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Old Tuesday, November 13, 2007
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Default Geography One - Oceans

CONTINENTAL MARGIN & THE SEA FLOOR

THE RELIEF OF THE OCEAN


If all water is removed from the ocean basins, the kind of surface revealed is not to be the quiet, subdued topography as was once thought, but a surface characterized by great diversity – towering mountain chains, deep canyons and flat plains. Infact, the scenery would be as varied as that on the continents.

FATHOMOMETER, a radar sounding and electric echo device is used to determine the depth of sea
Oceanographers studying the topography of the ocean basins have delineated three major units:
i. Continental Margins;
ii. The Ocean Basin Floor; and,
iii. Mid Ocean Ridges

I. Continental Margins
The zones that collectively make up the continental margin include the continental shelf, continental slope and continental rise.

1. Continental Shelf
The continental shelf is a gently sloping submerged surface extending from the shoreline towards the deep ocean basin. Since it is flooded by continental type crust, it is clearly a flooded extension of the continents. According to Goh Cheng Leong the shelf is in fact the seaward extension of the continent from the shoreline to the continental edge.
The depth of the continental shelf is conventionally taken as 100 fathoms (200 meters) but it is variable. Its width varies greatly. Off the coast of Ireland it stretches westward for a distance of 50 miles and off Siberia it reaches a maximum width of 800 miles. But it may be only a few miles, or it may be absent altogether. On the average, the continental shelf is about 80 km wide and 130 m deep at the seaward edge.
The width or extent of the shelf is determined by the adjoining landforms. In the case of plain coastal area, the shelf will be several miles in width. While if the coast is hilly and mountainous the continental shelf will be narrow.
The average inclination of the continental shelf is less than 1/10th of 1o, a drop of only about 2 m/km, (10 ft/mile), but seldom it is more than 2 or 3o (average), e.g. Ireland=5o, Cape Torinana=36o. The slope is so slight that it would appear to an observer to be a flat surface.

Origin of Continental Shelf
When compared with many parts of the deep ocean floor, the surface of the continental shelf is relatively feature less. This is not to say that the shelves are completely smooth. The most profound features are long valleys running from the coastline into deeper waters. Many of these valleys are seaward extensions of sea valleys on the adjacent landmass. Such valleys were excavated during the Pleistocene Epoch (Ice Age). During this time, great quantities of water were tied up in vast ice-sheets on the continents, causing sea level to drop by 90-120 m and exposing large portions of the continental central shelves. Due to this drop in sea level, rivers extended their courses, and land dwelling plants and animals inhabited the newly exposed portions of the continents. Today, these areas are again covered by the sea and inhabited by marine organisms. Dragging along the eastern coast of North America has produced the remains of numerous land dwellers, including mammoths, mastodons and horses. Bottom sampling has also revealed that fresh water peal bags existed, adding to the evidence that the continental shelves were once land areas.
Some smaller continental shelves may be formed by Wave Erosion or by the Deposition of land-derived or river-borne materials on the offshore terrace.
Shelves formed by erosion are found near Iceland and Faeroe Islands, Denmark. The breadth of erosional platform will depend upon:
i. the resistance of rocks;
ii. the strength of the waves and currents; and,
iii. the length of time during which the level of land and sea remains unchanged.
Glacial erosion has played a large part in the formation of shelves surrounding glaciated countries. These shelves exhibit the characteristic features – basins and troughs – resembling terrestrial features produced by ice. Such deposits have been modified by current or wave erosion. F. P. Shepard (Submarine Geologist) concludes;
“The most clear cut history of shelf development is found around the glaciated areas. The shelves in these places appear to have been greatly deepen specially on the inside by the movement of glaciers”.
The broken material due to the action of rain, rivers and waves of the sea also settles near the land. Off the mouth of Amazon it is said that the sea is sometimes discolored by mud at a distance of 300 miles. But still even the largest rivers must deposit most of their burden near the shore. The waves and currents may widen the settling area. The material derived from land accumulates as Sub-Marine Terrace upon the margins.
Beyond the edge of the continental terrace, the waves and currents cease to be effective. But as the terrace is built up, the water becomes shallower and the action of the transporting agents becomes more effective and they are able to carry the material farther than before. The terrace therefore grows gradually outwards by addition at its edge, in the same fashion as a railway embankment or the tip-heaps of a quarry.
Delta Growth is locally also an important factor in the formation of the shelf, e.g., Mississippi Delta. According to this view the continental shelf and slope are formed by deposits brought from the land. The deposition is smoothened and redistributed by waves and currents. The action of currents in general is said to cease at about 600ft. under the surface and as a result the shelf is usually found at this depth.
Shepard suggests that in some cases a combination of erosion and deposition may have produced wide shelves. Recent geophysical work on the origin of the shelf suggests that they are in part Gigantic Prism-Shaped Accumulation of sediments perhaps several thousand meters in thickness.
Significance
The continental shelves represent 7.5% of the total ocean area, which is equivalent to about 18% of the earth’s total land area. These areas have taken an increased economic and political significance.
They have been found to be sites of important mineral deposits, including large reservoirs of petroleum and gas (nearly 20%), as well as huge sand and gravel deposits.
The shallowness allows the sunlight to penetrate through the water, which encourages the growth of minute living organism. The plankton are a source of food to fishes. The waters of the continental shelf contain many important fishing grounds that are significant sources of food. For example the Grand Banks of New Foundland, the North Sea and the Sunda Shelf.
Their limited depth and gentle slope keep out cold undercurrents and increase the height of tides. This hinders shipping as ship can only enter and leave port on the tide. World’s greatest seaports are located on continental shelves, e.g., London, Singapore, Hong Kong.

2. Continental Slope
Making the seaward edge of the continental shelf is the continental slope, which leads into deep water and has a steep gradient compared to the continental shelf, i.e., about 1 in 20 (2o-5o), and extend upto 3,660 m. While the gradient varies from place to place, it has an average drop of about 70 meters/km (370 ft/mile). The continental slope represents the true edge of the continent.
Along some mountainous coasts, the continental slope descends abruptly into deep ocean trenches, which intervenes between the continent and ocean basin. In such cases, the shelf is very narrow or does not exist at all. The sides of this continental slope and the trench are essentially the same feature and grade into the adjacent mountains, which tower thousands of meters above sea level. This situation occurs along the West Coast of South America. Here the vertical distance from the high-peaks of the Andes Mountains to the floor of the deep Peru-Chile Trench, bordering the continent, exceeds 12,200 m (40,000 ft).
Perhaps the most significant features associated with continental slopes are deep, steep sided valleys known as Submarine Canyons. Originating on the outer continental shelf or on the continental slope, submarine canyons may reach water depths of 3 km. Although some appear to be seaward extension of valleys that were carved on the continental shelf during the Ice Age, others are not oriented in this manner. Further more, the canyons reach depths far below the maximum lowering of sea level, which indicates that they were created in some process that operates below the ocean surface. Most available information seems to favor the view that these deep scars on the continental slope are excavated largely by turbidity currents, which are dense slurries of sediment and water that flow downslope.

3. Continental Rise
In regions where trenches does exist, the steep continental slope merges into a more gradual incline known as continental rise. Here the gradient lessens to between 4 and 8 meters per kilometer (not more than 1o). It becomes very flat towards the deep sea floor and virtually merges with the Abyssal Plain. While the width of the continental slope averages 20 km. The continental rise may reach for hundreds of kilometers. This feature consists of a thick accumulation of sediments that moved downslope from the continental shelf to the deep ocean floor. More specifically, the sediments comprising the continental rise are delivered to the base of the continental slope by turbidity currents that follow submarine canyons. When these muddy currents emerge from the mouth of a canyon onto the relatively flat ocean floor, they deposit sediment that forms Deep Sea Fan. Deep sea fans have the same basic shape as alluvial fans, which form at the foot of steep mountain slopes on land. As fans from adjacent submarine canyons grow, they coalesce to produce the continuos apron of sediment at the base of the continental slope.

II. Ocean Basin Floor

Between the continental margin and oceanic ridge system, lies the ocean basin floor. The size of this region almost 30% of the earth’s surface is roughly comparable to the percentage of the surface that projects into the sea as land. Here we find ocean trenches, which are dramatically deep grooves in the ocean floor; remarkably flat regions, known as Abyssal Plains; and steep sided volcanic peaks, called Seamounts.

1. Deep Ocean Trenches
Deep ocean trenches are long, relatively narrow features that represent the deepest parts of the ocean. Several in the western Pacific approach or exceed depths of 10,000 meters and atleast a portion of one, the Challenger Deep in the Mariana Trench, southwest of the island of Guam is more than 11,000 m (39,960 ft.) below sea level.
Although deep ocean trenches represent only a very small portion of the ocean floor area, they are nevertheless very significant geological features. Trenches are the sites where moving crustal plates are destroyed as they plunge back into the mantle. In addition to the earthquakes created, as one plate descends beneath another, igneous activity also associated with trench regions. Trenches in the open ocean are paralleled by volcanic island areas, while volcanic mountains, such as the Andes, may be found paralleling trenches that are adjacent to the continents. Melting a descending plate produces the molten rock that leads to this igneous activity. Other examples of ocean deeps, which are around the shores of Pacific are; Mindanao Deep (35,000 ft.), Tonga Trench (31,000 ft.) and Japanese Trench (28,000 ft.); whereas the highest peak on land, Mt. Everest is only 8,848 meters (29,028 ft.).

2. Abyssal Plains
Abyssal plains are incredibly flat features, infact, these regions are likely the most level places on the earth. The abyssal plains found off the coast of Argentina, for example, have less than 3 meters (10 feet) of relief over a distance exceeding 1300 km (300 miles). The average depth of an abyssal plain is about 3,000-6,000 m and it covers nearly 40% of the ocean. The monotonous topography of abyssal plains will occasionally be interrupted by the protruding summit of a buried volcanic structure.
By employing seismic profiler instruments whose signals penetrate far below the ocean floor, the researchers have shown that abyssal plains consist of thick accumulations of sediments that were deposited atop the low, rough portions of the ocean floor. The nature of the sediment indicates that these plains consist of sediments transported far out to sea by turbidity currents. The turbidity deposits are inter-bedded with sediments composed of minute clay sized particles that continuously settle onto the ocean floor.
Abyssal plains are found as part of the sea floor in all of the oceans. However, they are more wide spread where there are no deep ocean trenches adjacent to the continents. Since the Atlantic Ocean has fewer traps to act as traps for the sediments carried down the continental slope, it has more extensive abyssal plains than the Pacific.

3. Seamounts
Dotting the ocean floor are isolated volcanic peaks called seamount that may rise hundreds of meters above the surrounding topography. These steep-sided conical peaks are found on the floors of all the oceans, but the greater number and density have been identified in the Pacific.
Many of these undersea volcanoes begin to rise near oceanic ridges, divergent plate boundaries where the plates of the lithosphere move apart, they continue to grow as they ride along on the moving plain. If the volcano rises fast enough, it emerges as an island. Examples in the Atlantic include the Azores, Ascension, Tristan de Cunha, and St. Helena. During the time when they existed as islands, some of these volcanoes are eroded to near sea level by running water and wave action. Over a span of millions of years, the islands gradually sink as the moving plate slowly carries them from the oceanic ridge area. These submerged, flat topped seamounts are called Guyots. In other instances guyots may be remnants of eroded volcanic islands that were formed away from the ridge crest, possibly by hot spot activity. Here subsidence occurs after the volcanic activity ceases and the sea floor cools and contracts.

III. Mid Ocean Ridges

The ocean floor’s continuity is interrupted by islands rising above the sea level. These are of various types according to their origin:
Volcanic: made of basaltic rocks, e.g., Hawaii, Pacific Coral Islands
Continental Fragments or Sundered Islands: Detached from parent island masses, e.g., Madagascar, New Foundland.
Orogenic Islands or Island Areas, e.g.; islands along the Asiatic corner of the Pacific Ocean.
Mid ocean ridges are found in all major oceans and represent more than 20% of the earth’s surface. They are certainly the most prominent topographic features in the oceans for they form an almost continuous mountain range, which extends for about 65,000 km. In a manner similar to seam on a baseball.
Although ocean ridges stand high above the adjacent deep ocean basins, they are much different from the mountains found on the continents. Rather than thick sequences of folded and faulted sedimentary rocks, oceanic ridges consist of layer upon layer of basaltic rock that have been faulted and lifted.
The term “Ridge” may also be misleading since these features are not narrow, but have widths from 500 to 5,000 km and, in places, may occupy as much as ½ of the total ocean floor area. Ridge crests are marked by deep clefts or rifts (cracks), and are flanked by ridges and lines of peaks that extend outward for hundreds of kilometers. Axes of the ridges are marked by frequent earthquakes and characterized by a much higher heat flow through the crust. The rifts at the center of the ridges are the sites where the new magma wells-up from the Asthenosphere below, continually creating new oceanic crusts. The rifts, therefore, represent divergent plate boundaries where sea floor spreading is taking place.
The primary reason for the elevated position of a ridge system is the fact that newly created oceanic crust is hot, and therefore occupies more volume than cooler rocks of the deep ocean basin. As the young lithosphere travels away from the spreading center, it gradually cools and contracts. This thermal contraction accounts in part for the greater ocean depths that exist away from the ridge. Almost 100 million years must pass before cooling and contraction cease completely. By this time, rock that was once a part of a majestic mountain system is located in the deep ocean basin where it is mantled by thick accumulations of sediments.
Echo soundings have revealed scores of underground mountains. These mountains in Pacific are mostly of a volcanic origin. Many have flat tops, which were eroded by waves at the surface before sinking to their present depths.
Down the middle of the Atlantic runs a formidable mountain range, the “Mid Atlantic Ridge”, sometimes identified as the lost continent of Atlantic.
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MOVEMENT OF THE OCEAN WATER

INTRODUCTION

The water of the ocean is never still; it is blown into waves by the winds; it rises and falls with the rides; and in many places there are definite currents either permanently in one direction, or changing with tide or with the season.
The water of the ocean is always being disturbed or being acted upon by certain factors, which give it movement. As a matter of fact, all these forces that come to play upon the water of the ocean, disturb its equilibrium and nature restores the equilibrium by giving movement to ocean waters.
The movement of ocean waters from one part to another part takes place due to the effects of the earth’s rotation, wind difference, salinity and density of sea waters.

MOVEMENTS

The water movement of the oceans is mainly horizontal, following the density stratification. It is only in high latitudes that there is fairly direct transference between surface and bottom. It is convenient to consider the circulation in two parts, viz. That of the upper layer and that of the lower layer.

1. Upper Layer

a. Winds
The movements of upper layers are mainly generated by the prevailing winds. The drag exerted by the winds blowing steadily across the ocean causes the surface layer of water to move. Thus, because winds are the primary driving force of surface layers, there is a relationship between the oceanic circulation and the general atmospheric circulation. When winds change direction, the surface layer movements also reverse direction.
The most regular winds at the surface of the earth are the south-east trade winds which towards Equator over the Atlantic Pacific and Indian Ocean and north-east trade winds which blow towards the Equator over the Atlantic and Pacific oceans. In the India Ocean, north of the equator, the north-east monsoon blows in winter and the south-east monsoon in summer. In the higher southern latitudes of all three oceans, the west winds blow continuously.
The trade winds generate the north and south equatorial currents, both flowing towards the west.

b. The Earth’s Rotation

Although winds are important in generating surface movements, other factors also influence the movement of ocean waters. The most significant of these is the earth’s rotation.
The earth rotates from west to the east at a tremendous speed. As a result of this, waters of the earth’s surface, particularly that of the middle part of the equatorial region, move from the east to the west.
In considering movements in the oceans, the effect of the rotation of the earth must be considered. This force, known as Coriolis Force, does not affect a particle at rest; but when the particle moves, its deflecting effect is proportional to the speed of the movement. Its value is zero at the equator, but increases towards the poles.
Due to the earth’s rotation, currents are deflected to the right of their path of motion in the Northern Hemisphere, and to the left in the Southern Hemisphere.
The Coriolis Effect is greater in high latitudes and diminishes toward the Equator. As a consequence, the direction of surface current does not coincide with the wind direction. In general the difference between wind direction and surface current direction varies from about 15o along shallow coastal areas to a maximum of 45o in the deep oceans.

2. Deep Water Movement

The circulation of deep water is maintained by changes of temperature and salinity taking place at the surface as well as by differences of wind stress.

a. Density
Unlike the wind-induced movements of surface and near-surface waters, deep ocean movement is governed by gravity and driven by density difference.
Two factors – temperature and salinity – are most significant in creating a dense mass of water. Sea water becomes denser with decreased temperature, increased salinity, or both. Consequently deep ocean circulation is called Thermohaline Circulation. Arctic and Antarctic represent the two major regions where dense water masses are created. Antarctic water is chilled in the winter. The temperatures here are low enough to form sea ice, and since salts are excluded from ice, the remaining water becomes saltier. The result is the densest water in all of the oceans.
The waters of the equatorial region are light, less dense because of high evaporation and high temperature, on the other hand, the density of sea water is quite high in the cold regions because of the low temperature and low evaporation. So waters of this region are heavy. In order to remove the density difference of these two regions, a circulation of water takes place.

b. Salinity
Salinity increases from one part of the ocean to another. It is quite possible that water with low salinity could be lighter than water with high salinity. This may result in a natural circulation of water.
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SALINITY OF THE OCEANS

Almost every known chemical element can be found in varying proportions in the oceans; whose most characteristic feature is the Salinity. It is the degree of saltiness of the ocean water. It is usually expressed as parts per thousand (ppt) or in % variations.
All sea water contains large amounts of dissolved mineral matter of which sodium chloride (NaCl) alone constitutes more than 77%. The other more important compounds include magnesium, calcium and potassium. Due to the free movement of ocean water, the proportions of different salts remain remarkably constant in all oceans and even to great depths. But the degree of concentration of the salt solution or salinity does vary appreciably in different areas.

DISTRIBUTION OF SALINITY

The distribution of salinity is expressed by “ISO-HALINES”, lines joining places having an equal degree of salinity.
Generally speaking the average salinity of the oceans is 35.2 per thousand, about 35 parts of salt in thousand parts of water. According to Dittmar, the composition is as:

Sodium Chloride--------NaCl------27.2 g
Magnesium Chloride----MgCl2-----3.8 g
Magnesium Sulphate----MgSO4---1.6 g
Calcium Sulphate-------CaSO4---1.26 g
Potassium Sulphate-----K2SO4---0.8 g
Calcium Carbonate------CaCO3---0.123 g
Magnesium Bromide-----MgBr2----0.07 g
Total-----------------------------35 g

The degree of concentration of salts varies appreciably in different areas. The variation is due to three causes:
1. Supply of fresh water
2. Rapidity of fresh water
3. Degree of water mixing by current

Open Seas

Since warm water will dissolve more of a given substance than cold water it would be expected that the areas of greatest salinity would be near the equator. This is not so sue to following reasons:

1. In the equatorial areas, the rainfall is heavy and occurs almost daily, and the relative humidity of the atmosphere is high so that there is little evaporation. In addition, there are rivers of large volume (e.g., Congo, Zaire and Amazon), which constantly supply fresh water. As a result, the salinity is not high but below normal, i.e., below 35 per thousand.

2. North and south of the equatorial area are the trade wind belts (trade winds are drying winds). Therefore, evaporation is rapid in these latitudes (about 20o to 30o north and south of the equator). On the land, in these latitudes, are the great deserts, sol that there are relatively few rivers to add a supply of fresh water. In these belts, the salinity rises to 37 per thousand and over. Where really large rivers enter the sea in these belts, the salinity is lower, viz. at the mouth of the Zambesi, the Ganges, the Mississippi and the rivers of Indo-China.

3. Proceeding polewards from the trade wind belt, the salinity gradually decreases until, in the poleward sections of the Arctic Ocean, it is only from 20 to 30 per thousand. Here there is less rapid evaporation, more rain, more rivers and a large supply of fresh water from melting ice.

Partially Enclosed Seas

The Mediterranean Sea has a very high salinity, i.e., 40 per thousand. It is partially enclosed and its waters do not circulate freely with those of the open ocean. It is in a region of rapid evaporation, particularly during the summer, when too many of the rivers become almost dry. The one really large river, the Nile, adds less and less fresh water year by year because of the increased demands for irrigation purposes. For similar reasons, very high salinities are found in Red Seas and Persian Gulf. In Red Sea, the average salinity increases to 39 per thousand.
The Baltic Sea contrasts with the Mediterranean. It, too, is nearly enclosed, but it is a cooler region, with a lower rate of evaporation. Some large rivers (Oder, Vistula) empty into the Baltic, and numerous streams flow from the snow-clad mountains of Scandinavia, bringing a constant supply of fresh water. As a result the salinity is only 2 per thousand, and the waters nearly fresh. One of the important results of this low salinity is that the Baltic Sea freezes readily.
The Baltic, Arctic and Antarctic waters have a salinity of less than 32 per thousand because of the colder climate with little evaporation and because much fresh water is added from the melting of icebergs, as well as by several large poleward-bound rivers, e.g., Ob, Lena, Yenisey and Mackenzie.

Inland Seas and Lakes

Enclosed seas which are areas of inland drainage, such as the Caspian Sea, the salinity is very high, 180 per thousand, and in the Dead Sea of Israel / Jordan, a salinity of 250 per thousand has been recorded. This is due to several features:
1. Temperature in the Dead Sea region are high
2. The rainfall and atmospheric humidity is low
3. The lake receives water from only a small drainage area
4. The rate of evaporation is high

The highest salinity is perhaps, that of Lake Van, in Turkey, with 330 per thousand. It is a Salt Lake, and salts are collected from its shores. The density of the water is so high that in Lake Van or the Dead Sea, it is almost impossible to sink. Beginner-swimmers will find it much easier to float here than any where else!
In enclosed seas and lakes, the composition of the dissolved salts is not the same as ordinary sea water.
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OCEAN CURRENTS

ATLANTIC OCEAN

Atlantic Ocean, the second largest of the earth's four oceans and the most heavily traveled. Only the Pacific Ocean is larger. It covers about twice the area of the Atlantic Ocean. The Atlantic is divided into two nominal sections: The part north of the equator is called the North Atlantic; the part south of the equator, the South Atlantic. The ocean's name is derived from Atlas, one of the Titans of Greek mythology.

BOUNDARIES AND SIZE

The Atlantic Ocean is essentially an S-shaped north-south channel, extending from the Arctic Ocean in the north to the Antarctic continent in the south and situated between the eastern coast of the American continents and the western coasts of Europe and Africa. The Atlantic Ocean proper has a surface area of about 82 million sq km (about 31,660,000 sq mi). Including its marginal seas—the Gulf of Mexico-Caribbean Sea, the Arctic Ocean, and the North, Baltic, Mediterranean, and Black seas—the total area is about 106,190,000 sq km (about 41 million sq mi).
The boundary between the North Atlantic and the Arctic Ocean is arbitrarily designated as lying along a system of submarine ridges that extend between the land masses of Baffin Island, Greenland, and Scotland. More clearly defined is the boundary with the Mediterranean Sea at the Strait of Gibraltar and with the Caribbean Sea along the arc of the Antilles. The South Atlantic is arbitrarily separated from the Indian Ocean on the east by the 20° east meridian and from the Pacific on the west along the line of shallowest depth between Cape Horn and the Antarctic Peninsula.

GEOLOGIC FORMATION AND STRUCTURAL FEATURES

The Atlantic began to form during the Jurassic period, about 150 million years ago, when a rift opened up in the supercontinent of Gondwanaland, resulting in the separation of South America and Africa. The separation continues today at the rate of several centimeters a year along the Mid-Atlantic Ridge. Part of the midoceanic ridge system that girdles the world, it is a submarine ridge extending north to south in a sinuous path midway between the continents. Roughly 1500 km (about 930 mi) wide, the ridge has a more rugged topography than any mountain range on land, and is a frequent site of volcanic eruptions and earthquakes. The ridge ranges from about 1 to 3 km (about 0.6 to 2 mi) above the ocean bottom.
Along the American, Antarctic, African, and European coasts are the continental shelves—embankments of the debris washed from the continents. Submarine ridges and rises extend roughly east-west between the continental shelves and the Mid-Atlantic Ridge, dividing the eastern and western ocean floors into a series of basins, also known as abyssal plains. The three basins on the American side of the Mid-Atlantic Ridge are more than 5000 m (more than 16,400 ft) deep: the North American Basin, the Brazil Basin, and the Argentina Basin. The Eurafrican side is marked by several basins that are smaller but just as deep: the Iberia, Canaries, Cape Verde, Sierra Leone, Guinea, Angola, Cape, and Agulhas basins. The large Atlantic-Antarctic Basin lies between the southernmost extension of the Mid-Atlantic Ridge and the Antarctic continent.
The Atlantic Ocean has an average depth of 3926 m (12,881 ft). At its deepest point, in the Puerto Rico Trench, the bottom is 8742 m (28,681 ft) below the surface.

ISLANDS

The largest islands of the Atlantic Ocean lie on the continental shelves. Newfoundland is the principal island on the North American shelf; the British Isles are the major island group of the Eurafrican shelf. Other continental islands include the Falkland Islands, the only major group on the South American shelf, and the South Sandwich Islands on the Antarctic shelf.
Oceanic islands, usually of volcanic origin, are less common in the Atlantic Ocean than in the Pacific. Among these are the island arc of the Antilles (including Puerto Rico, Hispaniola, Jamaica, and Cuba). In the eastern Atlantic, the Madeiras, Canaries, Cape Verde, and the São Tomé-Príncipe group are the peaks of submarine ridges. The Azores, Saint Paul's Rocks, Ascension, and the Tristan da Cunha group are isolated peaks of the Mid-Atlantic Ridge system; the large island of Iceland is also the result of volcanic action at the Mid-Atlantic Ridge. Bermuda rises from the floor of the North American Basin, and Saint Helena from the Angola Basin.

CURRENTS

Introduction
The circulatory system of the surface waters of the Atlantic can be depicted as two large gyres (turns), or circular current systems, one in the North Atlantic and one in the South Atlantic. These currents are primarily wind driven, but are also affected by the rotation of the earth. The currents of the North Atlantic, which include the North Equatorial Current, the Canaries Current, and the Gulf Stream, flow in a clockwise direction. The currents in the South Atlantic, among which are the Brazil, Benguela, and South Equatorial currents, travel in a counterclockwise direction. Each gyre extends from near the equator to about latitude 45°; closer to the poles are the less completely defined counterrotating gyres, one rotating counterclockwise in the Arctic regions of the North Atlantic and one rotating clockwise near Antarctica in the South Atlantic.
The Atlantic receives the waters of many of the principal rivers of the world, among them the Saint Lawrence, Mississippi, Orinoco, Amazon, Paraná, Congo, Niger, and Loire, and the rivers emptying into the North, Baltic, and Mediterranean seas. Nevertheless, primarily because of the high salinity of outflow from the Mediterranean, the Atlantic is slightly more saline than the Pacific or Indian oceans.

