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.
Biological weathering is the breakdown of rocks by plants and animals e.g. plant roots growing into cracks in the rock
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
There are four types of erosional processes:
Abrasion: eroded particles in the water scrape and rub against rock (either cliffs or river bed/banks), 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.
Erosional processes animation
Water transports material in four different ways.
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 is when the water ‘drops’ the sediment which it is carrying. This happens when water lacks sufficient energy to transport the load it is carrying. Deposition is encouraged by several factors:
- carrying a large load, providing more material to deposit
- a reduction in velocity e.g. inside of a meander, on entering lake/sea
- a fall in the volume of water e.g. at times of low flow during a period of drought, shallow water