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Unlock This CourseMechanical weathering is one of the most important processes shaping our coastlines. Unlike chemical weathering, which changes the actual composition of rocks, mechanical weathering physically breaks rocks apart without changing what they're made of. Think of it like smashing a biscuit - the pieces are still biscuit, just smaller!
At the coast, mechanical weathering works alongside waves, tides and other forces to gradually wear away cliffs and rocky shores. This process creates the dramatic landscapes we see around Britain's coastline, from the white cliffs of Dover to the rugged shores of Scotland.
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Coastal areas experience extreme conditions that make mechanical weathering very effective. The combination of saltwater, temperature changes and constant moisture creates the perfect storm for breaking down rocks. Add in the pounding of waves and you've got a recipe for rapid coastal erosion.
There are several different types of mechanical weathering that affect coastal areas. Each works in a slightly different way, but they all have the same result - breaking rocks into smaller pieces.
This is probably the most powerful type of mechanical weathering in coastal areas, especially in places like Scotland and northern England where temperatures regularly drop below freezing.
Here's how it works: Water gets into cracks in the rock during the day when it's warmer. At night, when temperatures drop below 0°C, this water freezes and expands by about 9%. This expansion puts enormous pressure on the rock - imagine trying to force a cork into a bottle that's too small!
When the ice melts the next day, the pressure is released, but the crack has been made slightly bigger. This process repeats over and over, gradually widening the crack until eventually a piece of rock breaks off completely.
Rainwater or sea spray gets into small cracks and joints in the rock face.
Temperature drops below 0°C and water freezes, expanding by 9% and putting pressure on rock.
Ice melts, pressure releases, but crack is now wider. Process repeats until rock breaks.
The granite cliffs along Scotland's northeast coast show excellent examples of freeze-thaw weathering. The combination of harsh winters and exposed rock faces means this process is particularly active. Local climbers often report finding fresh rock falls after particularly cold spells, showing how quickly this weathering can work.
This type of weathering is especially important at the coast because of all the saltwater around. It's sometimes called salt weathering or haloclasty (though you don't need to remember that fancy name!).
Salt crystallisation happens when saltwater gets into cracks in rocks and then evaporates. As the water evaporates, it leaves behind salt crystals. These crystals grow and put pressure on the surrounding rock, just like ice does in freeze-thaw weathering.
The process is most effective in areas where there are lots of wet and dry cycles - like the splash zone where waves regularly wet the rocks, followed by periods when they dry out in the sun and wind.
Salt crystallisation is most effective in the splash zone - the area just above high tide where waves regularly spray saltwater onto the rocks. This area gets wet and dry repeatedly, perfect for salt crystal formation.
Rocks expand when they heat up and contract when they cool down. At the coast, this happens every day as temperatures change between day and night and also with the seasons.
Different minerals in rocks expand and contract at different rates. This means that as a rock heats up and cools down, different parts of it are trying to change size by different amounts. This creates stress within the rock that can eventually cause it to crack and break apart.
Dark-coloured rocks are particularly affected because they absorb more heat from the sun. This is why you might notice that black rocks feel much hotter than white ones on a sunny day.
This type of weathering happens when rocks that were formed under pressure deep underground are exposed at the surface. It's particularly important where glaciers have carved out coastal valleys and then melted away.
When the pressure is removed, the rock expands slightly and cracks form parallel to the surface. These cracks make it easier for other types of weathering to get to work, speeding up the whole process.
You can see good examples of this in places like the Scottish Highlands, where glaciers carved deep valleys that are now filled by the sea (called fjords).
Several factors control how quickly mechanical weathering happens at the coast. Understanding these helps explain why some coastlines change quickly while others seem to stay the same for years.
Areas with lots of freeze-thaw cycles or wet-dry cycles experience faster weathering. Scotland's coast weathers faster than Cornwall's because of more freezing.
Rocks with lots of joints and cracks weather faster. Granite weathers slowly, while limestone with lots of cracks weathers much more quickly.
Cliffs facing the prevailing wind and waves experience more weathering. South-west facing coasts in the UK get more weathering than sheltered east-facing ones.
This chalk headland shows excellent examples of mechanical weathering in action. The chalk is relatively soft and has many natural joints, making it vulnerable to freeze-thaw weathering. The exposed position means it faces the full force of North Sea storms. Local records show the cliff retreating by an average of 2cm per year, with most retreat happening during winter months when freeze-thaw is most active.
Mechanical weathering doesn't just break rocks - it creates the raw materials for many other coastal processes and landforms.
When mechanical weathering breaks pieces off cliff faces, these pieces don't just disappear. They collect at the bottom of the cliff, forming slopes of broken rock called scree slopes or talus. You can see these at the base of many sea cliffs around Britain.
These loose rocks are then available for waves to pick up and use as weapons against the cliff face - a process called corrasion or abrasion.
By creating cracks and weakening rock faces, mechanical weathering makes cliffs more vulnerable to other forms of erosion. A cliff that's been weakened by freeze-thaw weathering will collapse much more easily when hit by storm waves.
Human activities can speed up mechanical weathering. Pollution can make rainwater more acidic, which helps water get into rock cracks. Coastal development can also change drainage patterns, affecting how much water reaches cliff faces.
It's important to understand that mechanical weathering doesn't work alone. It's part of a system of processes that all work together to shape our coastlines.
While mechanical weathering breaks rocks apart, marine erosion (the action of waves) removes the broken material. Chemical weathering changes the composition of rocks, making them weaker and more vulnerable to mechanical breakdown. Mass movement processes like landslides and rockfalls move the weathered material down to the sea.
Understanding how all these processes work together helps us predict how coastlines will change in the future - something that's becoming increasingly important as sea levels rise due to climate change.
This rapidly eroding coastline shows how mechanical weathering works alongside other processes. The boulder clay cliffs are particularly vulnerable to freeze-thaw weathering, which creates cracks that fill with water. When storm waves hit these weakened cliffs, large sections collapse. The coast here retreats by an average of 1.8 metres per year - one of the fastest rates in Europe.