Coastal Landscapes » Waves, Weathering and Mass Movement

Coasts: an optional topic

In the Physical Landscapes in the UK section of AQA GCSE Geography you do not study everything. You study two out of three topics: coastal landscapes, river landscapes and glacial landscapes. Check with your teacher which two your class is doing. This lesson starts the coasts topic.

The coast is shaped by a number of physical processes. Waves are the energy behind many of them, so we begin there.

Waves and fetch

Waves are made by wind blowing over the surface of the sea. The stronger the wind, the longer it blows and the further it blows, the bigger the waves. The distance of open water that the wind blows across is called the fetch. A long fetch means a lot of energy builds up in the waves before they reach the shore.

The UK faces the Atlantic Ocean to the south west. That is a very long fetch, so the south west coast gets large, powerful waves. A coast facing a short stretch of sea gets smaller waves.

Key terms:

• Wave: a movement of energy through the water, driven by the wind.
• Fetch: the distance of open water the wind blows across to make a wave.
• Swash: the movement of water up the beach when a wave breaks.
• Backwash: the movement of water back down the beach.

Two types of wave

Waves do not all behave in the same way. Geographers split them into two types: constructive and destructive.

Weathering

Weathering is the breakdown of rock where it is, without the rock being moved away. Changes in temperature and the chemistry of rainwater do it. It is not the same as erosion, which you will meet in the next lesson. On the coast, cliffs are weakened by two types of weathering.

Key terms:

• Weathering: the breakdown of rock in place, with no movement involved.
• Mechanical weathering: rock is broken into smaller pieces without its chemistry changing.
• Chemical weathering: rock is broken down or dissolved by a chemical reaction.

Mechanical weathering: freeze-thaw

Freeze-thaw happens when temperatures swing above and below 0°C. It works in five steps:

1. Rain water runs into cracks in the cliff.
2. At night the water freezes and turns to ice.
3. Ice takes up more space than water, so it expands and pushes the crack wider.
4. In the day the ice melts and the water sinks deeper into the crack.
5. This repeats again and again until pieces of rock break off.

Chemical weathering: carbonation

Rain water is slightly acidic because carbon dioxide from the air dissolves in it. Some rocks, such as limestone and chalk, are made of calcium carbonate. The weak acid reacts with the calcium carbonate and dissolves it. This is called carbonation. Over time the rock is weakened and small amounts are washed away. It works best on cliffs made of limestone or chalk.

Mass movement

Mass movement is the downhill movement of material under the pull of gravity. Weathering weakens a cliff, then gravity does the rest. Mass movement is most likely when rain soaks into the cliff. The water makes the cliff heavier and also makes the surface slippery, so material slides more easily. Waves that cut away the bottom of the cliff make it even less stable.

Key terms:

• Mass movement: the downhill movement of rock, soil and mud under gravity.
• Sliding: blocks of rock move in a straight line down a flat surface.
• Slumping: material moves down a curved surface, with a rotating movement.
• Rock fall: pieces of rock break off a steep cliff face and fall.

Exam-style question

(a) Describe one difference between constructive and destructive waves. [2 marks]

(b) Explain how freeze-thaw weathering can lead to a rock fall on a cliff. [4 marks]