Weathering is the breakdown of rocks where they are.
Erosion is when the rocks are broken down and carried away by something e.g. seawater.
Mechanical weathering: breakdown of rock without changing its chemical composition. There’s one main type affecting coasts = freeze-thaw weathering.
Happens when temperature alternates above and below 0 degrees (freezing point of water)
Water goes into rock that has cracks e.g. granite
When the water freezes it expands, putting pressure on the rocks
When the water thaws it contracts, releases pressure on the rock
Repeated freezing and expansions widens the cracks causing the rock to break up
Chemical weathering: breakdown of rock by changing its chemical composition. Carbonation weathering is a type of chemical weathering that happens in warm and wet conditions.
Rainwater has co2 dissolved in it, which makes it a weak carbonic acid
Carbonic acid reacts with rock containing calcium carbonate e.g. carboniferous limestone – so the rocks are dissolved by the rainwater.
Mass movement is when material shifts down a slope as one.
It is the shifting of rocks and loose material down a slope e.g. a cliff – it happens when the force of gravity acting on a slope id greater than the force supporting it
Causes coasts to retreat rapidly
More likely to happen when the material is full of water- it acts as a lubricant
2 types of mass movement: slides= happens when loosened rocks and soil suddenly tumble in a straight line down the slope but slumps= when rock is saturated with water and so material shifts with a rotation
Four processes of erosion
Hydraulic power- waves crash against rock and compress the air in the cracks. This puts pressure on the rock. Repeated compression widens the cracks and makes the bits of rock break off.
Abrasion- eroded particles in the water scrape and rub against rock, removing small pieces
Attrition- eroded particles in the water smash into each other and break into smaller fragments. Their edges also get rounded off as they rub together.
Solution- seawater dissolves rock like chalk and limestone because it is a weak carbonic acid.
Break as they approach shallow water. Swash is a wave surging up the beach but backwash is a wave washing back down a beach.
Destructive- remove beach material, they form in storm conditions with high winds, the waves are high relative to length, backwash is stronger than swash and they have a frequency of 11-15 per minute.
Constructive- deposit beach material, form in calm conditions with light winds, long relative to height, swash is stronger than backwash and they have a frequency of 6-9 per minute.
Transportation and deposition
Traction- boulders and cobbles are rolled along by powerful waves
Saltation- pebbles and sand are bounced around
Suspension- silt and clay are suspended in moving sea water
Solution- minerals are dissolved in sea water
Beach material comes from eroded cliffs, offshore sediment banks and river bedload. Where material is deposited by waves it might be sorted along or down a beach in order of size: boulders, cobbles, pebbles, sand, silt and clay.
The movement of beach material = longshore drift.
Waves cause most erosion at the foot of a cliff
This forms a wave-cut notch which 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.
Repeated collapsing results in the cliff retreating.
A wave-cut platform is the platform that’s left behind as the cliff retreats.
Beaches are often found in bays because they are more sheltered and there is a gentle gradient offshore, so waves have less energy and are more likely to deposit their load. Also when bays are curved, waves are refracted and this slows them down.
Deposition forms beaches
When material being carried by seawater is dropped on the coast
Coasts are built up when the…