Coasts (2/4)- Coastal Processes (Marine and Sub-Aerial)


Coastal Geomorphological Process

The coastline is affected by 2 main sets of processes

Marine processes: Operate upon a coastline and are connected with the sea, such as waves, tides and longshore drift.

Subaerial processes: Includes processes that slowly (usually) break down the coastline, weaken the underlying rocks and allow sudden movements or erosion to happen more easily. The material is broken down in situ, remaining in or near its original position. These may affect the shape of the coastline and include weathering, mass movement and runoff.


1. Erosion: Refers to the wearing away of the land surface and removal of materials by river and seabed, ice and wind.

  1. Hydraulic action

  2. Abrasion

  3. Wave quarries

  4. Attrition

  5. Solution

  1. Hydraulic Action

This refers to the impact on rocks of the sheer force of the water itself. This can exert enormous pressure upon a rock surface referred to as wave pounding

       2. Abrasion

The material the sea has picked up also wears away rock faces. Sand, shingle and boulders hurled against a cliff will do enormous damage. This is also apparent on intertidal rock platforms where sediment is drawn back and forth, grinding away at the platform.

       3. Wave quarries

A breaking wave traps air as it hits a cliff face. The force of water compresses this air into any gap in the rock face, creating enormous pressure within the fissure of joint.

As the water pulls back there is an explosive effect of the air under pressure being released. The overall effect of this over time is to weaken the cliff face. Storms may then remove large chunks of it. The term cavitation may also be used of the effects of pressure changes in areas of rapid flow in rivers for this process.

        4. Attrition

The rocks in the sea which carry out abrasion are slowly worn down into smaller and more rounded pieces.

        5. Solution

This is a form of weathering rather than erosion but also contributes to coastal erosion. The process of the solution includes the dissolving of calcium-based rocks (eg.limestone)- it is unlikely that seawater itself is the agent of this (as its pH is generally stale between 7.5 and 8.5, making it slightly alkaline.)

In localised areas where fresh water interacts with seawater conditions for a solution may occur, or carbon-based rocks at the coast may be broken down by water flowing from the land, or rainwater which is slightly acidic. The effects of evaporation of salts from water in the rocks to produce crystals of salts from water in the rocks to produce crystals. These expand when they form and put stress upon the rocks. Salt from seawater spray is capable of corroding several types of rock. This is most definitely a form of weathering.

The rate of Erosion depends on:

  1. Wave steepness and wave breaking point

The steeper the wave, the more energy so more erosive power



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