OCEANS SUBMARINE RELIEF :

OCEAN BASINS :

·  About 71 % of the earth’s surface is covered by water.

·  Oceans form a single, large, continuous body of water encircling all the landmass of the earth.

·  They account for four- fifth of the Southern Hemisphere and three fifth of the Northern Hemisphere.

·  They contain 97.2 percent of the world’s total water.

·  There are four principal oceans in the world which are separated largely on the basis of their geographical locations.

·  These are the Pacific Ocean, the Indian ocean, the Atlantic Ocean and the Arctic Ocean.

·  All the other seas, inland seas or the arms of the oceans, are counted within these four main oceans.

THE RELIEF OF THE OCEAN BASINS :

The Ocean basins are broadly divided into four major sub-divisions.

They are:

(a) Continental shelf;

(b) Continental slope;

(c) Abyssal plains and

(d) The ocean deeps.

(a) Continental Shelf :

·  There is no clear or well-defined line separating oceans from continents.

·  Infact, continents do not end abruptly at shoreline.

·  They slope seaward from the coast to a point where the slope becomes very steep.

·  The shallow submerged extension of continent is called the continental shelf.

·  The depth of this shallow sea water over the continental shelf ranges between 120 to 370 metres.

·  The width of the continental shelf varies greatly ranging between a few kilometres to more than 100 kilometres.

·  This variation can be seen even in the context of Indian peninsula.

·  The continental shelf off the eastern coast of India is much wider than that of the western coast.

·  Similar variations are seen all over the world.

·  They are much narrower or absent in some continents, particularly where fold mountains run parallel or close to the coast as along the eastern Pacific Ocean.

·   Most of the continental shelves represent land which has been inundated by a rise in sea level.

·  Many regard their formation due to the erosional work of waves or due to the extension of land by the deposition of river borne material on the off-shore terraces.

·   Off the coast regions which were once covered by ice sheets, they may have developed due to glacial deposits.

·  The continental shelves are of great importance to man.

·  The shallow water over the shelf enables sunlight to penetrate through the water to the bottom and encourages growth of microscopic plants and animals called planktons.

·   These planktons are the food for fishes.

·  Continental shelves are the source of fishes, mineral including sand and gravel.

·  A large quantity of the world’s petroleum and natural gas is obtained from these shelves.

·   The Bombay High and the recent discovery of petroleum in the Godavari basin are examples of on shore drilling on the continental shelf.

·  Coral reefs and lipoclastic materials are also common on continental shelves.

·  One of the striking features of the continental shelf is the presence of submarine canyons which extend to the continental slope.

·  These canyons are ‘steepsided valleys’ cut into the floor of the seas.

·  They are very similar to the gorges found on the continents.

·  Godavari Canyon in front of the Godavari river mouth is 502 metres deep .

·  One of the reasons for the formation of submarine canyon is underwater landslide.

·  The sediments collected on the continental shelves get dislodged by a storm or a earthquake.

·  The force of these moving sediments erode the slopes as they come down and as a result submarine canyons are carved out.

·  The continental shelf is generally considered to be territorial water extent of the nations to which it adjoins.

(b) Continental Slope :

·  The continuously sloping portion of the continental margin, seaward of  the continental shelf and extending down to the deep sea floor of the abyssal plain, is known as continental slope.

·  It is charactersied by gradients of 2.5 degrees.

·  It extends between the depth of 180 to 3600 metres.

·  Continental slopes, mainly due to their steepness and increasing distance from the land have very little deposits of sediments on them.

·  Sea life is also far less here than on the shelf.

·  Along the base of the continental slope is a deposit of sediments. This belt of sedimentary deposits form the continental rise.

·  In some regions the rise is very narrow but in others it may extend up to 600 km in width.

(c) Abyssal Plain :

·  Abyssal plains are extremely flat and featureless plains of the deepocean floor.

·  In fact, the abyssal plains are likely the most level areas on the earth.

·  Abyssal plains covering a major portion of ocean floor between the depth of 3000m to 6000m.

