Coastal Erosion Processes and Landforms

Erosion is the primary destructive process shaping cliffed coastlines. This lesson provides detailed coverage of the marine erosion processes and sub-aerial processes that combine to produce the distinctive erosional landforms found along high-energy coasts. It directly addresses Edexcel A-Level Geography Enquiry Question 1: Why are coastal landscapes different and what processes cause these differences?

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Marine Erosion Processes

Marine erosion is the removal of rock material by wave action. Five distinct processes operate, often simultaneously:

1. Hydraulic Action

Hydraulic action is the force of water itself — without any sediment load — impacting the rock face. When a wave strikes a cliff, water is forced into cracks, joints and bedding planes under pressure. As the wave retreats, the pressure is rapidly released, creating a partial vacuum. This repeated compression and decompression cycle weakens the rock and causes fragments to break away.

Hydraulic action is most effective on:

• Rocks with many joints and bedding planes (e.g., limestone, sandstone)
• Cliff faces that are directly exposed to high-energy waves
• During storms, when wave heights and energy are greatest

The pressures involved can be enormous. Storm waves can exert pressures exceeding 30 tonnes per square metre — sufficient to dislodge large blocks from well-jointed rock.

2. Abrasion (Corrasion)

Abrasion is the wearing away of rock by sediment carried in the waves. Sand, gravel and pebbles are hurled against the cliff face by breaking waves, acting like natural sandpaper. Abrasion is the most effective erosion process on most cliffed coasts.

The rate of abrasion depends on:

• Wave energy — higher energy waves carry larger, heavier sediment loads
• Sediment availability — beaches with abundant pebbles provide ammunition for abrasion
• Rock hardness — softer rocks are abraded more quickly
• Wave type — plunging breakers are particularly effective because they throw sediment directly at the cliff base

Abrasion also operates on the foreshore, smoothing and lowering the wave-cut platform as sediment is dragged across the rock surface by swash and backwash.

3. Attrition

Attrition is the wearing down of sediment particles as they collide with each other and with the rock surface during transport. It does not directly erode the coastline — instead, it reduces the size and angularity of beach material over time. Pebbles become progressively smaller, smoother and more rounded through repeated impacts.

Attrition explains why:

• Beach sediment is generally finer further from its source
• Pebbles on high-energy beaches are well-rounded and smooth
• Sand-sized particles are the eventual product of the breakdown of larger clasts

4. Solution (Corrosion)

Solution is the chemical dissolution of rock by seawater. It is particularly effective on limestone and chalk because calcium carbonate reacts with the weak carbonic acid naturally present in seawater and rainwater:

CaCO₃ + H₂CO₃ → Ca(HCO₃)₂ (calcium bicarbonate — soluble)

Solution operates continuously, even on calm days, and is enhanced by:

• Higher CO₂ concentrations in cooler water (which increase acidity)
• Biological activity (organisms produce acids)
• Longer contact time between water and rock (in pools and saturated zones)

Although solution is a slower process than hydraulic action or abrasion, its cumulative effect over decades and centuries is significant, particularly in widening joints and bedding planes, which then become more vulnerable to mechanical erosion.

5. Cavitation

Cavitation occurs when air bubbles trapped in turbulent water collapse against the rock surface, generating tiny but intense shock waves. These micro-scale impacts can cause surface pitting and weakening over time. Cavitation is most significant in highly turbulent conditions — at the base of plunging breakers and in narrow sea caves where water surges rapidly.

graph TD
    A["Marine Erosion Processes"] --> B["Hydraulic Action<br/>Force of water in cracks"]
    A --> C["Abrasion<br/>Sediment thrown at rock"]
    A --> D["Attrition<br/>Sediment particles collide"]
    A --> E["Solution<br/>Chemical dissolution"]
    A --> F["Cavitation<br/>Air bubble collapse"]
    B --> G["Most effective in jointed rock, storm conditions"]
    C --> H["Most effective erosion process on most coasts"]
    D --> I["Reduces sediment size, does not directly erode coast"]
    E --> J["Most effective on limestone and chalk"]
    F --> K["Minor role; operates in turbulent conditions"]

Exam Tip: When explaining erosional landforms, always identify which erosion processes are most responsible. Caves, for example, are formed primarily by hydraulic action exploiting joints and faults, assisted by abrasion and solution. Avoid simply listing all five processes for every landform — show understanding of which ones dominate in each case.

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Sub-Aerial Processes

Sub-aerial processes operate on the cliff face and top from above, weakening rock and preparing it for removal by waves. They include weathering (in situ rock breakdown) and mass movement (downslope movement of material under gravity). These processes are covered in full detail in Lesson 6, but their role in cliff recession is introduced here.

Weathering

• Freeze-thaw (frost shattering): Water enters cracks, freezes and expands (by ~9%), widening the crack. Repeated cycles shatter rock. Most effective where temperatures oscillate around 0°C.
• Salt crystallisation: Evaporation of seawater in rock pores causes salt crystals to grow, exerting pressure and causing granular disintegration. Important in the splash zone.
• Wetting and drying: Clay-rich rocks expand when wet and contract when dry, causing surface cracking and flaking. Important on shale and mudstone cliffs.
• Carbonation: Rainwater dissolves CO₂ to form weak carbonic acid, which dissolves calcium carbonate in limestone and chalk.
• Biological weathering: Plant roots widen cracks; burrowing organisms weaken rock; algae and lichens produce organic acids.

Mass Movement

• Rockfalls: Fragments of rock detach from a cliff face (often along joints) and fall under gravity. Common on steep, well-jointed cliffs (e.g., chalk cliffs at Beachy Head).
• Rotational slumping: A curved block of saturated material slides along a concave failure plane. Common on clay cliffs where water lubricates the slip surface (e.g., Holderness coast).
• Landslides (translational slides): A block of material slides along a planar failure surface (often a bedding plane). Common where permeable rock overlies impermeable rock.
• Mudflows: Saturated, fine-grained material flows downslope as a viscous mass. Fast-moving and potentially dangerous.

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Erosional Landforms