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In this article we will discuss about the denudation and depositional work of sea.
Denudation Work of Sea:
Like river water, sea water also performs denudation activity in the form of waves, currents and tides. Because of this various landforms are formed but obviously this process takes place only in coastal regions. Coastal regions are totally different to non-coastal regions.
The speed and velocity of waves depends on the speed of winds. Sea water rises because of direction, pressure and friction of winds. The part of water, which rises is ‘crest’ and the lower one is known as ‘trough’. Rocks, stones, sand, soil etc., present in high speed waves increase the erosion capacity of waves. Sea water also performs erosion, transportation and deposition activities.
Erosional Work of Sea Waves:
Erosional work depends on the speed and power of waves, gradient and height of the coast and moreover depth of sea. Erosion process is not uniform at all the places.
Erosion of hard rocks is slow but in case of soft rocks it is very fast. During rainfall water level increases which accelerates erosion. Plants and animals reduce the strength of rocks by making holes in them. Presence of sand, soil, rocks, stones etc. in waves increases the erosion.
Any hindrance in the way of waves gives it the name as ‘braker waves’. A number of braker waves when flow towards coast, these are known as ‘surfing waves’. At times the ‘might’ of these surfing waves is to such an extent that these exert a presence of 3000 kilometers per square meter to 30,000 km per sq. meter on the costal rocks.
Waves can perform erosion activity in four ways:
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i. Hydraulic Action:
Rocks are broken when waves having debris (rocks, soil, sand, stones etc.) strike with huge force.
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ii. Abrasion:
When both waves and currents break the rocks this happens when waves strike with rocks again and again. It is a frictional force and known as abrasion.
iii. Attrition:
It is because of waves, when the size of pieces of rocks decreases or they are grained, this process is known as Attrition.
iv. Solution:
Soluble rocks like limestone, dolomite, chalk etc. dissolve in sea water. This solvent action is limited to certain areas only.
When a wave strikes it starts erosion process, resulting in the formation of following forms:
i. Sea Cliff:
Waves erode the lower part of the coastal rocks, first because these are at sea level. Sometimes the lower parts of rocks are soft and get easily eroded. The upper part of these rocks look ‘high’, which is known as cliff. It has a steep gradient.
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Sometimes waves make a hole in lower part of the cliff with erosion activity, known as ‘notch’. The size of notch increases with the passage of time and sea caves are formed.
Cliffs situated on the western coast of India are the finest examples.
ii. Sea Caves:
With the passage of time, size of ‘Notch’ increases due to erosion and a large pit is formed in the lower part of cliff. This is known as sea cave.
iii. Arch or Natural Bridges:
If at any coast, Waves strike on both sides of caves and form a hole, such structure is known as sea arch, moreover it looks like a natural bridge.
iv. Stack:
Natural arch collapses due to erosion, leaving steep and often vertical columns of rock, which is known as ‘stack’.
v. Stumps:
When stacks get eroded by the waves, they collapse, leaving behind a stump. Stump usually forms a small rock island, low enough for a big tide to submerge.
vi. Spouting Horns or Blow Holes:
Due to erosion, the cracks of sea caves form a hole in the roof of the cave, which is known as spouting horn or blow hole. Actually when the water rises into sea cave, already available air starts dashing out through the hole in cave roof, creating whistling sound. This sound gives it the name, Spouting horn.
vii. Breaking of Sea Caves:
When high speed waves strike with huge force on cave, they create pressure on the internal air upto such extent that cave break down into pieces. In spite of this sometimes large number of blow holes are formed in caves because of which it loses its strength and collapses. With the breakdown of cave a narrow inlet is formed which is known as ‘geo’.
viii. Caves:
Sometimes on the coast, soft and hard rocks are situated parallel to each other. When soft rocks are eroded faster with the hard rocks surrounding them, small gulfs are formed which are known as caves.
ix. Creeks and Bays:
When soft and hard rocks are situated in vertical position and continuous erosion by waves erodes soft part of these rocks resulting in formation of deep and narrow inlets known as ‘creek’. With the passage of time their depth and breadth increases which is known as ‘Bay’.
x. Headland or Cape:
Sometimes a hard rock is surrounded by soft rocks and erosional activity of waves leaves only hard rock standing vertically ahead of it surrounding. Such form is known as headland or cape.
Transportation by Sea Waves:
Waves pick up the rocks, stones, soil etc. formed by the process of weathering and bring them in sea water. High speed waves bring shells and other materials on the coast, people collect these for manufacturing of material used for decoration purposes.
Shells are direct source of Calcium Carbonate and it is used for increasing the level of calcium in the soil, manufacturing of musical instruments, poultry feed and for making ornaments.
Depositional Work of Sea:
While transporting the debris, as carrying capacity of waves reduces, debris is deposited on the coast. Because of which various forms take shape.
i. Sea Beach:
Beaches are formed by deposition of soil, rocks, stones etc. on the coast by sea waves. The size of a beach depends upon the debris deposited, more the debris, larger will be the beach. Beaches may be of various types’ life carp beach, Gokarna and Kovalam, Linear Beaches; Marina beach Chennai, Rock beaches and Sand beaches. Marina beach is second longest beach in the world. Beach formation is possible during low velocity of waves or swift moving wave or thunder storms destroy beach deposits.
ii. Sandbar:
Sand bars are formed if sand gets deposited parallel to the waves. These are extended parallel on the front side of the hard rocks near the coast. When its size increases it is called as offshore or long shore sandbar. Sometimes gulf is blocked due to the huge deposition of soil, offshore bars which act or blockers get combined with each other and are known as Tied Island. The Islands also have various types e.g. Bay bar, Tombolo, Hook, Loop etc.
iii. Spit:
Deposition of sand and rocks at some distance from sea coast is known as spit. According to O.F. Evans, (1942), as pit is a “Ridge of embankment of sediment attached to land at one end and terminating in open at other”.
Spits are commonly found on the eastern and western coast of India. Around 50 KM long spit is situated on the mouth of Chilika lake and 60 KM long spit is situated in the east of Policot lake. When two spits confine, it is known as ‘Looped bar’.
iv. Lagoon:
Water collected between the sand bar and coast is known as Lagoon. These are very common in low coasts. ‘Chilka’ on the eastern coast and ‘Vembanad’ situated on the coast of Kerala are the important examples of ‘lagoons’.
v. Dunes:
Sand dunes are also get formed in coastal areas. Waves deposit the sand and winds transport this sand and deposit it at another place because of which sand dunes are formed. Such sand dunes are found on eastern and western coasts of India.
Sea waters carry unimaginable energy. Various land forms of the eastern and western coasts of India, formed by Oceanic waters are its biggest example. Total length of Indian coast is 7,516.6 kilometers, including Andaman and Nicobar (Bay of Bengal) and Lakshadweep (Arabian Sea).