Geomorphological processes at the coast

In this section you will learn about:

• weathering processes operating at the coast
• mass movement and slope processes
• marine erosion processes
• transportation and deposition
• factors affecting rates of coastal erosion

Coastal landscapes are shaped by a combination of interacting geomorphological processes. Energy from waves, tides, currents and gravity drives the movement of sediment and the erosion of coastlines, while weathering weakens rocks and cliffs over time.

The main processes operating at the coast are:

• weathering
• mass movement
• erosion
• transportation
• deposition

These processes are interconnected and together produce distinctive coastal landforms and landscapes.

Weathering

Weathering is the breakdown or decomposition of rock in situ (without movement). Weathering weakens cliffs and rocky coastlines, making them more vulnerable to erosion and mass movement.

At the coast, weathering is particularly effective because rocks are exposed to:

• saltwater
• repeated wetting and drying
• strong winds
• biological activity
• changing temperatures

Weathering can be divided into:

• mechanical (physical) weathering
• chemical weathering
• biological weathering

Mechanical (physical) weathering

Mechanical weathering involves the physical breakdown of rock without changing its chemical composition.

Freeze–thaw weathering

Water enters cracks and joints in rocks. When temperatures fall below freezing, the water expands by around 9% as it turns to ice. This exerts pressure on the surrounding rock.

Repeated freezing and thawing widens cracks until pieces of rock break away.

Freeze–thaw weathering is most effective:

• in upland and temperate climates
• where temperatures fluctuate around 0°C
• on exposed cliff faces

The rock fragments produced may later be used by waves in erosion processes such as abrasion.

Salt crystallisation

Sea spray and seawater enter cracks within rocks. As the water evaporates, salt crystals form and expand, exerting pressure on the rock.

Salt weathering is especially important in coastal environments because of the constant supply of saline water.

Over time, repeated crystal growth weakens and breaks apart the rock surface.

Wetting and drying

Certain rocks, particularly clays such as shale, expand when wet and contract when dry. Repeated cycles weaken the rock structure and increase instability.

This process contributes to cliff weakening and rotational slumping.

Chemical weathering

Chemical weathering involves chemical reactions that alter or dissolve minerals within rocks.

Carbonation

Rainwater absorbs carbon dioxide from the atmosphere, forming weak carbonic acid.

This reacts with carbonate rocks such as:

• limestone
• chalk

The rock gradually dissolves, widening joints and bedding planes.

Carbonation is particularly important on chalk and limestone coastlines.

Oxidation

Oxidation occurs when minerals react with oxygen. Iron-rich rocks may develop a rust-like surface as minerals break down.

This weakens the rock over time.

Solution

Solution occurs when soluble minerals dissolve directly in seawater or rainwater.

Rock salt and some carbonate rocks are especially vulnerable to this process.

Biological weathering

Biological weathering is caused by plants and animals.

Examples include:

• plant roots widening cracks in rocks
• burrowing animals disturbing sediment and cliffs
• organisms producing weak acids that chemically attack rocks

Biological weathering often works together with mechanical and chemical weathering.

Mass movement

Mass movement is the downslope movement of weathered material under the influence of gravity.

It is particularly important where cliffs are steep or heavily weathered.

Marine erosion often undercuts cliffs, removing support and increasing instability.

Water also plays a major role because saturated material becomes heavier and less stable.

Factors influencing mass movement

The likelihood of mass movement depends on:

• rock type and geological structure
• slope angle
• vegetation cover
• water content
• wave erosion at the cliff base

Rockfall

Rockfalls occur when fragments of rock break away from steep cliff faces and fall under gravity.

They are common where:

• cliffs are heavily jointed
• freeze–thaw weathering is active
• waves undercut the cliff base

The fallen debris may accumulate as scree or talus at the foot of the cliff.

Rotational slumping (landslip)

Rotational slumping occurs when saturated material moves downslope along a curved slip plane.

It is most common where:

• permeable rock overlies impermeable rock
• clay becomes saturated after rainfall
• wave erosion removes support at the cliff base

Slumping produces characteristic stepped cliff profiles.

Mudflows

Mudflows occur when saturated fine material flows downslope rapidly.

Heavy rainfall increases pore-water pressure within the sediment, reducing friction and causing material to flow.

Mudflows can transport large volumes of sediment into the coastal system.

Soil creep and solifluction

Soil creep

Soil creep is the very slow downhill movement of soil caused by repeated expansion and contraction.

Although movement is often only a few millimetres per year, over long periods it can significantly alter slopes.