·  They were once regarded as featureless plains but modem devices have shown that they are as irregular as the continental plain or surface.

·  They have extensive submarine plateaus, hills, guyots and seamounts.

·   The floor of the abyssal plain is covered by sediments.

·  The plains close to the continents are covered mostly by sediments brought down from the land.

·  But those seas which favour, an abundant growth of organisms have a thick layer of sediments, formed from the remains of living things.

·  These sediments are called oozes.

·  Some of the open seas do not support enough life to produce ooze on the floor.

·  They are covered with a type of sediment called red clay which is of volcanic origin or made up of tiny particles brought by wind and rivers.

(i) Submarine Ridges :

·  The lofty mountain systems which exist on the continents is also represented beneath the ocean waters.

·   These oceanic mountains are known as submarine ridges.

·  They are linear belts occurring near the middle of the oceans and are also called mid-oceanic ridges.

·  All the mid oceanic ridges constitute a world-wide system which is interconnected from ocean to ocean.

·   These ridges are intersected by faults.

·  The oceanic ridge is the site of frequent earthquakes.

·  Volcanism is common in ocean ridges and it produces many relief features.

·  The Mid-Atlantic Ridge is the largest continuous submerged mountain ridge which runs from north to south in the Atlantic-Ocean. It is in the shape of S.

·   At some places, the peaks, rise above the surface of water in the form of islands. Many of the islands are volcanic in origin.

·  The East Pacific Ridge and Carlsberg Ridge are some of the important submarine ridges.

(ii) Seamounts and Guyots :

·  Scattered over the entire sea floor are thousands of submerged volcanoes with sharp tops called seamounts.

·  Sometimes they rise above the sea as isolated Islands.

·  Hawaii and Tahiti Islands are the exposed tops of volcanoes.

·  Volcano rising above the ocean floor whose top has been flattened by erosion and is covered by water is called guyot.

(d) The Ocean Deeps  :

·  They are long, narrow, steep sided and flat-floored depressions on the ocean floor.

·  They are generally called submarine trenches.

·  These trenches are not always located in the middle of the ocean basins, as may be generally expected but are situated very close or parallel to the continents bordered by fold mountains.

·  They are usually found adjacent to the areas of volcanic and earthquake activity.

·  Great earthquakes and tsunamis are born in them. They occur in all the major oceans.

·  The Pacific Ocean has the largest number of trenches.

·  The Mariana Trench in ‘the Pacific Ocean is the deepest known part of the oceans.

LANDFORMS PRODUCED BY SEA WAVES

·  The work of sea waves as an agent of gradation includes erosion, transportation and deposition.

 (A) EROSION BY SEA WAVES :

·  Sea waves have a great erosive force.

·  In their role of an erosional agent they perform four functions.

·  When the sea water loaded with rock fragments and sand attack the coastal rocks it is called abrasion.

·  The rock particles present in the water hit against each other and break into progressively smaller particles. This process is called attrition.

·  Thirdly the broadening of cracks and crevices in the cliffs along the coast due to the attack of the sea waves is called the hydraulic action.

·  The rocks made up of limestone are subjected to solution action by the sea waves.

·  All these processes help in formation of new features on the coastal margins.

Landforms Produced by sea Wave Erosion :

·  Waves, like streams erode the coastal rocks with the help of rock fragments present in the water.

·  important features made through sea wave erosion are mentioned here:

(i) Sea Cliff

·  The maximum impact of the sea waves is observed on the lower part of the coastal rocks and consequently the lower part of the rocks is eroded more rapidly than the upper part.

·  This results in the formation of a hollow under the rock and with the passage of time this excavation in the lower part of the rock keeps on becoming larger.

·  The upper part of the rock is thus left projecting out towards the sea.

·  After sometime, this projecting part fall into the sea under its own weight. As a result a vertical wall is left. This vertical wall is called a cliff.

·  In India a number of sea cliffs are found along the Konkan Coast of India.

