Revision Notes - Water (Oceans) Notes | Study NCERT Textbooks in Hindi (Class 6 to Class 12) - UPSC

RELIEF OF THE OCEAN FLOOR

• The oceans are confined to the great depressions of the earth’s outer layer.

• The oceans, unlike the continents, merge so naturally into one another that it is hard to demarcate them.
• the Pacific, the Atlantic, the Indian and the Arctic. The various seas, bays, gulfs and other inlets are parts of these four large oceans.
• A major portion of the ocean floor is found between 3-6 km below the sea level.
• The ‘land’ under the waters of the oceans, that is, the ocean floor exhibits complex and varied features as those observed over the land.
• The floors of the oceans are rugged with the world’s largest mountain ranges, deepest trenches and the largest plains. These features are formed, like those of the continents, by the factors of tectonic, volcanicand depositional processes.

Divisions of the Ocean Floors

The ocean floors can be divided into four major divisions:

(i) the Continental Shelf; (ii) the Continental Slope; (iii) the Deep Sea  Plain; (iv) the Oceanic Deeps.

• Besides, these divisions there are also major and minor relief features in the ocean floors like ridges, hills, sea mounts, guyots, trenches, canyons, etc

1. Continental Shelf 

• The continental shelf is the extended margin of each continent occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean showing an average gradient of 1° or even less.
• The shelf typically ends at a very steep slope, called the shelf break.
• The average width of continental shelves is about 80 km.
• The shelves are almost absent or very narrow along some of the margins like the coasts of Chile, the west coast of Sumatra, etc. On the contrary, the Siberian shelf in the Arctic Ocean, the largest in the world, stretches to 1,500 km in width. The depth of the shelves also varies. It may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m.
• Massive sedimentary deposits received over a long time by the continental shelves, become the source of fossil fuels.

Continental Slope

• The continental slope connects the continental shelf and the ocean basins.
• It begins where the bottom of the continental shelf sharply drops off into a steep slope.
• The gradient of the slope region varies between 2-5°.
• The depth of the slope region varies between 200 and 3,000 m. The slope boundary indicates the end of the continents.
• Canyons and trenches are observed in this region.

Deep Sea Plain

Deep sea plains are gently sloping areas of the ocean basins. These are the flattest and smoothest regions of the world.

• The depths vary between 3,000 and 6,000m. These plains are covered with fine-grained sediments like clay and silt.

Oceanic Deeps or Trenches

• These areas are the deepest parts of the oceans.
• The trenches are relatively steep sided, narrow basins.
• They are some 3-5 km deeper than the surrounding ocean floor.
• They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes.
• That is why they are very significant in the study of plate movements.
• As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean and 6 in the Indian Ocean.

Minor Relief Features

• Apart from the above mentioned major relief features of the ocean floor, some minor but significant features predominate in different parts of the oceans.

Mid-Oceanic Ridges

• A mid-oceanic ridge is composed of two chains of mountains separated by a large depression.
• The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface.
• Iceland, a part of the mid- Atlantic Ridge, is an example.

Submarine Canyons

• These are deep valleys, some comparable to the Grand Canyon of the Colorado river.
• They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers.
• The Hudson Canyon is the best known submarine canyon in the world.

Guyots

• It is a flat topped seamount.
• They show evidences of gradual subsidence through stages to become flat topped submerged mountains.
• It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.

Atoll

• These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression.
• It may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline water.

TEMPERATURE OF OCEAN WATERS

Factors Affecting Temperature Distribution

1. Latitude : the temperature of surface water decreases from the equator towards the poles because the amount of insolation decreases poleward.
2. Unequal distribution of land and water : the oceans in the northern hemisphere receive more heat due to their contact with larger extent of land than the oceans in the southern hemisphere.
3. Prevailing wind : the winds blowing from the land towards the oceans drive warm surface water away form the coast resulting in the upwelling of cold water from below. It results into the longitudinal variation in the temperature. Contrary to this, the onshore winds pile up warm water near the coast and this raises the temperature.
4. Ocean currents : warm ocean currents raise the temperature in cold areas while the cold currents decrease the temperature in warm ocean areas. Gulf stream (warm current) raises the temperature near the eastern coast of North America and the West Coast of Europe while the Labrador current (cold current) lowers the temperature near the north-east coast of North America.