Solifluction

Solifluction occurs in cold environments where surface layers thaw during summer, but deeper ground remains frozen.

The saturated upper layer slowly flows downhill under gravity.

Marine erosion

Marine erosion involves the wearing away and removal of rock by wave action.

Wave energy is the main driver of coastal erosion, particularly during storms.

Hydraulic action

Hydraulic action occurs when waves force water and compressed air into cracks in cliffs.

As waves retreat, pressure is suddenly released. Repeated compression and decompression weaken the rock until fragments break away.

This process is especially effective on cliffs with many joints and bedding planes.

Cavitation

Cavitation occurs when waves break against a cliff and trap air bubbles within the water. As the pressure changes rapidly, the bubbles collapse or implode, producing tiny shock waves.

Although each implosion releases only a small amount of energy, repeated cavitation can weaken rock surfaces over time and contribute to cliff erosion.

Cavitation often works alongside hydraulic action and is most effective in high-energy environments where powerful waves repeatedly strike the coastline.

Wave quarrying

Wave quarrying occurs when the force of breaking waves dislodges blocks of rock from cliff faces.

This process is most effective during high-energy storm conditions.

Abrasion (corrasion)

Abrasion occurs when waves throw rock fragments against cliffs and shore platforms.

The sediment acts like sandpaper, wearing away the rock surface.

Abrasion is particularly important where beaches contain large quantities of coarse material.

Attrition

Attrition occurs when rock fragments carried by waves collide with one another.

Over time:

• particles become smaller
• edges become smoother and rounder

This helps explain why beach sediment often becomes finer further along the coast.

Solution (corrosion)

Acidic seawater can dissolve soluble rocks such as chalk and limestone.

Solution often works together with carbonation and other weathering processes.

Factors affecting coastal erosion

Rates of coastal erosion vary considerably depending on several interacting factors.

Wave energy

High-energy waves generated by:

• strong winds
• long fetches
• storm conditions
• increase erosion rates.

Storm waves are particularly powerful because they contain large amounts of energy.

Rock type (lithology)

Rock resistance strongly influences erosion rates.

Soft rocks such as clay and sand tend to erode rapidly, while resistant rocks such as granite erode more slowly.

Geological structure

Cliffs with many:

• joints
• faults
• bedding planes

are generally more vulnerable to erosion because water and wave energy can penetrate lines of weakness.

Presence or absence of beaches

Beaches absorb wave energy and help protect cliffs.

Where beaches are narrow or absent, waves attack cliffs more directly, increasing erosion.

Subaerial processes

Weathering and mass movement weaken cliffs and supply sediment for marine processes.

Subaerial processes, therefore, interact closely with marine erosion.

Transportation

Transportation is the movement of sediment within the coastal system.

The main transportation processes are:

• traction
• saltation
• suspension
• solution

Traction

Large rocks and boulders are rolled along the seabed by waves and currents.

Traction occurs mainly in high-energy environments.

Saltation

Small pebbles and sand grains bounce along the seabed.

Saltation is one of the most common coastal transport processes.

Suspension

Very fine sediment, such as silt and clay, is carried within the water column.

Suspended sediment may travel large distances.

Solution

Dissolved minerals are transported within seawater.

Longshore drift

Longshore drift is one of the most important sediment transfer processes at the coast.

Waves approach the coastline at an angle because of prevailing winds. Swash carries sediment up the beach at this angle, while backwash moves material downslope at 90° under gravity.

This creates a zig-zag movement of sediment along the coastline.

Longshore drift transports large volumes of material between different parts of sediment cells.

Deposition

Deposition occurs when waves and currents lose energy and can no longer transport sediment.

This commonly happens:

• in sheltered bays
• where wave energy decreases
• in estuaries
• behind spits and bars
• where constructive waves dominate

Factors encouraging deposition

Deposition is more likely when:

• wave velocity decreases
• water becomes shallower
• sediment supply is high
• coastlines are sheltered
• beaches have gentle gradients

Fine sediment such as silt and clay may settle in low-energy environments to form mudflats and salt marshes.

Depositional landforms

Deposition contributes to the formation of:

• beaches
• spits
• bars
• tombolos
• dunes
• mudflats
• salt marshes

These landforms are most common in low-energy coastal environments.

Interactions between geomorphological processes

Coastal processes are closely interconnected.

For example:

• Weathering weakens cliffs
• Marine erosion undercuts the cliff base
• Mass movement occurs
• Sediment enters the coastal system
• Transportation redistributes material
• Deposition forms new landforms elsewhere

This demonstrates the dynamic nature of coastal systems and the transfer of energy and sediment through time.