Shape and Size
The Atlantic Ocean extends over about 16.5% of the total area of the earth. This percentage does not include that of its marginal seas. The area of this ocean is about 50% of the Pacific Ocean. The most striking feature about the bottom of the Atlantic is the presence of the Mid- Atlantic Ridge that divides this ocean in half.
This ridge extends 2.5 km above the deep-sea plains on either side. This submarine ridge slopes gently towards the deep-sea plain on either side. It is S-shaped following the general trend of the coastlines.
On the eastern side, the coast of Saharan Africa bulges towards the west, while the north coast of South America recedes into the Caribbean Sea; while the Cape Sao Roque projects eastwards, the Gulf of Guinea recedes in the same direction.
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From the nature of the shape of the respective coasts together with other geological and bio-logical evidences, it becomes amply clear that in the remote past the continents on both the sides of the present ocean must have been parts of a single land-mass.
This ocean basin becomes narrower towards the equator. The distance between the Liberian coast of Africa and Cape Sao Roque is only 2560 km. To the north of equator the North Atlantic Basin is 4800 km wide in latitude 40HN.
While the South Atlantic Basin is 5920 km. wide in latitude 35°S. Whereas the South Atlantic opens broadly into the Southern Ocean, the North Atlantic is rather enclosed by Greenland and Iceland.
Bottom Relief:
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The Continental shelf:
Barring a few areas, the Atlantic Ocean is characterized by having a broad continental shelf all around. The continental shelves of the North Atlantic are almost broad and flat-surfaced everywhere. As we know, the shelves along such coasts as are backed by mountains or plateaus are very narrow, while those on the coasts flanked by plains are quite broad and level.
That is why the continental shelves adjoining the north-east coast of the United States of America and the west European coast are quite broad. On the contrary, the continental shelf is rather narrow from the Bay of Biscay to the Cape of Good Hope. The width of the shelf of the Labrador coast varies from 240 to 400 km.
But the shelf near the Brazilian coast is very narrow because of the presence of a plateau flanking the sea coast. Excepting the east coast of the southern part of South America, where the Falkland Islands are found on the continental shelf, the continental shelf of the South Atlantic Ocean is generally narrow. The coastal areas of Africa and Brazil being plateaus, the narrowness of the shelf is quite natural.
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Mid-Oceanic ridges and rises:
As stated earlier, the S-shaped Mid-Atlantic Ridge occupies the middle part of the ocean basin. This ridge extends from the Bear Islands in the north to Bouvet Island in the south, over a distance of about 15,000 km. It may be stated that with the exception of the Bermuda Islands, all the oceanic islands of this ocean belong to this ridge and other transverse ridges rising from it.
The presence of this longitudinal ridge is the most characteristic feature of the bottom relief of this ocean. The average depth of water above the ridge is about 1700 fathoms.
The northern part of this ridge is known as the Dolphin Rise, and the southern part as the Challenger Rise. In the North Atlantic, the Mid-Atlantic Ridge is relatively wider and forms the broad Telegraph Plateau. This submarine plateau extends from Ireland to Labrador.
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To the east and west of the Mid-Atlantic Ridge, there are several transverse ridges in this ocean basin. The Walvis Ridge extends from the neighbourhood of Tristan da Cunha to the west coast of Africa. It runs in a north-easterly direction. Another ridge, called the Rio Grande Ridge, originates in the same area and runs to the South American coast.
From the Telegraph Plateau a broad ridge extends from northern Scotland to south-eastern Greenland, and is covered by 550 fathom-deep water. In fact, the Faeroes and Iceland are the higher parts of this ridge rising above the sea surface. The submarine ridge located between northern Scotland and Iceland is called the Wyville-Thompson Ridge.
In the South Atlantic the Mid- Atlantic Ridge is 960 km broad, and several transverse ridges rise from it on both the sides. The continuity of the Mid- Atlantic Ridge is broken by the Romanche Deep which lies near the equator and extends down to 7,370 m.
This break in the ridge is very important because it makes possible the rejuvenation of the bottom water in the Eastern Atlantic Basin. In the absence of this break, the bottom water could not mix laterally with water from higher southern latitudes because of its blocking by the Walvis Ridge, located between the Gough Island and the coast of South Africa.