(ii) Sea Caves :

·  When the upper part of the coastal rock is hard and the lower part is soft, the erosion is not uniform.

·  The lower part of the rock in such circumstances is eroded much faster than the upper part.

·  Due to differential erosion a hollow is created in the lower part of the rock.

·  When the waves pound against this hollow, air present

in the hollow gets compressed.

·  When the wave comes out of the hollow, the pressure on air is also released and it expands.

·  Due to continuous compression of the air in the hollow, the rocks are subjected to a great pressure and they break.

·  In this process, the hollows in the lower part of the rock keep on enlarging.

·  With passage of time they attain the form of caves and are known as sea caves.

(iii) Sea Arches :

·  When a part of coast extends to some distance into the sea, sea waves working from opposite directions cut a passage through the soft rocks.

·  In the initial stages, this passage is a narrow hole but it enlarges into a broad arch.

·  These broad doorlike features are called sea arches or natural bridges.

(iv) Sea Stacks :

·  When the roof of an arch is broken by erosion or under its own weight or due to any other reason a part of the original rock remains standing as a solitary mass.

·  It may be the rock forming the side of the arch. This type of a feature is called a seastack.

·  Sometimes they take the shape of islands but such islands are not

permanent.

·  Small underwater stacks are known as stumps.

(B) Transportation by Sea Waves :

·  Sea waves, currents and tides are the main agents of transportation of eroded material in the coastal regions.

·  However, the role of waves is more important in connection with the formation of coastal relief features.

·  The material deposited on the coasts by the rivers and glaciers etc. is removed and transported by the waves.

Transportation by sea waves is carried out in two ways:

(i) Removal and transportation, towards the sea, of the material deposited by river etc. on the coast.

(ii) Carrying of material found in the sea to the coastal areas. During this process, the oceanic materials like pearls, conches and other shells are brought to the coast.

(C) Deposition by Sea Waves :

·  Sea waves are helpful in the deposition of the material eroded from the coastal areas.

·  Oceanic current are also helpful in deposition of the transported material.

·  Deposition of the material along the coast is selective.

·  The larger particles are deposited first therefore they are found near the coast.

·  On the other hand, the finest particles are deposited last and they are deposited generally away from the coast.

·  This selective deposition is sometimes altered or affected by a change in the intensity or force of the waves.

·  Thus it is sometimes possible to find very fine particles deposited near the coast where generally larger particles are deposited.

·  A number of topographical features are formed due to deposition by waves and currents.

Some of these topographical features are discussed here:

(i) Beach :

·  Most of the material eroded and picked up by the waves is deposited near the coast.

·  Due to this deposition, the sea becomes shallow and a part of the coastal area is raised above the water level.

·  This raised portion is almost like a flat plain of a platform formed of gravel and sand.

·  This type of depositional features along the coast is called a beach. Beaches are centres of tourist attraction.

·  Marina Beach of Chennai and Kovalam Beach of Thiruvananthapuram are the famous beaches of India.

(ii) Sand Bar :

·  Sometimes the deposits of sand and gravel laid down by waves and currents form embankment, separating shoreline from the sea.

·  They thus form barriers between the sea and the mainland.

·  Such deposits are called sand bars.

·  They sometime pose difficulties in navigating.

(iii) Spit Earth :

·  When one end of a bar is attached to the coast and other extends into the sea, it is called a spit.

·  These spits are formed by the accumulation of materials brought by waves like sand and gravel.

(iv) Lagoon :

·  Sometimes due to deposition of waves and currents both the ends of the bar join to enclose a part of the sea water between the coast and the bar.

·  This enclosed part of the sea forms a lake of saline water.

·  This saline water lake is called a lagoon.

·  Sometimes the lagoons are formed due to wave erosion also.

·  A lagoon is generally connected with the sea through a narrow passage. The Chilka and Pulicate lakes on the north-eastern coast and lake Vembanad on Kerala coast are examples of lagoon lakes in India.

·  Topographical features like beaches, bars, spits and lagoons are formed by the action of waves.

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