Horizontal and Vertical Distribution of Temperature

• The temperature-depth profile for the ocean water shows how the temperature decreases with the increasing depth.
• The profile shows a boundary region between the surface waters of the ocean and the deeper layers.
• The boundary usually begins around 100 - 400 m below the sea surface and extends several hundred of metres downward
• This boundary region, from where there is a rapid decrease of temperature, is called the thermocline.
• About 90 per cent of the total volume of water is found below the thermocline in the deep ocean.
• In this zone, temperatures approach 0° C.
• three-layer system from surface to the bottom. The first layer represents the top layer of warm oceanic water and it is about 500m thick with temperatures ranging between 20° and 25° C. This layer, within the tropical region, is present throughout the year but in mid latitudes it develops only during summer.
• The second layer called the thermocline layer lies below the first layer and is characterised by rapid decrease in temperature with increasing depth. The thermocline is 500 -1,000 m thick.
• The third layer is very cold and extends upto the deep ocean floor.
• In the Arctic and Antartic circles, the surface water temperatures are close to 0° C and so the temperature change with the depth is very slight. Here, only one layer of cold water exists, which extends from surface to deep ocean floor.
• The average temperature of surface water of the oceans is about 27°C and it gradually decreases from the equator towards the poles. The rate of decrease of temperature with increasing latitude is generally 0.5°C per latitude.
• The oceans in the northern hemisphere record relatively higher temperature than in the southern hemisphere. The highest temperature is not recorded at the equator but slightly towards north of it.
• This variation is due to the unequal distribution of land and water in the northern and southern hemispheres.
• the heat is transmitted to the lower sections of the oceans through the process of convection.

SALINITY OF OCEAN WATERS

• Salinity is the term used to define the total content of dissolved salts in sea
• It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater.
• It is usually expressed as parts per thousand or ppt.
• Salinity of 24.7 %has been considered as the upper limit to demarcate ‘brackish water’.

Factors affecting ocean salinity

1. The salinity of water in the surface layer of oceans depend mainly on evaporation and precipitation.
2. Surface salinity is greatly influenced in coastal regions by the fresh water flow from rivers, and in polar regions by the processes of freezing and thawing of ice.
3. Wind, also influences salinity of an area by transferring water to other areas.
4. The ocean currents contribute to the salinity variations. Salinity, temperature and density of water are interrelated. Hence, any change in the temperature or density influences the salinity of water in an area.

: Spatial pattern of surface temperature (°C) of the oceans Highest salinity in water bodies Lake
Van in Turkey (330 o/oo ),
Dead Sea (238 o/oo),
Great Salt Lake (220 o/oo

• Dissolved Salts in Sea Water (gm of Salt per kg of Water) –
• 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
• Strontium 0.01

HORIZONTAL DISTRIBUTION OF SALINITY

• The salinity for normal open ocean ranges between 33o/oo and 37 o/oo .
• In the land locked Red Sea, it is as high as 41o/oo , while in the estuaries and the Arctic, the salinity fluctuates from 0 - 35 o/oo , seasonally.
• In hot and dry regions, where evaporation is high, the salinity sometimes reaches to 70 o/oo.
• The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent.
• Salinity decreases from 35 o/oo - 31 o/oo on the western parts of the northern hemisphere because of the influx of melted water from the Arctic region.
• The highest salinity is recorded between 15° and 20° latitudes.
• Maximum salinity (37 o/oo) is observed between 20° N and 30° N and 20° W - 60° W.
• It gradually decreases towards the north.
• The North Sea, in spite of its location in higher latitudes, records higher salinity due to more saline water brought by the North Atlantic Drift. Baltic Sea records low salinity due to influx of river waters in large quantity.
• The Mediterranean Sea records higher salinity due to high evaporation.
• Salinity is, however, very low in Black Sea due to enormous fresh water influx by rivers.
• The average salinity of the Indian Ocean is 35 o/oo. The low salinity trend is observed in the Bay of Bengal due to influx of river water. 
• On the contrary, the Arabian Sea shows higher salinity due to high evaporation and low influx of fresh water. 

Vertical Distribution of Salinity Salinity