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.
(a) Continental shelf;
(b) Continental slope;
(c) Abyssal plains and
(d) The ocean deeps.
· 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 .
· 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.
· 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.
(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.
· 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.
(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.
· 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.