Processes and Mechanism of Marine Erosion | Geography

Sea waves resort to erosion of the coastal land and backshore zone through the processes and mecha­nism of marine erosion, corrasion or abrasion, attri­tion, corrosion or solution and water pressure. When the sea waves break at plunge line, the potential energy of the waves is converted into kinetic energy and the resultant breakers or swash or surf currents strike the coast land with enormous power and erode the geomaterials in different manner as stated above.

It may be pointed out that the coastal rocks are im­mensely affected by weathering processes resulting into disintegration and decomposition and thus weak­ening of rocks. Such weakened rocks are easily plucked and eroded away by the hydraulic pressure and turbu­lence of breaking waves (swash).

The nature and magnitude of coastal erosion are affected and determined by the following factors:

(1) Wavelength, wave velocity, wave frequency and wave period. Long enduring waves with longer wavelength and high velocity become effective erosive agent.

(2) Structure and composition of bedrocks of coast land. Well jointed and fractured rocks are more easily plucked, quarried and abraded by sea waves. Rock types (lithological characteristics) determine the nature of erosion.

(3) More or less stable coastline is subjected to more erosion than unstable coastline.

(4) Vertical coast land (cliffs) having deep water is less eroded because the sea waves are reflected back without causing much harm to the cliff. On the other hand, the cliffs, which rise moderately from wide basal platform and if the sea water is of shallow depth, are prone to more hydraulic action and plucking because the breakers or swash strike the cliff with great ferocity and enormous power and thus hammer the rocks.

(5) Availability of erosion tools (sands, gravels, pebbles and cobbles and sometimes boulders) and

(6) Duration of marine erosion.

Hydraulic action refers to the impact of moving water on the coastal rocks. Large storm waves attack the coastal rocks with erormous hammer blows amount­ing to 50 kgf per square centimetre (gravity force (f) is 9.81 and hence sea waves, normally, hurl a force of 50 kgf per square centimetre of the coastal rocks).

Re­peated blows of striking sea waves enlarge the incipi­ent joints, fracture patterns and thus help in breaking the rocks into smaller joint-bounded blocks. The waves are capable of dislodging larger fragments of rocks weighing several tonnes in weight. This process of displacement of rock fragments is also called as quar­rying and plucking. In fact, wave quarrying and wave plucking caused by the hydraulic pressure and turbu­lence of breaking waves is very effective mechanism of erosion of weathered and joint-bounded fresh bed­rocks.

The striking breaking waves also exert enor­mous pressure on the air trapped in the crevices and hollows within the coastal rocks. Thus, alternate proc­ess of compression (when the waves strike the rocks as swash) and decompression (when the waves return as backwash) causes pressure changes and weakens the rocks to break into the blocks of several tonnes.

Abrasion or corrasion is another effective mecha­nism of coastal erosion by marine waves with the help of tools of erosion (coarse sands, pebbles, cobbles and sometime boulders). High-energy storm waves charged with large cobbles drill out circular potholes and abrade the standing bedrocks. Attrition involves mechanical tear and wear and consequential breakdown of frag­ments due to their mutual collision effected by back­wash and rip currents which remove the fragments from the cliff base and transport them towards the sea.

Corrosion or solution refers to the chemical al­teration of rocks mainly carbonate rocks (lime-stones, dolomites and chalks) due to their contact with sea water. Besides hydraulic action, abrasion and corrosion, coastal rocks are also weakened and disintegrated due to alternate processes of wetting (hydration) and drying (dehydration) because these promote a wide range of chemical processes which help in the disintegration and decomposition of coastal rocks. Alternate freeze and thaw actions in the foreshore zones in the cold climates cause disintegration of joint-bounded rocks.

It may be mentioned that lithological character­istics of coastal zones and their layout largely control the mechanism of marine erosion. It is argued that basalts and obsidian weather far more in marine water than in freshwater. This factor explains the unusual width of continental shelf west of the Deccan basalt region of Peninsular India.

The west coast of Maharashtra is characterized by rias, coves, caves, stacks, inlets etc. because the waves strike the joints and fissures of basalts transversely and thus have caused differential erosion while the south coast of Kathiawar having the same lithology (basalt) is almost devoid of such fea­tures because the waves do not attack the coast trans­versely as they move parallel to the coast.