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Beyond the Cape of Good Hope the Mid-Atlantic Ridge is called the Atlantic- Antarctica Ridge. The origin of the Mid- Atlantic Ridge has been the most controversial problem which still remains unresolved.
A group of scientists is of the view that this ridge came into existence because of the tensional forces produced by the tectonic movements. On the contrary, other scientists are of the opinion that compressional forces were responsible for the origin of the ridge. There are still other viewpoints regarding the origin of the Mid-Atlantic Ridge.
Basins:
There are numerous basins surrounded by ridges and rises on the floor of the Atlantic Oceans:
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Labrador Basin :
Situated between Greenland in the north and Newfoundland in the South this basin is approximately 4,000 m deep.
North-east Atlantic Basin :
This elongated basin extends from north-east to south-west between 38°N and 50°N latitudes. The average depth of the basin is a little more than 5,000 m.
North-west Atlantic Basin :
This is the largest basin in the North Atlantic extending from 12°N to 40°N latitude. The average depth of the basin also exceeds 5,000 m. This basin is located between the Mid-Atlantic Ridge and the African west coast. The latitudinal extent of the basin is from 10°N to 38° latitude. Its depth varies from 5,000 to 7,000 meters.
Cape Verde Basin :
This basin is located between the mid-Atlantic Ridge and the African west coast. The latitudinal extent of the basin is from 10° N to 38° N lat. Its depth varies from 5,000 m to 7,000 m.
Brazil Basin :
This basin is found in the South Atlantic Ocean. It extends from the equator to 30°S latitude, and its average depth is more than 6,000 meters.
South-east Atlantic Basin:
This basin extends from the Guinea’s Projection to the Walvis Ridge. Its average depth is 6,700 m.
Cape Basin :
The Cape basin is located to the west of Africa, and extends from 28°S to 41°S.
Argentina Basin :
Situated between 35°S and 41°S, the average depth of this basin exceeds 5,000 m. However, in the southern part of this basin 7,000 m depth has been measured.
Agulhas Basin :
This basin is situated to the south of the Cape of Good Hope. Between 40°S and 50°S latitudes, the basin is relatively wider. In the latitude of 60°S the basin has east- west extension to the north of Antarctica.
Atlantic-Antarctic Basin :
In the latitude of 60° S this basin has east-west extension to the north of Antarctica.
Oceanic Deeps:
The Atlantic Ocean floor is characterized by a general absence of linear deeps. This is due to the absence of rather recent fold lines along the Atlantic coast. However, there are a few very important deeps found on the ocean floor. These oceanic deeps are usually called “deep-sea trenches” or “troughs”.
According to Defant, this has reference only in a morphological sense and not to its origin. They are very closely connected with folding processes in the earth’s crust. Just to the north of Puerto Rico there is a very deep trough called Puero Rico Deep. It is the deepest of all the troughs or deeps found on the floor of the Atlantic Ocean. It is 4812 fathoms deep.
Some important deeps are found in the vicinity of West Indies. As stated earlier, Romanche Deep is situated in the middle of the Mid-Atlantic Ridge which measures 4030 fathoms.
Another deep, which is more than 4000 fathoms, is located near the South Sandwich Island and is curvilinear in shape. It is termed “the South Sandwich Trench”. The depth of this trench is 4,545 fathoms.
Islands in the Atlantic Ocean:
Only a few islands are found in the Atlantic Ocean. The British Isles and Newfoundland are merely the slightly raised parts of the continental shelf. Thus, these islands are actually parts of an adjacent continental landmass.
The waters separating these continental islands from the mainland are relatively shallow since they cover the continental shelf. There is evidence that in the Pleistocene ice ages, when there was a fall in the sea-level, these islands were connected with the mainland of Europe and North America.
The West Indies comprises a few island arcs that are found near the mainland. Iceland and the Faeroes are actually the highest peaks or the higher parts of the mid-oceanic ridge between northern Scotland and Greenland.
In the extreme south of the South Atlantic Ocean there is a large number of the island groups namely, the Sandwich Islands, Georgia Islands, Shetlands Island, Falklands Island, South Orkneys Island etc. These islands are the higher parts of various submarine ridges and plateaus that are present between the southernmost tip of South America and the Graham-land Peninsula of Antarctica.
