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Chapter 17. Oceanography (Geography Notes)


Oceans and Seas
➤ The oceans comprise more than 70.1% of the earth’s surface.
➤ They exert a great influence upon the global and regional climates.
➤ They help to modify the distribution of temperature along the coastal areas.
➤ They help to sustain the hydrological cycle.
➤ Oceans are the repositiories of a large number of useful metallic and non-metallic minerals such as petroleum, gas, common salt, gold, diamond etc.
➤ The sea, as a biological environment is, the easiest available and an inexhaustible source of food and other products of value to man.
➤ Of all the marine resources, fish is the most abundant and important.
Sodium Chloride (Nacl) – 77.8%
Magnesium Chloride – 10.9 % (MgCl2)
Magnesium Sulphate (MgSO4) – 4.7%
Calcium Sulphate – 3.6% (CaSO4)
Potossium Sulphate – 2.5% (K2SO4)
Chlorine 18.97 > Sodium 10.47 > Sulphate 2.65 > Magnesium 1.28 > Calcium 0.41 > Potassium 0.38 > Bicarbonate 0.14 > Bromine 0.06 > Borate 0.02

Ocean Relief

Continental Shelf : It is the seaward extension of the continent from the shoreline to the continental edge.
➤ The continental shelves provide the richest fishing ground in the world e.g. the Grand Banks off Newfound land, the North sea and the Sunda shelf.
➤ The shelves are also potential mining sites for minerals.
➤ A significant proportion of the world production of petroleum and natural gas comes from them.
➤ The average width of shelves is about 70 km and the mean slope is less 1.
Continental slope
➤ It lies at the edge of the continental shelf, generally up to the depth of 2000 fathoms (3660 mt.) from the mean sea level.
➤ It has a steep slope with the angle of slope varying from 2° to 5°.
➤ It covers about 8.5% of the total oceanic area and individually about 12.4% of the Atlantic Ocean, 7.1% of the Pacific ocean and 6.5% of the Indian ocean.
➤ The continental blocks are supposed to end at the site of continental slope.
➤ The continental slope along the many coasts of the world is furrowed by deep canyons like trenches terminating as fanshaped deposits at the base.
➤ There are five types of continental slopes-(a) fairly steep with elongated hills and basins, (b) gentle slope with elongated hills and basins,
(c) faulted slope, (d) slope with terraces & (e) slopes with seamounts.
Continental Rise
➤ The place where the continental slopes end, the gentle sloping continental rise begins.
➤ The average slope is between 0.5° and 1° & its general relief is low.
➤ With increasing depth the Continental Rise becomes virtually flat and it merges with abyssal plain.
Abyssal Plains
➤ Beyond the Continental Rise it is found at the depth of 3000 to 6000 mt.
➤ They cover about 40% of the total ocean floor and generally are bounded by hills on the seaward side.
➤ They are almost flat with a gradient less than 1:100.
➤ They are covered by sediments both of terrigenus and shallow water origins. The irregular topography of the abyssal plain are burried forming relatively flat areas due to large supply of sediments.
Submarine Ridges
➤ These are the mountain ranges in the ocean.
➤ A large number of submarine ridges are placed centrally in the oceans.
➤ At some places their summits may rise above the sea level forming islands.
Abyssal Hills
➤ A deep sea floor also contains numerous isolated abyssal hills, sea-mounts and guyots.
➤ A submarine mountain peak rising more than 1000 mt. above ocean floor are known as “seamount”.
➤ Flat topped seamounts are known as “Guyots”.
➤ All the above features are volcanic in origin.
Submarine Trenches or Deeps
➤ A long narrow and steep sided depression on the ocean floor is called trench. These are the deepest part of the ocean.
➤ They are usually 5500 mt. in depth and lie along the fringes of the deep sea plains and usually run parallel to the bordering fold mountains or the island chains.
➤ They are believed to have resulted from faulting or down folds of the earth crust and so tectonic in origin.
➤ They are most common in the Pacific ocean like “Mariana Trench”, off the Guam Island Chain which is deepest (11022 mt.) in the world.
Submarine Canyons
➤ These are the deep gorges on the ocean floor and are restricted to the continental shelves, slopes and rises.
➤ Some canyons begin at the edge of the continental shelf and extend down the continental slope.
For example-“Oceanographer Canyon” near New-England.
➤ Some canyons have dendritic appearance like off the east coast of Southern California.
➤ Some begins at the river mouth and extends over the continental shelf as “Zaire”, Mississipi” and “Indus” canyons.
Bank, Shoal and Reef
➤ These are the marine features which are formed through the processes of erosion, deposition and biological activity.
➤ Banks are almost flat topped elevation located in the continental margin.
➤ The depth of water is relatively small but adequate for navigation and are sites of rich fishing e.g.
“Dogger Bank” (North Sea), “Grand Bank” (off Newfoundland).
➤ Shoales are detached elevation with shallow depth.
➤ They are dangerous for navigation as they only 18-20 mt. below the sea level.
➤ Reefs are the mound or rocky elevation like ridges made of organic
➤ ‘Great Barrier Reef’ (off Queensland, Australia) is the largest reef in the world.
Coral Reefs : In tropical seas many kinds of coral animals and marine organisms such as coral polyps, calcareous algae, shellforming creatures and lime-secreting plants live in large colonies.
➤ Though they are very tiny creatures, their ability to secrete calcium carbonate within their tiny cells has given rise to peculiar type of marine land form.
➤ They exist in numerous species of many forms, colours and shapes Under favourable conditions, they grow in great profusion just below the water level.
➤ Taking coral animals as a whole, the polyps are most abundant and also the most important.
➤ Each polyp resides in tiny cup of coral and helps to form coral reefs.
➤ When they die, their lime skeletons are cemented into coralline limestone.
➤ There are also non-reef building species such as the ‘precious corals’ of the pacific oecan and the ‘red coral’ of the Mediterranean which may survive in the colder and even the deeper waters.
➤ As a rule they thrive well only in the warmer tropical seas.
Coral Bleaching : The loss of algae from the corals resulting into the white colour which indicates the death of corals is considered as coral Bleaching.
➤ The coral bleaching during 1997 -98, has been recorded as the most catastrophic event.
➤ Global warming has been reported as the major factor of coral bleaching.
➤ Large scale coral bleaching accounting for 70% death of the corals of the coasts of Maldives, Andamans and Lakshadweep Islands in Indian ocean and Kenya.


