Weathering, Mass Movement and Erosion

Before studying specific coastal and river landforms, it is essential to understand the processes that break down and move rock material. Weathering, mass movement, and erosion work together to shape every landscape in the UK. This lesson examines each group of processes in detail.

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What Is Weathering?

Weathering is the breakdown of rocks in situ — that is, the rock breaks apart where it is, without being transported away. Weathering weakens rock and produces loose material that can then be moved by erosion or mass movement.

There are three types of weathering: mechanical (physical), chemical, and biological.

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Mechanical (Physical) Weathering

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

Freeze-Thaw Weathering

This is the most important type of mechanical weathering in the UK, particularly in upland areas.

How it works:

graph LR
    A["Water enters cracks<br/>in the rock"] --> B["Temperature drops<br/>below 0°C"]
    B --> C["Water freezes and<br/>expands by ~9%"]
    C --> D["Ice exerts pressure<br/>on crack walls"]
    D --> E["Repeated cycles widen<br/>the crack"]
    E --> F["Rock eventually<br/>shatters into fragments"]
    F --> G["Angular scree<br/>collects at the base"]

1. Water enters cracks, joints, or pores in the rock surface.
2. When the temperature drops below 0°C, the water freezes. Ice occupies approximately 9% more volume than liquid water.
3. The expansion exerts enormous pressure on the surrounding rock — up to 2,100 kg per cm².
4. When the temperature rises, the ice melts and water seeps deeper into the widened crack.
5. Repeated freeze-thaw cycles progressively widen the crack until pieces of rock break off.
6. The broken fragments collect at the base of slopes as scree (also called talus).

Freeze-thaw weathering is most effective where temperatures fluctuate around 0°C frequently — for example, in the Pennines, Lake District, and Scottish Highlands during winter and spring.

Other Types of Mechanical Weathering

Type	Process	Where It Occurs
Thermal expansion	Rock expands when heated and contracts when cooled; repeated cycles cause surface layers to peel off (exfoliation or onion-skin weathering)	More common in hot climates, but occurs in the UK during summer heatwaves on south-facing rock faces
Salt crystallisation	Seawater enters cracks; when it evaporates, salt crystals grow and exert pressure, shattering the rock	Common on UK coastlines, especially in porous rocks like sandstone
Wetting and drying	Certain clay-rich rocks expand when wet and contract when dry, causing them to crack and crumble	Clay cliffs on the east coast of England (e.g., Holderness)
Pressure release	When overlying rock or ice is removed, the rock beneath expands and cracks parallel to the surface	Upland areas following glacial retreat

Exam Tip: In Edexcel B exams, freeze-thaw weathering is the mechanical weathering type you are most likely to be asked about. Make sure you can describe the process step by step, including the detail that water expands by approximately 9% when it freezes.

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Chemical Weathering

Chemical weathering involves a chemical reaction that changes the mineral composition of the rock, weakening or dissolving it.

Carbonation

This is the most important type of chemical weathering in the UK because it affects limestone and chalk, which are widespread.

How it works:

1. Rainwater absorbs carbon dioxide (CO₂) from the atmosphere, forming a weak carbonic acid (H₂O + CO₂ → H₂CO₃).
2. This acid reacts with calcium carbonate (CaCO₃) in limestone or chalk.
3. The calcium carbonate dissolves to form calcium bicarbonate, which is soluble and is carried away in solution.
4. Over time, joints and bedding planes in the rock widen, creating features such as limestone pavements (clints and grykes), caves, potholes, and gorges.

The chemical equation: CaCO₃ + H₂CO₃ → Ca(HCO₃)₂

Other Types of Chemical Weathering

Type	Process	Effect
Oxidation	Iron-bearing minerals react with oxygen and water to form iron oxide (rust)	Rock becomes weaker and crumbly; visible as orange/brown discolouration on cliff faces
Hydrolysis	Minerals (especially feldspar in granite) react with water to form clay minerals	Granite decomposes into a crumbly material called kaolin (china clay) — commercially quarried in Cornwall
Acid rain	Polluted rainfall (containing sulphuric and nitric acids) accelerates chemical weathering	Particularly affects limestone buildings and monuments

Exam Tip: Chemical weathering is faster in warm, wet conditions because chemical reactions speed up at higher temperatures and require water. This is why tropical regions often have deeper weathered profiles than the UK.

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Biological Weathering

Biological weathering is caused by living organisms breaking down rock. It can involve both physical and chemical processes.

Mechanism	Description	Example
Root growth	Tree and plant roots grow into cracks in rock, gradually widening them as the roots thicken	Very common on cliff faces and in river gorges throughout the UK
Burrowing animals	Rabbits, worms, and other animals burrow into weathered rock and soil, loosening material	Widespread in chalk and clay landscapes
Organic acids	Decaying vegetation releases organic acids that dissolve minerals in the rock	Peat bogs in upland Britain accelerate chemical weathering of the underlying rock
Algae and lichens	Micro-organisms colonise rock surfaces and produce acids that slowly dissolve the minerals	Visible on almost every exposed rock surface in the UK

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Mass Movement

Mass movement is the downslope movement of material under the influence of gravity. Unlike erosion, mass movement does not require a transporting agent (such as water, wind, or ice) — gravity alone causes the material to move.

Mass movement is a major process shaping both coastal cliffs and river valley slopes.

Types of Mass Movement

graph TD
    A["Mass Movement"] --> B["Slow"]
    A --> C["Fast"]
    B --> D["Soil Creep"]
    B --> E["Solifluction"]
    C --> F["Landslide / Slide"]
    C --> G["Slump / Rotational Slip"]
    C --> H["Rockfall"]
    C --> I["Mudflow"]