Sign up to access the complete lesson and track your progress!
Unlock This CourseCoastal erosion is the wearing away of land and rocks along the coastline by the sea. It's a natural process that has been shaping our coasts for millions of years, creating dramatic cliffs, caves and other landforms. Understanding these processes is crucial because they affect millions of people living near the coast and can cause significant economic damage.
Coastal erosion happens through four main processes that work together to break down and remove coastal materials. These processes are constantly at work, though they speed up during storms and rough weather.
Key Definitions:
Waves are the main driving force behind coastal erosion. The bigger the wave, the more energy it has to erode the coast. Storm waves can be incredibly powerful - a single large wave can exert pressure of up to 30 tonnes per square metre!
Marine erosion refers to the processes carried out directly by the sea. These are the most important erosion processes along most coastlines and include hydraulic action, abrasion, attrition and solution.
This is the sheer force of waves hitting the coast. When waves crash against cliffs, they compress air in rock cracks. As the wave retreats, this compressed air expands rapidly, creating explosive pressure that can split rocks apart. It's like nature's own pneumatic drill!
Direct force of waves hitting rock faces can create pressures of several tonnes per square metre.
Air trapped in rock cracks gets compressed by incoming waves, then expands explosively.
Repeated pressure cycles gradually widen cracks until rock fragments break away.
This process involves waves picking up sand, pebbles and rocks and hurling them against the coast. It's like natural sandblasting - the constant bombardment of rock particles wears away the cliff face. Beach material acts like tools in the hands of the waves.
Abrasion is most effective during storms when waves have more energy to pick up larger rocks and throw them with greater force. The process creates distinctive features like wave-cut notches at the base of cliffs.
As rocks and pebbles are moved around by waves, they knock into each other and gradually become smaller and more rounded. Sharp edges get worn away and large rocks eventually become smooth pebbles, then sand. This process doesn't directly erode the coastline but it creates the tools that waves use for abrasion.
Some rocks, particularly limestone and chalk, can be dissolved by seawater. The slightly acidic nature of seawater (especially when it contains dissolved carbon dioxide) can chemically break down certain minerals in rocks. This process is slow but continuous.
The Holderness Coast is Europe's fastest-eroding coastline, losing about 2 metres per year. The soft boulder clay cliffs are particularly vulnerable to hydraulic action and abrasion. The village of Happisburgh has lost over 250 metres of land since 1600, with several streets disappearing into the sea. Storm surges make erosion even worse - during the 2013 storm surge, some areas lost 5 metres of cliff in just one night.
Weathering weakens rocks before marine erosion removes them. It's like softening up the target before the main attack. Coastal rocks face both physical and chemical weathering in the harsh seaside environment.
Physical weathering breaks rocks apart without changing their chemical composition. The most important type at the coast is freeze-thaw weathering.
Water gets into rock cracks and freezes overnight. Ice takes up about 9% more space than water, so it expands and widens the cracks. When it melts, more water can get in and the cycle repeats until the rock splits apart.
Salt weathering is another important process. Salt crystals grow in rock cracks as seawater evaporates, creating pressure that can split rocks apart. This is particularly effective in hot, dry climates.
Chemical weathering changes the chemical composition of rocks, making them weaker and more easily eroded. The main types affecting coastal rocks are:
Once rocks have been weakened by weathering, gravity can cause them to move downhill. This is called mass movement and it's an important way that weathered material reaches the sea where it can be eroded away.
The type of mass movement depends on the rock type, slope angle and water content. Each type moves at different speeds and affects the coastline differently.
Individual rocks break away from cliff faces and fall freely. Common on steep, well-jointed cliffs like chalk and limestone.
Large sections of cliff slide down along a slip plane. Often triggered by heavy rain that lubricates rock layers.
Saturated soil and rock debris flows downhill like thick liquid. Common on clay cliffs after heavy rainfall.
Several factors make mass movement more likely:
Lyme Regis sits on unstable cliffs made of different rock layers. The underlying clay becomes slippery when wet, causing the harder limestone above to slide. In 2008, a major landslide destroyed part of the coastal path and threatened homes. The town has spent millions on sea defences and cliff stabilisation, including drainage systems to remove water from the cliff and rock bolts to hold unstable sections together.
Not all coastlines erode at the same rate. Several factors determine how quickly erosion happens and understanding these helps explain why some coasts change rapidly while others remain stable for centuries.
The geology of the coast is the most important factor affecting erosion rates. Hard rocks like granite resist erosion much better than soft rocks like clay.
Granite, basalt and some sandstones are hard and resist erosion. They form rugged, slowly-changing coastlines with dramatic cliffs and rocky shores.
Rock structure also matters. Rocks with many joints and cracks erode faster because waves can exploit these weaknesses. Rocks that dip towards the sea are more unstable than those dipping inland.
Coasts facing prevailing winds and storms experience more erosion. The fetch (distance over which wind blows) affects wave size - longer fetch means bigger waves and more erosion.
Seasonal variations are important too. Winter storms cause most erosion, while summer conditions allow beaches to build up and protect cliffs. Climate change is increasing storm intensity in many areas, accelerating erosion rates.
Beaches act as natural shock absorbers, soaking up wave energy before it reaches cliffs. Wide beaches provide better protection than narrow ones. When beaches are eroded away, cliff erosion accelerates rapidly.
Human activities can affect erosion rates both positively and negatively. Sea defences can reduce erosion locally but may increase it elsewhere. Coastal management decisions have long-term consequences for erosion patterns.