INTRODUCTION
Oceans covers about 70% of the Earth’s surface. The oceans contain almost 97% of the Earth’s water supply.
The oceans of the earth are unique in our solar system. Life on earth is originated in the seas
and the oceans continue to be home to an incredibly diverse web of life.
RELIEF FEATURES OF OCEAN
- The floors of the ocean are rugged and complex with long mountain ranges, deep trenches and large plains.
- The ocean floor can be divided into four major divisions.
(a) Continental Shelf:
- The continental shelf is the gently sloping part of a continent that lies submerged below the sea.
- The shelf is generally formed due to a relative rise in sea level or marine deposition beneath the water.
- The average width of continental shelf is about 70 km, while the average depth is about 200 m.
- In all, about 7.5 percent of the total area of the oceans is covered by the continental shelves.
- The North Sea and the Baltic Sea are example of seas that lie on the continental shelf. Such seas are known as epicontinental or shelf seas.
- About 20 percent of the world production of petroleum and gas comes from shelves.
(b) Continental Slope:
- The continental slope is the comparatively steep slope that descends from the edge of the continental shelf to the deep-sea platform.
- There are five type of slopes:
(i) Fairly steep with the surface dissected by canyons,
(ii) gentle slope with elongated hills and basins,
(iii) Faulted slopes,
(iv) slopes with terraces, and
(v) slopes with sea-mounts
(c) Continental Rise:
- The continental rise apron is the gently inclined slope which leads up from the deep sea floor (abyssal zone) to the foot of the continental slope.
- In other words, where the continental slope ends, the continental rise begins.
- The continental rise has an average slope of between 0.5° to 1° and the general relief is low.
(d) Abyssal Plain:
- The abyssal plains or abyssal floors are the areas of deep-ocean floor found at a depth of 3,000 to 6,000 metres.
- Abyssal plain occupy about 40 percent of the ocean floor and are present in all major oceans and several seas of the world.
(e) Submarine Ridges:
- Submarine ridges are the mountain ranges on the sea floor that rise above the general level of the abyssal plain.
(f) Abyssal hills:
Areas of upland smaller than a mountain are found on the deep sea floor. These upland areas are called abyssal hills.
(g) Seamount:
- Seamount is an isolated mountain rising abruptly some 1,000 m from the ocean floor but without extending above sea level.
- Seamounts are of volcanic origin, and are common in the Pacific Ocean.
(h) Guyots:
- Flat-topped sea- mounts are known as guyots.
- The summits of guyots are almost entirely at depths of more than 1,000 m but rise upto 3 km from the ocean floor.
- The conical shape of the guyot suggests a volcanic origin . Lake seamounts, guyots are fund in abundance in the Pacific Ocean.
(i) Trench or deep:
- A long, narrow and steep-sided depression on the bottom of the ocean is called a trench or deep.
- Trenches or deeps are the deepest parts of the oceans with their bottoms far below the average level of the ocean floors.
- They are most common in the Pacific Ocean. Deepest trench in the world is the Mariana
- Trench in Pacific Ocean southwest of Guam, 11,033 metres below surface.
(j) Submarine Canyons:
- Submarine canyons are deep gorges on the ocean floor.
- They occur around all the coasts of the world and are mainly restricted to the continental shelf, slope and rise.
- Hudson Canyon is the best known canyon in the world.
- The largest canyons in the world occur in the Bering Sea off Alaska. They are the Bering, Pribilof and Zhemchung canyons.
(k) Bank:
- A bank is a more or less flat-topped elevation located in the continental margins
- The depth of water over a bank is relatively small.
(l) Shoal:
A shoal is a detached elevation with shallow depth. The shoal is not composed of a rock or coral.
(m) Reef:
- A reef is predominantly organic deposit made by living or dead organisms that forms a mound or a rocky elevation like a ridge.
- The largest reef in the world is found off the Queensland coast of Australia.
OCEAN FACTS
- The shape of the Pacific Ocean is roughly triangular with its apex in the north at the Bering Strait.
- The Northern Pacific is the deepest part of the Pacific Ocean.
- Among the trenches of the Pacific Ocean, the Mariana off the Guam Islands is the deepest.
- The Atlantic Ocean resembles the letter’S in shape.
- The most striking feature of the Atlantic Ocean is the presence of the mid-Atlantic Ridge,
- which divides the Atlantic into two deeper basins on either side.
- The Indian Ocean may be considered a half ocean, because, unlike the Pacific and the Atlantic,
- it does not open out northward into the Atlantic Ocean.
- Mauritius and Reunion Island to the east of Madagascar are of volcanic origin.
- Most of the islands of the Pacific Ocean are of volcanic or coral origin.
- The name ‘Pacific Ocean’ was coined, by Ferdinand Magellan.
- Of all the oceans, the Atlantic Ocean has the longest coastline.
OCEAN WATER AND THEIR CIRCULATION
- The temperature and salinity are two important properties of the ocean water,
- which determine the movements of large masses of water, their characteristics and the types, and also the types of marine flora and fauna.
- Temperature decreases according to the increasing depth of the ocean.
- In general, temperature in ocean waters varies from below –5° C to over 33° C.
- The annual range of temperature is greater in the Atlantic than in the Pacific Ocean due to the difference in their size.
- The amount of salinity also varies from one part of the ocean to another. The areas of the highest salinity are found near the tropics.
- The salinity decreases both towards the equator and towards the poles.
- In the polar seas, there is less salinity - between 20 percent and 32 percent- due to very low amount of evaporation.
(a) Movements in the Oceanic Water:
The movement of ocean water takes place in three different way: waves, currents and tides.
(i) Ocean Waves:
- Waves are oscillatory movements in water, mainfested by an alternate rise and fall of the sea surface.
(ii) Ocean Currents:
- The ocean currents are the most important of the movements in the oceanic water.
- Ocean currents can broadly be divided into two types:
- (i) warm current, and (ii) cool or cold currents.
- Currents that flow from equatorial regions to poles have a higher surface temperature and are warm currents.
- Those currents that flow from polar regions to equator have a lower surface temperature and are cold currents.
- In the Northern Hemisphere, the currents move to their right and in the Southern Hemisphere to their left.
- However, as an exception, in the northern part of the Indian Ocean,
- the direction of currents changes in response to the reversal of the monsoon winds.
- Warm currents move towards the cold seas and cold currents towards the warm oceans.
- The direction of currents in the Northern and Southern Hemispheres is determined by Coriolis force or Ferrel’s law.
(a) Pacific Ocean:
(i) North Equatorial Current:
- A warm current, it flows westwards in the north of the equator.
- This current is produced by the north-east trades.
(ii) South Equatorial Current:
- A warm current, it flows westwards in the south of the equator.
- This current is produced by the south-east trades.
(iii) Kuro Siwo Current:
- The north-east trade winds below the north equatorial current off the coasts of the
- Philippines and Formosa into the East China Sea as the Kuro Siwo or Kuroshio or Japan current.
- It is a warm current.
(iv) North Pacific Current:
- From the south-east coast of Japan, the current flows right across the ocean from west to east as the North Pacific current.
- North Pacific current is a warm current.
(v) Alaska Current:
- A cold current, Alaska flows flows anti-clockwise along the coast of British Columbia and Alaska.
- This current is a branch of the North Pacific current.
(vi) Californian Current:
- A cold current, the Calcifornian current flows southwards along the Pacific coastline of the USA.
(vii) Peru or Humboldt Current:
- Reaching the south western coast of South America, the South Pacific Current turns northward as Peru current.
- It is a cold current.
(viii) Centre Equatorial Current:
- Between the North and the South Equatorial Currents, a current flows from west to east which is known as the Counter Equatorial Current.
- It is a warm current.
(ix) Oya Siwo or Oyashio Current:
The Cold Alaskan current or Bering current creeps southwards from the narrow
Bering strait and is joined by the Okhotsk current (a cold current) to meet the warm Japan current as the Oya Siwo or Oyashio (a cold current).
(b) Atlantic Ocean:
- In the Atlantic Ocean, the steady trade winds in north and south of the equator drive two streams of surface water westward.
- They are known as North and South Equatorial Currents. They are warm currents. Other important currents of the Atlantic Ocean are as follows:
(i) Guinea Current:
A current called the Equatorial Counter Current flows from west to east betwee
the two main equatorial currents. It is known as the Guinea current off the West African coast.
(ii) Brazil Current:
The south Equatorial Current is split into two branches near Cape de Sao Roque (Brazil). One branch turns south as the warm Brazilian current.
(iii) Gulf Stream:
- It is a warm current of sea water. It originates in the eastern
- Gulf of Mexico before flowing past Florida and the eastern seaboard of the USA.
- The warm Gulf stream combines with the prevailing south-westerly
- winds to produce the temperate climate of north-west Europe and keeps the Norwegian coast ice-free during the winter months.
(iv) Canary Current:
- It is a cold current which flows southwards past Madeira and the Canaries off the Atlantic coast of North Africa.
(v) Labrador Current:
- The Labrador current (warm) flows southwards along part of the east coast of Canada and meets the warm Gulf Stream.
(vi) Benguela Current:
- It is a cold ocean current flowing northwards off the coast of South West Africa.
(vii) Florida Current:
It is one of the important ocean currents of the North Atlantic commences as a branch of the North Equatorial Current,
which enters the Caribbean and then returns to the Atlantic Ocean through the Florida Straits as the Florida current.
(c) Indian Ocean:
The current circulation in the Indian Ocean is different from those of the Atlantic and the Pacific Oceans Important Indian Ocean currents are:
(i) South-West Monsoon Drift:
In summer, when the dominant wind is the south-west monsoon,
the currents are blown from a south-westerly direction as the South-West Monsoon Drift.
(ii) North-East Monsoon Drift:
In winter, the dominant northeast monsoon blows the currents from the north-east as the North-East Monsoon Drift.
(iii) West Wind Drift:
- A cold current, the West Wind Drift moves across the ocean in the higher latitudes from west to east.
