
Subaerial processes
Erosion from the sea is reinforced by weathering (mechanical, chemical and biological breakdown of rock in place) and mass movement (rockfall, slumping, landslides) on the cliff face. These weaken the cliff from above and behind, so cliff retreat is a partnership between marine attack at the base and subaerial processes on the face.
Cliffs and wave-cut platforms
Destructive waves concentrate energy at the cliff base between the tides, eroding a wave-cut notch. As the notch deepens, the overhang collapses under gravity and the cliff retreats. The eroded base left behind is a gently seaward-sloping wave-cut platform. As the platform widens, waves must cross more of it and lose energy to friction, a negative feedback that slows further retreat.
The cave-arch-stack-stump sequence
Where a resistant headland contains lines of weakness (joints, faults, bedding planes), erosion exploits them:
- Hydraulic action and abrasion widen a weakness into a cave.
- A cave driven through a headland, or two caves meeting, forms an arch.
- Continued basal erosion and weathering of the roof enlarge the arch until the roof collapses, leaving an isolated stack.
- The stack is undercut and eventually reduced to a stump, often submerged at high tide.
Wave refraction concentrates energy on headlands, which is why this sequence develops there.
Examples in context
Example 1. The Twelve Apostles, Australia. Along the Victorian coast, powerful Southern Ocean waves have eroded the soft limestone cliffs into a wave-cut platform fringed by stacks, the Twelve Apostles, formed as arches collapsed. The site is a textbook display of the cave-arch-stack-stump sequence and of rapid retreat in weak rock, with stacks periodically collapsing as undercutting continues.
Example 2. Erosion on chalk coasts such as Old Harry Rocks. On the chalk coast of Dorset in England, the headland has been eroded along joints into caves, an arch and the Old Harry stack and stump. The white chalk shows fresh erosion clearly, and the contrast with the adjacent clay bays illustrates how rock resistance and lines of weakness control where erosional landforms develop.
Try this
Q1. Distinguish between abrasion and hydraulic action. [2 marks]
- Cue. Abrasion is the wearing of the coast by sediment hurled at it by waves; hydraulic action is the breaking of rock by the force of water and by compression and release of air trapped in joints.
Q2. Explain why a wave-cut platform slows the rate of cliff retreat over time. [2 marks]
- Cue. As the platform widens, waves must travel across more of it before reaching the cliff, losing energy to friction, so less energy reaches the cliff base and retreat slows.
Q3. Outline the sequence by which a stack forms from a headland. [3 marks]
- Cue. Waves erode a line of weakness into a cave; the cave is driven through to form an arch; the arch roof is enlarged and collapses, leaving an isolated stack separated from the headland.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original10 marksExplain the formation of a wave-cut platform.Show worked answer →
Argument: a wave-cut platform forms through the repeated undercutting and collapse of a cliff, leaving a gently sloping rock surface exposed at low tide.
Process sequence to explain: destructive waves concentrate their energy at the base of the cliff between high and low tide, where they erode by hydraulic action (the compression of air in joints) and abrasion (rock fragments hurled at the cliff). This cuts a wave-cut notch at the base. As the notch deepens, the overhanging rock above is left unsupported and collapses under gravity. The cliff retreats landward, and this is repeated. The eroded base left behind is a wave-cut platform, gently sloping seaward, across which later waves must travel, losing energy by friction, which eventually slows further retreat.
Evaluation and marks: a strong answer sequences notch, undercutting, collapse, retreat and platform, names the marine processes responsible, and notes the negative feedback as the widening platform dissipates energy. Markers reward the notch-collapse-retreat mechanism, named erosion processes, and the platform as the product of cliff retreat.
Original10 marksExplain how a headland is progressively eroded into caves, arches, stacks and stumps.Show worked answer →
Argument: where waves attack lines of weakness in a resistant headland, erosion exploits and widens them in a recognised sequence from cave to stump.
Sequence to explain: waves focus their energy on joints, faults or bedding planes in the headland. Hydraulic action and abrasion widen a weakness into a cave. Where caves form on both sides of a narrow headland and meet, or where a single cave is driven right through, an arch results. Continued erosion at the base and subaerial weathering of the roof enlarge the arch until the roof collapses, leaving an isolated pillar, a stack, separated from the headland. The stack is undercut at its base and eventually collapses to a stump, often visible only at low tide.
Evaluation and marks: a strong answer ties the sequence to the exploitation of lines of weakness and to wave refraction concentrating energy on headlands, and uses the cave-arch-stack-stump progression with named processes. Markers reward the role of weaknesses, the ordered sequence, and the processes driving each step.
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