Sub-Aerial Processes

Sub-aerial processes — weathering and mass movement — operate on the cliff face and cliff top from above, weakening rock and delivering material to the foreshore where it can be removed by waves. On many coasts, sub-aerial processes are at least as important as marine erosion in determining the rate and style of cliff retreat. This lesson provides comprehensive coverage of weathering types, mass movement mechanisms, and the role of vegetation and groundwater in slope stability, directly addressing Edexcel A-Level Geography Enquiry Question 1.

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Weathering

Weathering is the in situ breakdown of rock — the material is broken down where it sits, without being transported. Weathering weakens rock and prepares it for erosion and mass movement. Three categories of weathering operate on coastal cliffs:

Mechanical (Physical) Weathering

Mechanical weathering breaks rock into smaller fragments without changing its chemical composition.

Freeze-Thaw (Frost Shattering)

Water enters cracks, joints and pore spaces in the rock. When the temperature drops below 0°C, the water freezes and expands by approximately 9% in volume. This expansion exerts pressure on the surrounding rock (up to 2,100 kPa in ideal conditions). When the ice melts, the crack has been widened. Repeated freeze-thaw cycles progressively shatter the rock.

Freeze-thaw is most effective where:

• Temperatures regularly oscillate around 0°C (common on UK coastal cliffs during winter)
• Rock has high primary or secondary permeability (many joints, pores or bedding planes)
• Water supply is abundant (rainfall, sea spray, snowmelt)

On chalk cliffs (e.g., Seven Sisters, Sussex), freeze-thaw widens joints and bedding planes, creating blocks that eventually fall away as rockfalls.

Salt Crystallisation (Haloclasty)

Seawater is absorbed into porous rock in the splash zone (the area above the high-tide mark that receives salt spray). As the water evaporates, salt crystals grow within the pore spaces. Crystal growth exerts pressure on the pore walls, causing granular disintegration — the surface of the rock crumbles into individual grains.

Salt crystallisation is most effective on:

• Porous rocks (sandstone, some limestones)
• In the splash zone where wetting and drying cycles are frequent
• During warm, dry weather when evaporation is rapid

The process can also operate in the intertidal zone, where repeated tidal wetting and drying provides the necessary cycles.

Wetting and Drying (Slaking)

Clay minerals in shale, mudstone and glacial till absorb water and swell when wet. When they dry out, they shrink and crack. Repeated wetting and drying cycles progressively weaken the rock, causing the surface to flake and crumble. This process is particularly important on the Holderness coast, where the glacial till cliffs contain significant amounts of clay. The wetting-drying cycle contributes to the instability that makes these cliffs so vulnerable to collapse.

Mechanical Weathering Type	Mechanism	Most Effective On	Coastal Relevance
Freeze-thaw	Water freezes and expands in cracks	Jointed rock; temperatures oscillate around 0°C	Rockfalls on chalk and limestone cliffs
Salt crystallisation	Salt crystals grow in pores	Porous rock in splash zone	Surface crumbling on sandstone
Wetting and drying	Clay minerals swell and shrink	Clay-rich rocks (shale, till)	Cliff instability on Holderness

Chemical Weathering

Chemical weathering alters the chemical composition of minerals, producing new, weaker minerals or dissolved products.

Carbonation

Carbon dioxide dissolves in rainwater to form weak carbonic acid (H₂CO₃). This acid reacts with calcium carbonate in limestone and chalk:

CaCO₃ + H₂CO₃ → Ca(HCO₃)₂

The product — calcium bicarbonate — is soluble and is carried away in solution. Carbonation widens joints, bedding planes and fractures, weakening the rock structure and making it more vulnerable to mechanical erosion and mass movement.

Carbonation is enhanced in coastal settings because:

• Seawater naturally contains dissolved CO₂
• Cooler water holds more CO₂ than warm water (UK coasts have relatively cool sea temperatures)
• Biological activity on rocky shores produces additional CO₂ and organic acids

Oxidation

Oxidation occurs when minerals containing iron react with oxygen (in air and water), producing iron oxides (rust). This process weakens the mineral structure and causes discolouration (the characteristic orange-brown staining on coastal rocks). Oxidation is most important in rocks containing iron-bearing minerals such as pyrite (iron sulphide) — when pyrite oxidises, it produces sulphuric acid, which further accelerates chemical weathering.

Hydrolysis

Hydrolysis is the reaction between minerals and water itself (H₂O). Hydrogen ions (H⁺) from water replace metal cations (e.g., K⁺, Na⁺, Ca²⁺) in the crystal structure of silicate minerals, producing clay minerals. Feldspar in granite, for example, is converted to kaolinite (a clay mineral) through hydrolysis:

2KAlSi₃O₈ + 2H₂CO₃ + 9H₂O → Al₂Si₂O₅(OH)₄ + 4H₄SiO₄ + 2K⁺ + 2HCO₃⁻

This process weakens granite over long timescales, contributing to the breakdown of cliff faces on granitic coasts.

graph TD
    A["Chemical Weathering"] --> B["Carbonation<br/>CO₂ + water dissolves<br/>CaCO₃ in limestone/chalk"]
    A --> C["Oxidation<br/>Iron minerals react<br/>with O₂ → rust/weakness"]
    A --> D["Hydrolysis<br/>Water reacts with<br/>silicate minerals → clay"]
    B --> E["Widens joints, bedding planes<br/>Weakens cliff structure"]
    C --> F["Discolouration, mineral<br/>decomposition"]
    D --> G["Converts strong minerals<br/>to weak clay minerals"]

Biological Weathering

Biological weathering involves the physical or chemical breakdown of rock by living organisms: