The coast is shaped by weathering, mass movement, erosion, transportation and deposition.
BBC Bitesize coastal processes video
Weathering
Mechanical weathering is the breakdown of rock without changing its chemical make-up. The main type of mechanical weathering that affects coasts is freeze-thaw weathering as outlined below.
Chemical weathering is the breakdown of rock by changing its chemical composition. Carbonation commonly happens in coastal environments. Rainwater has carbon dioxide dissolved in it which makes it a weak carbonic acid. This then reacts with rocks that contain calcium carbonate (e.g. limestone) and the rainwater makes the rock dissolve.
Mass movement
Mass movement is the downslope movement of rocks and loose material. It happens when the force of gravity on a slope is greater than the force supporting it. Mass movement can causes coasts to retreat rapidly.
Slides – material moves downslope along a straight line
Slumps – material moves downslope in a rotational manner
Erosion
There are four types of erosional processes on a coast:
Abrasion: eroded particles in the water scrape and rub against rock, removing small pieces
Attrition: rocks and pebbles being carried by the water crash against each other, wearing them down to become smaller, rounded pebbles.
Solution: chemical erosion of the rocks by the slightly acidic water especially, chalk and limestone.
Hydraulic Action: water forces air to be trapped and pressured into cracks in the rocks. This constant pressure eventually causes the rocks to crack and break apart.
Transportation
Waves transport material in four different ways (the same way as rivers do).
The amount of load being carried depends on:
- the volume of water – the greater the volume, the more load it can carry
- the velocity – fast-flowing water has more energy to transport and can move larger particles
- the local rock types – some rocks (e.g. shales) are more easily eroded than others (e.g. granite)
Waves transport material along the coast by longshore drift.
Deposition
Deposition is when the waves ‘drops’ the sediment which it is carrying. This happens when waves lack sufficient energy to transport the load it is carryingThe amount of material that’s deposited on an area of coast is increased when:
- there’s lots of erosion elsewhere on the coast so there’s lots of material being carried
- there’s lots of transportation of material into an area
Types of waves
There are two main types of wave:
Constructive waves deposit more material than they erode. Destructive waves are those that carry out erosional processes.
