coastal erosion

The erosion of the land by the constant battering of the sea, primarily by the processes of hydraulic action, corrasion, attrition, and corrosion. Hydraulic action occurs when the force of the waves compresses air pockets in coastal rocks and cliffs. The air expands explosively, breaking the rocks apart. It is also the force of the water on the cliff. During severe gales this can be as high as 6 tonnes/cm³ – the force of a bulldozer. Rocks and pebbles flung by waves against the cliff face wear it away by the process of corrasion, or abrasion as it is also known. Chalk and limestone coasts are often broken down by solution (also called corrosion). Attrition is the process by which the eroded rock particles themselves are worn down, becoming smaller and more rounded.

Frost shattering (or freeze–thaw), caused by the expansion of frozen water in cracks, and biological weathering, caused by the burrowing of rock-boring molluscs and plants, also lead to the breakdown of rock.

Where resistant rocks form headlands, the sea erodes the coast in successive stages. First it exploits weaknesses such as faults and cracks to form caves. Then it gradually wears away the interior of the caves and enlarges them. In some cases the roofs may be broken through to form blowholes. In other cases the caves at either side of a headland may unite to form a natural arch. When the roof of the arch collapses, a stack is formed. This may be worn down further to produce a stump. There are good examples of stacks at The Needles, Isle of Wight, England.