Coastal Zones

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Mechanical and Chemical Weathering

Weathering is the breakdown of rocks where they are, but erosion is when rocks are broken down and carried away by something, e.g. Seawater.

Mechanical weathering is the breakdown of rock without changing its chemical composition; Freeze-thaw weathering is the main type that affects coasts. It happens when the temperature fluctuates around 0°C. Water gets into rocks with cracks (such as Granite). The water freezes and expands, putting pressure on the rock. Then the water melts, and trickles further into the crack before it freezes again. As this process repeats the cracks widen and cause the rock to break up.


Carbonation weathering happens in warm and wet areas – as rainwater is a weak carbonic acid due to carbon dioxide dissolved in it. Carbonic acid reacts with calcium carbonate to dissolve the rocks.

Mass Movement

 Mass movement is where lots of material slides down a slope or cliff at once, and happen when the force of gravity is stronger than the supporting force – they cause coasts to retreat very quickly.

They are more likely to happen when the material is full of water as it acts as a lubricant.

Slides occur when material shifts in a straight line.

Slumps occur when material shifts with a rotation.

Erosion of Waves

Hydraulic power is where waves crash against rock and compress air in cracks, putting pressure on them. Repeated compression weakens the rock further.

Abrasion is where eroded particles in the water scrape and rub against rock, removing small pieces.

Attrition is where smaller particles in the water erode each other as they collide, becoming smaller, smoother and rounder.

Solution is where weak carbonic acid in seawater dissolves rock such as chalk and limestone.

Destructive waves carry out erosion processes, as their strong backwash carried sediment away from the beach. They are high, steep, and more frequent than constructive waves (10-14 waves a minute).


 Wave Cut Platforms

Waves cause most erosion at the foot of a cliff, which forms a wave cut notch that is enlarged as erosion continues. The rock above the notch becomes unstable, and eventually collapses.

The collapsed material is washed away, and a new wave cut notch starts to form. The repeated collapsing leads to the cliff retreating, leaving behind only a wave cut platform.

Headlands and Bays

Some rocks are more resistant to erosion than others, so headlands and bays form where there are alternating bands of resistant and less resistant bands of rock along a coast.

The less resistant rock (e.g. Clay) forms a bay with gently sloping edges.

The resistant rock (e.g. Chalk) is eroded much slower and is left jutting out forming a headland.

Headland Erosion

Headlands are usually made of resistant rocks that have weaknesses such as cracks. Waves crash into the headland and enlarge the cracks, by hydraulic power and abrasion. Repeated erosion…


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