The oceanic islands like the Azores in the north and the Ascension and Tristan da Cunha Islands in the south are the higher parts of the Mid-Atlantic Ridge rising above the water surface. To the east of the Mid-Atlantic Ridge lies the St. Helena Island which has risen from the abyssal plain and its slopes are steep.
Similarly, on the western side of the ridge in the latitude 20° south the island of Trinidad has also risen from the ocean floor. These islands appear to be seamounts which are high enough to break the ocean surface and appear as islands.
The Bermudas in the North Atlantic are actually coral islands which have been built on the submarine volcanic cones. Madeira Island off the coast of Morocco is of purely volcanic origin. The highest peak of this mountainous island is Pico Ruivo.
The Canaries group of islands, the Cape Verde Islands and most of the other Atlantic islands rise from the shelf-like extension of the mainland.
Marginal Seas :
The South Atlantic Ocean is characterized by the absence of marginal seas. On the contrary, the European and the American sides of the North Atlantic have a number of broad and extensive marginal seas.
On the eastern side of the North Atlantic it was the submergence of the continental margins of Europe that produced a highly indented coastline. The marginal seas of the European side comprise the Baltic Sea, the North Sea, and the Mediterranean Sea.
The Mediterranean Sea extends from east to west, and has a very irregular coastline dividing it into sub-seas. It is bounded by Europe and Asia Minor on the north and east respectively, and Africa to the south.
It is surrounded by land except for a narrow connection with the Atlantic Ocean through the Strait of Gibraltar. It is also connected with the Black Sea through the Bosporus Strait which is very narrow. The Suez Canal, which is man-made, connects it with the Red Sea.
A sill with 400 meter depth, extending from Sicity to the coast of Tunisia, divides the Mediterranean Sea into two major basins. Atlantic Ocean water flows into the Mediterranean through the Strait of Gibraltar as a surface flow. In fact, the Atlantic water replaces the surface water evaporated in the very dry eastern section of the Mediterranean Sea.
The circulation between the Atlantic Ocean and the Mediterranean Sea is characteristic of closed basins that lie in very arid areas where evaporation exceeds precipitation. Because of the high rate of evaporation of the water that comes in from the open ocean, the salinity of the Mediterranean water is increased considerably. This causes the sinking of water which returns to the open ocean as a subsurface flow through the strait of Gibraltar.
In the Black Sea, which is situated in the north of the Mediterranean Sea, precipitation as well as the fresh water brought in by the rivers exceeds the amount of evaporation. The Bosphorus Strait actually provides access to the Sea of Marmara, which is connected to the Mediterranean Sea through the Strait of Dardanelles.
The surface water of the Black Sea and the Sea of Marmara is less saline due to the high amount of precipitation and run off. This low salinity surface water flows into the Mediterranean through the narrow straits of Bosphorus and Dardanelles. Due to the narrowness of the straits, there is high velocity flow of water into the Mediterranean Sea which causes turbulence and vertical mixing.
On the western margin of the Atlantic Ocean, the Caribbean Sea and the Gulf of Mexico together form the “American Mediterranean”. The Caribbean Sea is separated from the open ocean by the islands of Cuba, Hispaniola, Puerto Rico and Jamaica, which form the northern boundary of this sea. This chain of islands is called the Greater Antilles.
The Lesser Antilles, on the other hand, extends from the Virgin Islands to the continental shelf of South America near the Island of Trinidad. The Caribbean Sea is divided into four basins namely, Venezuela, Colombia, Cayman and Yucatan Basins. The depth of all these basins exceeds 4000 meters.
The Gulf of Mexico is a simple structure with broad continental shelf all around its margin. The maximum depth of this broad basin exceeds 3600 meters. The Yucatan Strait connects the Gulf of Mexico with the Caribbean Sea. However, the Gulf of Mexico is connected with the Atlantic Ocean only through the Strait of Florida. The depth of this strait is nearly 1000 meters.
Other marginal seas on the western margin of the Atlantic Ocean are Hudson Bay, Baffin Bay, and Davis Strait. Hudson Bay and Baffin Bay are shallow basins the major parts of which being less than 100 fathoms deep. Laying between Greenland and Baffin Island the Davis Strait connects the Atlantic Ocean with the Arctic Ocean.
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Deep-sea basins Ridges and rises
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