➤ Salinity is expressed as the number of grams of dissolved salts in 1000 grams of sea water.
➤ The average salinity of oceans is about 35 per thousand or 35%°.
➤ The amount of salinity determines the composition and movements of the sea water and the distribution of various marine lives.
➤ Salinity varies both horizontally and vertically.
➤ The average salinity of N-Hemisphere is 34%o while for S-Hemisphere it is 35%o.
➤ The greatest proportion of salt is found in two areas which lie about the Tropic of Cancer and the Tropic of Capricorn. From these regions the salinity decrease both towards equator and the poles.
➤ In general there is low salinity in equatorial zone, high in tropical belt, low in temperate zone and minimum in sub-polar zone.
Worlds Highest Saline bodies
Van Lake 330
Dead Sea 238
Great Salt lake 220
Sambhar Lake 205
Red Sea 40

Ocean Movements

➤ Ocean currents are swift moving, large masses of surface water that move in a fairly defined direction.
➤ Ocean currents are of two typesthe warm currents and the cold currents.
➤ In the Northern Hemisphere, the current move to their right and in
➤ In Northern Indian Ocean the currents change their direction in response to the reversal of the monsoon winds.
➤ The periodic phenomenon of alternate rise and fall in the sea levels is known as Tide.
➤ Tides are produced due to gravitational interaction of Earth, Moon and Sun.
➤ Moon exerts the strongest influence on tides because of its closeness to Earth.
➤ On the full moon and the new moon tides are the highest (Spring Tides).
➤ On the Ist and 3rd quarters, tides are lower than the usual (Neap tides).
➤ Waves are oscillatory movements in water mainly produced by winds.
➤ Tsunamis
The movement of a mass of oceanic water parallel to the coast is called as ocean current.
Currents are of two types on the basis of temperature.
(i) Warm current
(ii) Cold current

East Australian Peruvian/Warm
Florida North AtlanticWarm
Gulf StreamWarm
West AustralianCold

➤ Warm and Cold currents maintain the temperature of the place as per its temperature.
➤ Because of warm current, ports are open in Polar region.
➤ Winds that are in contact with the warm current are full of humidity and thus rainfall occurs in coastal areas.
➤ Due to cold current, deserts develop.
➤ A dense fog is created where two different currents meet.
➤ Warm current melts the iceberg.


(i) Territorial waters : over which a state has full sovereignty.
They are recognised as extending for 12 nautical miles from the coastline.
(ii) Contiguous Zone or pursuit zone : extends for further 12 nautical miles in which the coastal state can take action against those who break the law within the true territorial waters.
(iii) Exclusive Economic Zone
(EEZ) : extends for 200 nautical miles, starting from the same base line as the territorial waters. Within the EEZ, the coastal state has the right to exploit all economic resources.
(iv) High Seas lie beyond all these zones, and are free to navigation by vessels of all nations.

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