Weathering (Mechanical & Chemical)

Mechanical Weathering

Mechanical weathering breaks down rocks physically without changing their chemical composition. It is important in shaping coastal landscapes by weakening rocks, making them more vulnerable to erosion.

Freeze-thaw Process

This occurs when water enters cracks in rocks and freezes. Water expands by about 9% when it freezes, exerting pressure on the rock. Repeated freezing and thawing causes the cracks to widen and eventually pieces of rock break off.

This process is common in coastal areas of the UK during winter, especially where temperatures fluctuate around $0^\circ\text{C}$.

For instance, if water freezes in a crack and expands by 9%, the pressure exerted can be enough to fracture the rock over many cycles.

Salt Crystallisation

Salt crystallisation happens when salty seawater enters cracks in rocks. As the water evaporates, salt crystals form and grow. These crystals exert pressure on the rock, causing it to break apart.

This is especially common in coastal areas where waves splash saltwater onto rocks, such as the chalk cliffs of Dover.

Biological Weathering Effects

Living organisms contribute to mechanical weathering:

• Plant roots: Roots grow into cracks in rocks, forcing them apart as they expand.
• Burrowing animals: Creatures like rabbits or crabs dig into cliffs, loosening rock fragments.
• Human activity: Walking or construction can also cause physical breakdown.

Chemical Weathering

Chemical weathering changes the chemical composition of rocks, weakening them and making them easier to erode. It is important in coastal landscapes, especially where rocks like limestone and chalk are exposed.

Carbonation

Carbonation occurs when rainwater absorbs carbon dioxide from the atmosphere, forming a weak carbonic acid:

$$\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3$$

This acid reacts with calcium carbonate (a mineral found in rocks like limestone and chalk) dissolving them slowly:

$$\text{CaCO}_3 + \text{H}_2\text{CO}_3 \rightarrow \text{Ca}^{2+} + 2\text{HCO}_3^{-}$$

This process causes rocks to weaken and form features such as cracks and caves.

Oxidation

Oxidation happens when oxygen reacts with minerals in rocks, especially those containing iron. This reaction forms iron oxides (rust), which weakens the rock and makes it crumbly.

For example, iron-rich rocks like some sandstones can turn reddish and break down more easily due to oxidation.

Hydrolysis

Hydrolysis is a chemical reaction between minerals in rock and water, producing new minerals and soluble salts. It commonly affects silicate minerals in rocks like granite, causing them to break down into clay minerals.

This weakens the rock structure and makes it more prone to erosion.

Weathering Effects on Coastal Landscapes

Weathering plays a vital role in shaping coastal landscapes by breaking down rocks and creating sediment. This sediment can then be transported by coastal processes like waves and longshore drift.

Rock Breakdown

Mechanical weathering causes rocks to fracture and break into smaller pieces, while chemical weathering weakens rock minerals. Together, these processes make cliffs more unstable and prone to collapse.

Formation of Sediment

Weathering produces sediment of various sizes, from large rock fragments to fine sand and clay. This sediment is essential for forming beaches and other depositional landforms.

Influence on Coastal Erosion

Weathered rock is easier for waves to erode. For example, cliffs weakened by freeze-thaw or carbonation are more likely to be worn away by hydraulic action and abrasion.

Therefore, weathering indirectly accelerates coastal erosion and influences the shape of the coastline.

For instance, chalk cliffs along the southern coast of England often show cracks widened by freeze-thaw weathering, making them vulnerable to collapse during storms.