Coastal Erosion Processes

Spec mapping (AQA 7037): Paper 1, §3.1.3 Coastal Systems and Landscapes — geomorphological processes: marine erosion (hydraulic action, wave quarrying, corrasion/abrasion, attrition, solution); sub-aerial processes (weathering — mechanical, chemical, biological; and mass movement — landslides, slumping, rockfall). These are the transfers that move material from the cliff store into the beach store in the systems model (Lesson 1). The lesson links synoptically to §3.1.4 Glacial Systems (weathering and mass movement operate in glacial environments too, and till — a glacial deposit — is the easily-eroded input at Holderness) and to §3.1.5 Hazards (mass-movement events such as the Holbeck Hall slump are themselves geomorphic hazards). The dominant Assessment Objectives are AO1 (precise knowledge of each named process and its mechanism) and AO2 (explaining how processes combine and vary in importance with rock type, energy and time). The erosion-rate exercise exercises AO3 (calculating and interpreting rates of retreat).

Why this lesson matters. Examiners report that the most frequent error in cliff-retreat answers is the conflation of erosion with weathering. Weathering breaks rock down in situ with no transport; erosion involves both breakdown and removal. Cliffs retreat through the interaction of the two — sub-aerial weathering weakens the rock, marine erosion removes it. Get this distinction crisp and the rest of the lesson, and most of Lesson 4, falls into place.

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Marine Erosion Processes

Marine erosion occurs where waves attack the coastline. There are four principal mechanisms:

1. Hydraulic Action (Wave Quarrying)

Hydraulic action is the sheer force of water striking rock surfaces. When waves crash against a cliff face, water is driven into cracks, joints and bedding planes under enormous pressure. As the wave retreats, the compressed air within these cracks expands explosively, progressively widening the fissures and eventually causing blocks of rock to fracture and detach.

• The pressure exerted by storm waves can exceed 30 tonnes per m² (measured by Thomas Stevenson in the 1840s using a wave dynamometer at Skerryvore lighthouse, Scotland)
• Modern measurements by Wolters and Müller (2004) at the Elbe estuary recorded pressures of up to 100 kPa during storm events
• Hydraulic action is most effective in well-jointed rocks such as limestone, chalk and sandstone
• It is the dominant erosion process on exposed, high-energy coastlines

Key Definition: Hydraulic action is the erosive force of water and compressed air being driven into cracks and joints in rock by wave impact, causing mechanical fracturing and removal of material.

2. Abrasion (Corrasion)

Abrasion is the sandpapering effect of sediment carried by waves being hurled against rock surfaces. Pebbles, sand and gravel act as tools (the load) that scour, scrape and chip away at the cliff face and wave-cut platform.

• The effectiveness of abrasion depends on: the hardness of the load, the energy of the waves, and the resistance of the rock being eroded
• Abrasion is responsible for the smoothing and polishing of wave-cut platforms — studies by Trenhaile (1987) showed that platform surfaces are typically lowered by 1-3 mm per year through abrasion
• It is often the dominant erosion process where there is an abundant supply of sediment (e.g., beaches backed by cliffs)
• Robinson (1977) used micro-erosion meters on the Yorkshire coast and recorded abrasion rates of up to 3.2 mm/year on limestone platforms

3. Attrition

Attrition is the process by which the sediment itself is worn down. As rocks and pebbles are transported by waves, they collide with each other and with the sea bed, becoming progressively smaller, smoother and more rounded.

• Attrition does not directly erode the coastline, but it reduces the size of the erosive tools available for abrasion
• It also produces the progression from angular boulders at the base of cliffs to rounded pebbles and eventually fine sand further along the beach
• Wentworth's (1922) grain size classification remains the standard scheme for describing sediment:

Category	Grain Size
Boulder	> 256 mm
Cobble	64-256 mm
Pebble	4-64 mm
Granule	2-4 mm
Sand	0.0625-2 mm
Silt	0.004-0.0625 mm
Clay	< 0.004 mm

4. Solution (Corrosion)

Solution is the chemical dissolution of rock by seawater. It is most significant where the rock contains calcium carbonate (CaCO₃), which dissolves in the mildly acidic conditions created when CO₂ dissolves in seawater.

CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂

• Solution is the dominant erosion process on chalk and limestone coastlines — for example, at the White Cliffs of Dover and along the Jurassic Coast in Dorset
• The rate of solution increases with: lower water temperature (cold water holds more dissolved CO₂), higher wave energy (greater surface area of rock exposed), and higher biological activity (organisms produce CO₂)
• Trudgill (1985) measured limestone dissolution rates of 0.5-1.0 mm/year on exposed coastal surfaces in County Clare, Ireland
• Solution can create distinctive landforms such as limestone pavements, karren (solution channels) and enlarged joints

Exam Tip: In exam answers, always use the correct terminology — "hydraulic action" not "hydraulic power," "abrasion" not "erosion by waves." Examiners reward precise use of geographical vocabulary. Also, remember to name specific researchers and data to support your points.

Which marine process dominates, and where?

A frequent higher-level demand is to explain why a particular process dominates on a given coast — the four marine processes are not equally important everywhere:

• Hydraulic action dominates on high-energy, well-jointed rock coasts (chalk, jointed sandstone) where storm waves can drive water and compressed air into abundant fissures. It is the leading process on the exposed Atlantic-facing cliffs of Cornwall and the chalk of the south coast.
• Abrasion dominates where there is an abundant supply of mobile sediment to act as tools — a cliff backed by a shingle beach is abraded far faster than an identical cliff with no beach, because the waves arrive armed. This is why a partial beach can paradoxically increase erosion of the cliff foot through abrasion even as a full beach protects it by absorbing energy.
• Solution dominates on carbonate rock (chalk, limestone) and is relatively independent of wave energy, proceeding even in sheltered conditions; it is more sensitive to water temperature and biological activity than to wave power.
• Attrition dominates the sediment, not the cliff, everywhere that material is in transport — it never attacks the coast directly but governs the calibre of the abrasive load.

The dominant process therefore depends on the intersection of rock type, sediment supply and wave energy — another instance of the interacting-controls theme. A complete answer specifies which process leads on which coast and why, rather than listing all four indiscriminately.

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Sub-aerial Processes

Sub-aerial processes operate on the land surface and cliff face, weakening rock and preparing it for removal by marine processes. They include weathering and mass movement.

Weathering

Weathering is the in situ breakdown of rock — material is broken down but not transported. Three types of weathering affect the coast:

Mechanical (Physical) Weathering

Process	Mechanism	Significance at the Coast
Freeze-thaw	Water enters cracks, freezes (expands by ~9%), thaws, refreezes — progressively widening the crack	Important on chalk and limestone cliffs in winter; responsible for scree slopes at the base of cliffs
Salt crystallisation	Seawater enters pores and cracks; evaporation causes salt crystals to grow, exerting pressure on the rock	Very effective in the splash zone; particularly damaging to porous rocks like sandstone
Wetting and drying	Repeated cycles of water absorption and loss cause swelling and contraction, leading to flaking	Important for clay-rich rocks and shales; contributes to cliff instability
Thermal expansion	Repeated heating and cooling of rock surfaces causes stress and eventual fracturing	Most significant in hot climates but also operates on south-facing cliffs in the UK
Pressure release	Removal of overlying rock (through erosion) reduces confining pressure, causing rock to expand and fracture	Important in areas of recent cliff retreat

Chemical Weathering

Process	Mechanism	Rocks Affected
Carbonation	CO₂ dissolves in rainwater forming weak carbonic acid (H₂CO₃), which reacts with calcium carbonate	Limestone, chalk
Oxidation	Oxygen reacts with iron-rich minerals, weakening the rock structure	Rocks containing iron (e.g., some sandstones, basalt)
Hydrolysis	Water reacts with minerals (especially feldspars) to form clay minerals	Granite, gneiss
Hydration	Water molecules are absorbed into the mineral structure, causing expansion	Clays, some igneous rocks

Biological Weathering

• Root action — plant roots grow into cracks and joints, exerting pressure and widening them. Studies by Gabet and Mudd (2010) demonstrated that tree roots can generate pressures of up to 15 MPa
• Burrowing organisms — piddocks (boring bivalves) drill holes in soft rock; sea urchins scrape rock surfaces; worms and crabs burrow into sediment
• Algal and lichen growth — produce organic acids that dissolve rock surfaces through bioerosion
• Guano — bird droppings contain phosphoric acid, which attacks rock surfaces

Key Definition: Weathering is the in situ breakdown of rock by mechanical, chemical or biological processes without the involvement of transport. It differs from erosion, which involves both breakdown and removal of material.

The relative importance of the three weathering families varies with climate and rock type, and noting this variation demonstrates analytical control rather than rote listing. In the cool, wet, temperate climate of the UK coast, mechanical weathering is dominant where freeze-thaw can operate (chalk and limestone cliffs in winter) and salt crystallisation is especially aggressive in the splash zone of porous rocks, while chemical weathering (carbonation) is significant but slower, dissolving carbonate rocks over longer timescales. In hot climates, by contrast, chemical weathering accelerates (reaction rates roughly double with each ~10 °C rise) and freeze-thaw becomes negligible. Biological weathering — root prising, boring organisms, algal bioerosion — operates everywhere but is rarely the leading process. Crucially, the families reinforce one another: salt crystallisation opens micro-cracks that admit water for freeze-thaw and acids for carbonation, so the total weathering effect exceeds the sum of the parts. A coast's weathering regime is therefore the product of its climate and its lithology acting together — the interacting-controls principle once again.

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Mass Movement

Mass movement is the downslope transfer of material under the influence of gravity. It is a critical process in cliff retreat, as it delivers weathered material from the cliff face to the beach, where it is removed by wave action.

Types of Mass Movement

graph TD
    A["Mass Movement Types"] --> B["Rockfall"]
    A --> C["Landslide"]
    A --> D["Rotational Slump"]
    A --> E["Mudflow"]
    A --> F["Soil Creep"]
    B --> B1["Rapid, free-fall of rock fragments from steep cliff"]
    C --> C1["Rapid sliding of rock mass along a planar surface"]
    D --> D1["Curved failure plane; block rotates backward"]
    E --> E1["Saturated material flows downslope"]
    F --> F1["Very slow, imperceptible downslope movement"]

Type	Speed	Material	Failure Plane	Coastal Example
Rockfall	Very rapid	Rock blocks/fragments	Free fall from vertical/near-vertical cliff	Beachy Head chalk cliff falls, East Sussex
Landslide	Rapid	Coherent rock mass	Planar — along bedding plane or joint	Black Ven landslide complex, Dorset
Rotational slump	Moderate	Saturated clay/soft rock	Curved — material rotates backward	Holbeck Hall, Scarborough (1993)
Mudflow	Moderate to rapid	Saturated fine sediment	No distinct plane — viscous flow	Barton-on-Sea, Hampshire
Soil creep	Very slow (< 1 cm/year)	Surface soil and regolith	No distinct plane — continuous slow movement	Widespread on vegetated cliff tops

Case Study: Holbeck Hall, Scarborough (1993)

The Holbeck Hall landslide of June 1993 is one of the most dramatic examples of rotational slumping in the UK:

• The four-star Holbeck Hall Hotel, perched on a cliff overlooking South Bay, Scarborough, was destroyed when a massive rotational slump carried approximately 1 million tonnes of glacial till and boulder clay downslope
• The landslide occurred over four days (3-7 June 1993) and extended the coastline seaward by 135 m
• The failure was triggered by a combination of factors: prolonged dry weather followed by heavy rainfall, marine erosion at the cliff base removing toe support, and the weight of the hotel itself
• The event demonstrated the interaction between marine and sub-aerial processes in cliff retreat
• Scarborough Borough Council was subsequently sued by the hotel owners, resulting in a landmark legal ruling on coastal management liability

The mechanics of slope failure: pore-water pressure

Understanding why saturated cliffs fail is what lifts a mass-movement answer into the top band. A slope remains stable while the shear strength of the material (its resistance to sliding, derived from friction and cohesion) exceeds the shear stress acting on a potential failure plane (the downslope component of gravity). Water destabilises a slope through two mechanisms. First, it adds mass, increasing the shear stress. Second, and more importantly in clays, rising pore-water pressure pushes apart the grains on the failure plane, reducing the friction between them and so lowering the shear strength. When prolonged rainfall raises pore-water pressure enough that shear stress finally exceeds the now-reduced shear strength, the slope fails — often suddenly, as a rotational slump along a curved plane. This is exactly the Holbeck Hall sequence: drought had desiccated and cracked the boulder clay, heavy rain then infiltrated the cracks and saturated the mass, pore-water pressure rose, marine erosion had already removed the supporting toe, and the slope failed catastrophically. The same mechanism explains why soft-cliff failures cluster in wet winters rather than in storms alone — the trigger is frequently hydrological, not directly marine.

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Factors Affecting Rates of Erosion

The rate of coastal erosion varies enormously — from virtually zero on resistant granite coastlines to over 10 m/year on some soft cliff coasts. The key factors are:

Rock Type and Resistance