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In this article we will discuss about the marine cycle of erosion along a shoreline of submergence and emergence.
Marine Cycle of Erosion along a Shoreline of Submergence:
Initial Stage:
The initial stage of marine cycle of erosion of shoreline of submergence begins with the submergence of coastal land under sea water.
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Submergence of coast land takes place in two ways viz.:
(i) Either due to rise in sea level (positive change in sea level), or
(ii) Subsidence of coast land so that most of the coast land is submerged under sea water due to its transgression on main land.
Rise in sea level may be either due to rise in the oceanic floor due to tectonic factors or due to return of melt-water locked in the form of ice sheets on the continents during ice age. The initial form of shoreline of submergence may be a ria coast or a fiord coast.
The lower segments of the rivers at their mouths are dismembered due to submergence of coast land. The initial submerged coastline is highly irregular characterized by numerous embayments, coves, bays, headlands, inlets, islands etc. (fig. 20.16).
Youth:
Marine waves mainly swash or breaker waves or surf currents erode the exposed coastal land through the mechanisms of hydraulic action and corrasion (abrasion). The uprushing high-energy storm waves bombard the densely jointed rocks and dislodge larger rock blocks. Consequently, the coastline is highly indented and becomes crenulated and irregular. Numerous caves and headlands are formed due to differential erosion of coastal rocks.
The breaker waves notch the rocks at water level and thus initiates the formation of sea cliffs. In the beginning the cliffs are of low height and are imperfectly developed. Gradually, the cliffs are sharpened due to regular erosion at the cliff base.
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Wave-cut platforms (shore platforms) are formed in front of cliffs due to regular landward recession of cliffs. Though the early youth is dominated by erosional work but some depositional features are also developed such as beaches in the back shore zone. Wave-cut platforms are characterized by several coastal features such as arch, stacks, caves, natural chimneys etc.
Late youth is characterized by maximum development of wave-cut platforms as they become most extensive due to progressive recession of cliffs. Most of the erosional features start disappearing and numerous depositional features are formed e.g., bars, offshore bars, connecting bars, spits, hooks, loops, looped bars, tombolo, beaches etc. Several types of beaches such as shore beaches, headland beaches, bay-head beaches, berms, cusp beaches etc. are developed. Most of the bays are enclosed by bars and thus lagoons are formed. Cliffs are fully developed and thus the coast becomes almost vertical.
Maturity:
Most of the features developed during youthful stage are obliterated. Profile of equilibrium is attained due to balance in the rate of erosion and deposition. Most of the depositional features are destroyed by late maturity and thus the coast line becomes almost straight and regular. The height and gradient of coastland decrease significantly.
Old Stage:
Coast and shore are significantly lowered in height because of continued weathering and erosion. Adjoining land areas are eroded down to sea level. Thus, the coast and shore become straight and slope very gently towards the sea.
It may be pointed out that this may be possible only when there is crustal stability for long period. It is obvious that the conditions of old stage are only theoretically possible because coasts are affected by emergence and submergence due to diastrophic forces.
Marine Cycle of Erosion along a Shoreline of Emergence:
Initial Stage:
Shoreline of emergence is evolved in two ways viz.:
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(i) Due to upliftment of coastland in relations to sea level because of tectonic events, or
(ii) Due to fall in sea level because of subsidence of oceanic floor.
Initially, shorelines of emergence are straight and regular.
Coastal plains extend for greater distance in sea water but their gradient is quite gentle. Since the depth of water is shallow, most of the sea waves break in off-shore zone. The breakers erode the coast to form ‘notch’ and small cliffs. Such small cliffs are called ‘nips’. Submarine bars are formed due to deposition of sediments on submerged coastal plains. These submarine bars gradually grow in height and are aligned parallel to the shoreline.
Youth:
The youthful stage of marine cycle of erosion on shoreline of emergence begins with the appearance of submarine bars above sea level. Numerous independent small bars are united and thus form offshore bars. These offshore bars protect the coast from wave erosion. Sea water locked between the coast and offshore bars becomes lagoon (fig, 20.17).
Sea waves break offshore bars at some places and thus make their way to the lagoons. Such openings in the offshore bars are called tidal inlets. Legoons are filled with sediments brought by the rivers (which debouch in the lagoons) and wind. Sometimes, lagoons become swamps and marshes because of vegetation.
Late youth is characterized by migration of offshore bars towards the coast. Seaward side of these bars is steepened due to their erosion by storm waves. Eroded materials are deposited by sea waves on the landward side of these bars. This process causes gradual shifting of offshore bars coastward with the result lagoons become narrower.
Maturity:
Offshore bars, lagoons, swamps and marshes, tidal inlets etc., are destroyed by the beginning of mature stage. Sea waves erode the submerged coastal platform upto wave base. Most of the coastal irregularities are obliterated and the coastline becomes simple and straight. The slope of the coast becomes steep and the depth of water increases.
Old Stage:
The development of old stage of marine cycle of erosion on shoreline of emergence can be deduced theoretically only as its practicability is not possible.