North Atlantic Current

At the shoulder of Brazil, the Cayenne Current and the Brazilian Current arise from the South Equatorial Current.
The Cayenne Current is joined by the North Atlantic Current at Caribbean Sea. It emerges from the Florida Strait as the Florida Current. Rest of the equatorial water flows northwards to join the Gulf Stream off the southeastern USA.

Gulf Stream, warm current of the North Atlantic Ocean, flowing in a generally northeastern direction from the Straits of Florida to the Grand Banks, east and south of Newfoundland. The term is often extended to include the North Atlantic Drift, which flows from the Grand Banks to the shores of western Europe, Scandinavia, and the islands of the Arctic Ocean. The Gulf Stream is of great climatological importance because of its moderating effects on the climate of western Europe.

The sources of the Gulf Stream are the two equatorial currents: the North Equatorial Current, which flows west, roughly along the tropic of Cancer; and the South Equatorial Current, which flows from the coasts of southwestern Africa to South America and then north into the Caribbean. The fusion of these two warm currents and a certain amount of water from the Gulf of Mexico forms the Gulf Stream.

In the straits that separate Florida from the Bahamas and Cuba, the Gulf Stream has a maximum width of about 80 km (about 50 mi) and a depth of about 640 m (about 2100 ft). The surface temperature is about 25° C (about 77° F) and the surface current averages about 5 km/h (about 3 mph). Farther north the stream gradually widens and is approximately 480 km (about 300 mi) wide off New York. Between the stream and the northeastern coast of the United States lies an area of colder water, sometimes called the Cold Wall.

The Gulf Stream is deflected eastwards at Cape Hatteras (35oN) under the influence of the Westerlies and the earth rotation. It reaches Europe as the North Atlantic Drift.

This current with the speed of approximately 1 km/hr. carries the warm water for over 1,000 km to the coasts of Europe. From here it divides in three parts; one going northwards to the Arctic, one going eastwards to Britain, and another on southwards along the Iberian Coast as the cool Canaries Current.
Researches shows that almost 2/3rd of Gulf Stream warm water is returned by dense, cold polar water through creeping.

The Canaries Current merges with North Equatorial Current, thus completing the cycle. An area in the middle of the Atlantic has no perceptible current. A large amount of floating sea weeds gather and the area is called Sargasso Sea.

There are also currents that enter North Atlantic from the Arctic Regions. These cold waters are blown south by the out flowing polar winds. Irminger Current or East Greenland Current flows between Iceland and Greenland and cools the North Atlantic Drift at the point of convergence. The cold Labrador Current drift southeastwards between West Greenland and Baffin Island to meet the warm Gulf Stream of New Foundland. From here it divides into two branches; the smallest one called Cobot Current enters the St. Lawrence River and the Strait Current curves southwestwards to join the Gulf Stream. At 50oN, the icebergs carried south by the Labrador Current melts.

All the way north from its source to the region of the Grand Banks, the Gulf Stream has special physical characteristics, including a markedly bright blue color and a high salinity. After the Gulf Stream mixes with the Labrador Current, the characteristic color is lost, but the water of the North Atlantic Drift remains markedly salty.

South Atlantic Current

The same pattern is followed but the direction is anti-clockwise. The collection of sea weeds is not so distinctive. The South Equatorial Current moves south as the Brazilian Current. Its blue waters are easily distinguishable from the muddy waters of Amazon. At about 40oS, the influence of the Westerlies and the rotation of the earth propel the current to merge with the west wind drift as the South Atlantic Current. Reaching the cost of Africa, the current is diverted northwards as the Cold Benguela Current. This is driven by the southeast trade winds to join the south equatorial current. The Atlantic circulation is thus completed.

Between the north and the south equatorial currents, there is the east flowing Equatorial Counter Current. This current is the direct result of the slope produced by the northeast and southeast trade winds continually carrying water to the American side. It has been shown that the slope is about 4 cm per 1000 km. It involves a thin layer of surface water. It is best developed in the northern summer.

TEMPERATURES

The Atlantic Ocean may be described as a bed of water colder than 9° C (48° F)—the cold-water sphere—within which lies a bubble of water warmer than 9° C—the warm-water sphere. The warm-water sphere extends between latitude 50° north and latitude 50° south and has an average thickness of about 600 m (about 2000 ft). The most active circulation is found in the uppermost layer of warm water. Below this, circulation becomes increasingly sluggish as the temperature decreases.
Surface temperatures range from 0° C (32° F), found year-round at the Arctic and Antarctic margins, to 27° C (81° F) in the broad belt at the equator. At depths below 2000 m (about 6600 ft), temperatures of 2° C (36° F) are prevalent; in bottom waters, below 4000 m (about 13,200 ft), temperatures of -1° C (30° F) are common.

MARINE RESOURCES

The Atlantic Ocean contains some of the world's most productive fisheries, located on the continental shelves and marine ridges off the British Isles, Iceland, Canada (especially the Grand Banks off Newfoundland), and the northeastern United States. Upwelling areas, in which the nutrient-rich waters of the ocean depths flow up to the surface, as in the vicinity of Walvis Bay off southwestern Africa, also have abundant sea life. Herring, anchovy, sardine, cod, flounder, and perch are the most important commercial species. Tuna is taken off northwestern Africa and northeastern South America in increasing numbers. The catch per unit area is much higher in the Atlantic than in the other oceans.

A remarkable example of plant life is found in the Sargasso Sea, the oval section of the North Atlantic lying between the West Indies and the Azores and bounded on the west and north by the Gulf Stream. Here extensive patches of brown gulfweed (Sargassum) are found on the relatively still surface waters.

