Coastal Systems: How Weathering Affects the Coastline

As well as wave energy, other geomorphological processes can modify the coastline. Chemical, mechanical and biological weathering loosen rocks, in advance of their removal by waves and mass movement also provides loose material.

Mechanical Weathering

There are two main types of mechanical weathering which occur at the coast.

Freeze-thaw weathering occurs when the daytime temperature is different than the night-time temperature either side of freezing point on rock faces.

During the day, water enters cracks in the rock and at night when the temperature drops the water expands as it freezes. The increase in the volume of ice exerts pressure on fissures in the rock and weakens it internally. During the day, when the temperature rises again, the ice melts. The process re-occurs repeatedly until the rock breaks apart. The material is then loose to fall onto the shore.

The other type of mechanical weathering is exfoliation. This occurs when the daytime and night-time temperatures are distinctively different on dry rock. During the day, the sun heats up the surface of the rock and at night the outer layers cool down. This repeated expansion and contraction of the surface layers results in the gradual disintegration of the top layers.

Biological Weathering

This type of weathering occurs when plants or animals break down the rock.

On cliff faces, both animal and plant action can loosen large amounts of rock material. Trees and plant roots can prise apart rocks by growing in between the bedding planes and joints enlarging them as they increase in size. Equally biological damaging is when small animals burrow underground and loosen rocks such as sand martins. Microorganisms in the soil are also able to break down the rocks; particularly carbonate rocks.

Below high tide level, seaweed will often attach itself to rocks and when waves crash on the shore, seaweed will be subject to powerful currents and forces that will pull pieces of the rock away with it.

Chemical Weathering

The chemistry that makes up coastal rocks can be affected by rain and sea water leading to the gradual disintegration of solid rocks.

Hydrolysis occurs in certain rocks with minerals that are reactive to water or chemicals in the water. This is where particular minerals break down due to their contact with water leading to loss of rock coherence and structure.

Oxidation can also occur on cliff faces and shoreline rocks where iron elements are present within rocks. The ‘rusting’ of the iron content may cause rocks to disintegrate.