Waves are caused by wind, tides are caused by gravity from the moon and the sun, and currents are caused by tides, winds & temperature & density differences in different areas of the oceans.
- The most powerful agents of marine erosion are waves, which originate due to sweeping of winds over the water surface, setting up a series of undulating swells surging forward
- On approaching shallow water near the shores, their speed is reduced & the waves are curved or refracted against the alignment of the coast
- The shallow water, when is less than the height of the waves, check their forward movement, the wave crest curl over & breaks into the shore
- Water that finally rushes up the beach & hurls rock debris against the land is termed as swash, with the water that retreats or sucked back called backwash
- Another element in offshore drift is undertow, which flows near the bottom away from the shore
- This current exerts the pulling effect which can be dangerous for sea bathers
- Marine agent of erosion operates as corrasion, attrition, hydraulic action & solution to transform the coastal landscape
Coastal Features of Erosion
Capes & Bays
- On exposed coasts, the continual action of waves on the rocks of varying resistance causes the coastline to be eroded irregularly.
- This is particularly pronounced where hard rocks occur in alternate band with softer rocks.
- The softer rocks are worn back into inlets or bays & the harder ones persist as headlands or capes.
- Even where the coastline consists of one rock type, irregularities will be caused by variation within the rock.
Cliffs & Wave cut platforms
- Generally, any steep rock platform adjoining the coast forms a cliff, whose rate of recession will depend on its geological structure
- Means the stratification & jointing of the rocks & their resistance to wave attack.
- If the bed dips seaward, large blocks of rock will be dislodged & fall into the sea & cliff will rise in a series of steps.
- On the other hand, if the beds dip landward, the cliff will be more resistant to wave erosion.
- At the base of the cliff the sea cuts a notch, which gradually undermines the cliff, so that it collapses.
- As the cliff recedes landwards under the pounding of waves, an eroded base is left behind called a wave cut platform.
- The platform, upper part of which is exposed at low tides, slopes gently seawards, with its surface strewn with rock debris from the receding cliff.
- Further the abrasion continues until the pebbles are swept away in the sea with eroded material deposited on off shore terrace.
Cave, Arch, Stack & Stump
- Prolonged waves attack on the base of the cliff & excavate holes in regions of local weakness called called
- When 2 caves approach each other from either side of headland & unite, they form an
- Further erosion by waves will lead to total collapse of the arch.
- The seaward portion of the headland will remain as a pillar of rock known as
- With the course of time, these stubborn stacks will gradually be eroded, leaving behind the stumps, which are only just visible above the sea level.
Geos & Gloups (blow-holes)
- The occasional splashing of the waves against the roof of a cave may enlarge the joints when air is compressed & released repeatedly inside them.
- A natural shaft is thus formed which may eventually pierce through the surface.
- Waves breaking into the cave may force water or air out of this hole. Such a shaft is termed as Gloup or blow hole.
- The enlargement of blow-holes & continual action of waves weakens the cave roof.
- When the cave roof collapses, a long, narrow creek may develop known as Geos
Coastal Features of Deposition
- Sands & gravel loosened from the land are moved by waves to be deposited along the shore as beaches.
- The eroded material is transported along the shore in several distinct ways.
- Long shore drifts which comes obliquely to the coast carries material along the shore in the direction of the dominant wind.
- At the same time, backwash removes part of the material seawards, along the bed of the sea, & deposits it on the off-shore terrace & even beyond.
- The constant action of the waves automatically sorts out the shoreline deposits in a graded manner.
- The coarser materials are dropped by the waves at the top of the beaches & the finer materials, carried down the beach by the backwash, are dropped closer to the sea.
Spits & Bars
- The debris eroded by waves is continually moved by long shore drift & where there is indentation in the coast, such as at the mouth of the river or a bay; material may continue to be deposited across the inlet
- As more materials are added, they will pile up into a ridge or embankment of shingle forming a spit, with one end attached to the land & other projecting into the sea
- When a ridge of shingle is formed across the mouth of a river or the entrance to a bay, it is called a bar
- Such a connecting bar that joins two land masses is known as Tombolo
Marine Dunes & Dune Belts
- With the force of on-shore winds, a large amount of coastal sand is driven landwards forming extensive marine dunes that stretches into dune belts
- Their advance inland may engulf farms, roads & even the entire villages;
- Hence to arrest the migration of dunes, sand binding species of grass & shrubs, such as marram grass & pines are planted.