Actively mined mineral resources in the Atlantic include titanium, zircon, and monazite (phosphates of the cerium metals), off the eastern coast of Florida, and tin and iron ore, off the equatorial coast of Africa. The continental shelves and slopes of the Atlantic are potentially very rich in fossil fuels. Large amounts of petroleum are already being extracted in the North Sea and in the Caribbean Sea-Gulf of Mexico region; lesser amounts are extracted off the coast of Africa in the Gulf of Guinea.
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PACIFIC OCEAN

Pacific Ocean, largest and deepest of the world's four oceans, covering more than a third of the earth's surface and containing more than half of its free water. It is sometimes divided into two nominal sections: the part north of the equator is called the North Pacific; the part south of the equator, the South Pacific. The name Pacific, which means peaceful, was given to it by the Portuguese navigator Ferdinand Magellan in 1520.

BOUNDARIES AND SIZE

The Pacific Ocean is bounded on the east by the North and South American continents; on the north by the Bering Strait; on the west by Asia, the Malay Archipelago, and Australia; and on the south by Antarctica. In the southeast it is arbitrarily divided from the Atlantic Ocean by the Drake Passage along 68° west longitude; in the southwest, its separation from the Indian Ocean is not officially designated. Apart from the marginal seas along its irregular western rim, it has an area of about 165 million sq km (about 64 million sq mi), substantially larger than the entire land surface of the globe. Its maximum length is about 15,500 km (about 9600 mi) from the Bering Strait to Antarctica, and its greatest width is about 17,700 km (about 11,000 mi) from Panama to the Malay Peninsula. Its average depth is 4282 m (14,049 ft). The greatest known depth in any of the world's oceans is 11,033 m (36,198 ft) in the Mariana Trench off Guam.

GEOLOGIC FORMATION AND STRUCTURAL FEATURES

The Pacific is the oldest of the existing ocean basins, its oldest rocks having been dated at about 200 million years. The major features of the basin and rim have been shaped by the phenomena associated with plate tectonics. The coastal shelf, which extends to depths of about 180 m (about 600 ft), is narrow along North and South America but is relatively wide along Asia and Australia. The East Pacific Rise, a mid ocean ridge system, extends about 8700 km (about 5400 mi) from the Gulf of California to a point about 3600 km (about 2235 mi) west of the southern tip of South America, and rises an average of about 2130 m (about 7000 ft) above the ocean floor. Along the East Pacific Rise molten rock upwells from the earth's mantle, adding crust to the plates on both sides of the rise. These plates, which are huge segments of the earth's surface, are thus forced apart, causing them to collide with the continental plates adjacent to their outer edges. Under this tremendous pressure, the continental plates fold into mountains, and the oceanic plates downbuckle, forming deep trenches, called subduction zones, from which crust is carried back into the mantle. The stresses at the areas of folding and subduction are responsible for the earthquakes and volcanoes that give the rim of the Pacific basin the name “ring of fire”.

ISLANDS


The Pacific Ocean contains more than 30,000 islands; their total land area, however, amounts to only one-quarter of one percent of the ocean's surface area. The largest islands, in the western region, form volcanic island arcs that rise from the broad continental shelf along the eastern edge of the Eurasian Plate. They include Japan, Taiwan, the Philippines, Indonesia, New Guinea, and New Zealand. The oceanic islands, collectively called Oceania, are the tops of mountains built up from the ocean basin by extruding molten rock. The mountains that remain submerged are called seamounts. In many areas, particularly the South Pacific, the land features above the sea surface are accretions of shell material. Along the eastern edge of the Pacific, the continental shelf is narrow and steep, with few island areas. The major groups are the Galápagos at the equator, which rise from the Nazca Plate, and the Aleutians in the north, which are part of the North American continental shelf.

CURRENTS

The driving forces for ocean currents are the earth's rotation, wind friction at the surface of the water, and variations in seawater density due to differences in temperature and salinity. The interaction between wind and current has a major effect on climate and is studied for long-range weather prediction and for sea travel.
The pattern of circulation is similar to that of Atlantic except in modifications arising from the greater size and open nature of the Pacific.

North Pacific Current

The surface currents of the North Pacific consist of two gyres, or circular systems. In the extreme north the counterclockwise Sub-Arctic Gyre encompasses the westward-flowing Alaska Current and the eastward-flowing Sub-Arctic Current. The main body of the North Pacific, however, is dominated by the huge North Central Gyre, which circulates clockwise. It encompasses the North Pacific Current, flowing east; the California Current, flowing southeast; and the Kuroshio, or Japan, Current, flowing north up the coast of Japan. The California Current is cold, broad, and slow-moving; the Kuroshio is warm, narrow, and rapid, similar to the Gulf Stream. Close to the equator at 5° north latitude, the eastward-flowing Equatorial Counter Current separates the North and South Pacific systems but sends most of its waters into the North Equatorial Current.

The North Equatorial Current flows westwards with a compensating Equatorial Counter Current running in the opposite direction. The volume of water is much greater than the Atlantic Equatorial Current. Due to the action of the north-east trade winds, the North Equatorial Currents turns off the coasts of Philippines and Taiwan into the East China Sea as the Kuroshio or Kuro Siwo or Japan Current. The warm current moves towards the pole as North Pacific Drift, keeping the Alaskan coast-ice free. The cold water sinks beneath the North Pacific Drift, part of which moves towards south as cool Californian Current, along the coasts of Western USA and coalesces with the North Equatorial Current.

From the Arctic, the Cold Bering Current or Alaskan Current creeps down southwards from the narrow Bering Strait and is joined by the Okhotsk Current to meet the warm Japan Current as the Oyashio off Hokkaido.

