GEOLOGY

Geology

Geology

Question 1

Cliff Face

The four main types of wave action are:

Hydraulic action - occurs when waves striking a cliff face compress air in cracks on the cliff face. This exerts pressure on the surrounding rock, and can progressively splinter and remove pieces of rock. Over time, the cliff face cracks can grow, sometimes forming a cave. The rock from the cliff face which was removed falls to the sea bed where it is subjected to further wave action.

Attrition - occurs when waves causes loose pieces of rock debris (scree) to collide with each other. The effect is that pieces of scree collide, grinding and chipping each other, progressively becoming smaller, smoother and rounder. Scree also collides with the base of the cliff face, and can chip small pieces of rock from the cliff or have a corrasion (abrasion) effect, similar to sandpapering.

Corrasion - (abrasion) occurs when the waves break on the cliff face pounding the cliff face and slowly erode it. As the sea pounds cliff faces it also uses the scree from other wave actions to batter and break off pieces of rock from higher up the cliff face which can be used for this same wave action and to attrition.

Corrosion - or solution occurs when the sea's pH (anything below pH 7.0) corrodes rocks on a cliff face. Limestone cliff faces, which have a high pH, are particularly affected in this way. Wave action also increases the rate of reaction by removing the reacted material.

The ability of waves to cause erosion of the cliff face depends on number of factors, including:

The hardness or 'erodibility' of the rocks exposed at the base of the cliff

The key factors in determining erodibility include the rock strength and the presence of fissures, fractures, and beds of non-cohesive materials such as silt and fine sand.

The rate at which cliff fall debris is removed from the foreshore

Debris removal from the foreshore is dependent on the power of the waves crossing the beach. This energy must reach a critical level or to remove material from the debris lobe. On many cliffs debris lobes can be very persistent and may take many years to completely disappear.

The presence/absence of a beach at the cliff base.

Beaches dissipate wave energy on the foreshore and can provide a measure of protection to the cliff from marine erosion.

The stability of the foreshore, or its resistance to lowering

Lowering of the beach or shore platform through wave action is a key factor controlling the rate of cliff recession. If the beach is not lowered the foreshore should widen and become more effective at dissipating the wave energy, so that fewer and less powerful waves reach the cliff.

The adjacent bathymetry

The nearshore bathymetry controls the wave energy arriving at the coast, and can have an important influence on the rate of cliff erosion.

The supply of beach material in the coastal cell from updrift

The provision of updrift material coming onto the foreshore beneath the cliff helps ensure a stable ...