South Pacific Current

The South Pacific is dominated by the counterclockwise-moving South Central Gyre, which encompasses the South Equatorial Current flowing east and south, the South Pacific Current flowing west, and the Mentor Current flowing north, parallel to South America. Located in the extreme south is the Antarctic Circumpolar Current (West Wind Drift), which encircles the globe, merging the waters of the Pacific, Atlantic, and Indian oceans. It is the most important source of deep-sea circulation. From it flows the broad, cold Peru, or Humboldt, Current, which travels north along the coast of South America and sends its waters into the South Equatorial Current.

The South Equatorial Current, driven by the south-east trade winds flow southwards along the coast of Queensland as East Australian Current, bringing warm equatorial water into temperate waters. Under the influence of the Westerlies, it moves towards New Zealand in the Tasman Sea and merges with the west wind drift forming the South Pacific Current. It is obstructed by the tip of Chile and turns northwards along the South American coast as the Peruvian Current. It eventually links up with the South Equatorial Current and the circulation is completed.

WIND SYSTEMS

The outstanding wind systems of the Pacific Ocean are the twin belts of Westerlies, which blow from west to east between 30° and 60° latitude, one in the northern hemisphere and one in the southern hemisphere. These winds vary in seasonal patterns. The stormy and unpredictable westerly of the North Central Pacific is being studied for its possible controlling effect on global weather patterns. Between the Westerlies are the much more steady trade winds, which move from the east in the northern hemisphere and from the west in the southern hemisphere. Violent tropical storms, called typhoons in the western Pacific and hurricanes in the southern and eastern Pacific, originate in the trade wind belt in late summer and early autumn. At the equator are the equatorial doldrums, light winds with seasonal cyclonic activity. At the highest latitudes of the Pacific, the winds have little direct effect on climate and water currents.

RESOURCES

Much of the plant and animal life of the Pacific Ocean is concentrated along its margins. Nutrient-rich waters from the deep Antarctic Circumpolar Current upwell to the surface in the Peru Current along the coast of Chile and Peru, and the area sustains a large population of anchovetas that is of great importance as a world food resource. A large guano industry has been established from droppings of the seabirds that feed upon the anchovetas. The northwestern Pacific, including the Sea of Japan (East Sea) and the Sea of Okhotsk, is another major world fishery. Coral reefs rich with sea life reach their peak in the Great Barrier Reef, which extends for about 2000 km (about 1250 mi) along the northeastern coast of Australia. Tuna is another important Pacific resource, bringing fleets of many nations in search of the schools that migrate over much of the ocean. The Pacific has also begun to be exploited for its vast mineral resources. The continental shelves off the coasts of California, Alaska, China, and the Indonesian area are known to contain large reserves of petroleum. Patches of the ocean floor are covered with “manganese nodules,” potato-sized concretions of iron and manganese oxides that sometimes also contain copper, cobalt, and nickel. Programs are under way to examine the feasibility of mining these deposits.
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EFFECTS ON WEATHER AND CLIMATE

The weather is determined largely by the ocean behavior and the relationship that exists between sea and air.

The ocean act as Thermo-Accumulator or Bank of Heat. The solar radiation is stored in the oceans and released slowly in cold times so that the climate of the adjacent land gets a tempering touch. To understand the weather one has to understand the oceans and conversely to understand the oceans one has to understand the circulation of the atmosphere.

The effect of atmosphere is evident from the fact that 90% of the surface ocean currents are wind-driven. Indeed the storage of heat in the ocean waters is due to the stirring effect caused by the winds. The study of Meteorology alone will not give a full picture of the heat and water balance of the atmosphere. Water and heat affect man the most.

The circulation of the oceans is also limited with the climate. Oceans have a capacity to retain heat in contrast to the atmosphere. The British Climate is completely dependent on the warm currents from equator, without which the equator would be much hotter and the poles much colder.

Oceans are the Blood Streams which distribute and regulate the Earth’s heat.

The drastic effect on climate of a change in ocean currents is demonstrated in Peru. The normal climate there tends to be cool, foggy, but almost rainless during a cool ocean current from the Antarctic. But every 10 years or so a warm current no more than 10 ft. deep slides down the coast brining tropical rain, flood and havoc. The current is called El Nino and occurs near 12o south of equator.

It is the Peruvian Current which renders the Chilean and Peruvian Coasts rainless. The animals and plants that attract huge shoals of fish. Consequently millions of sea birds gather here to feed on fish. Their droppings completely whiten the coastal cliffs and islands, forming thick deposits of Guano, a valuable source of fertilizer.

The Gulf Stream is so warm that its warming effect is also noticeable in the arctic ocean. It modifies the temperature of the cold regions that it washes, and does not allow the waters of the oceans to freeze and supplies the winds that blow over them with warmth and moisture so that when they reach the continents, they are able to bring enough rainfall. The British Isles and the Western Europe derive the greatest benefit from these warm currents directly through the North Atlantic Drift and indirectly through the Westerlies that reach the continent after having passed over them.

The Labrador Current brings the great fall in temperature at the eastern coast of North America. It brings the cold Arctic water which merges with the warm waters of the Gulf Stream. At this point (near New Foundland) a lot of fog is produced which is dangerous for shipping. Besides, it brings many icebergs; floating blocks of ice, which often collide with the ships and cause disasters. So because of obstruction to navigation, this is known as the “Cold Wall” near New York.

The exchange of energy between the sea and the air is not yet understood completely, and more knowledge would lead to great improvements on long range weather forecasting. The ocean currents have a controlling effect on fisheries also.
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I tried but could not upload the "currents" diagram, please find it on some online cyclopaedia.
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