Water (Oceans) Class 11 Notes Geography Chapter 12 - CBSE

Chapter : 12

What Are Water (Oceans) ?

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Water is an essential component of all life forms that exist over the surface of the earth.
Water is life; it is a rare commodity in our solar system.
The earth has abundant supply of water on its surface, thus called the ‘Blue Planet’.

Hydrological Cycle

The hydrological cycle describes the movement of water on, in, and above the earth.
The water cycle has been working for billions of years and all the life on earth depends on it.
The hydrological cycle is the circulation of water within the earth’s hydrosphere in different forms, i.e., the liquid, solid, and gaseous phases.
It also refers to the continuous exchange of water between the oceans, atmosphere, land surface and
subsurface, and the organisms.
About 71% of the planetary water is found in the oceans.
Nearly 59% of the water that falls on land returns to the atmosphere through evaporation from over the oceans as well as from other places. This leads to water crisis in different parts of the world – spatially and temporally.

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. A major portion of the ocean floor is found between 3–6 km below the sea level.

Divisions Of The Ocean Floors

The ocean floors can be divided into four major divisions:

(i) Continental Shelf

The continental shelf is the extended margin of each continent occupied by relatively shallow seas and gulfs.
The width of the continental shelves vary from one ocean to another.
The average width of continental shelves is about 80 km.
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.

(ii) 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.

(iii) 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 6,000 m.
These plains are covered with fine-grained sediments such as clay and silt.

(iv) 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.

(v) 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.
Example: Iceland.

(vi) Seamount

It is a mountain with rounded tops that rises from the ocean's floor but does not touch the water's surface.
Volcanoes are the cause of seamounts.
They may be 3,000–4,500 m high.
A notable example is the Emperor seamount which is an outgrowth of the Hawaiian Islands in the Pacific
Ocean.

(vii) Submarine Canyons

These are deep valleys, some of which are equivalent to the Colorado River's Grand Canyon.
They frequently originate from the mouths of big rivers, but they can occasionally be seen cutting over
continental shelves and slopes.
The Hudson Canyon is the world's most well-known undersea canyon.

(viii) Guyots

It has a seamount with a flat top.
They exhibit signs of slow sinking over a number of stages to become mountains with flat tops that are submerged.
More than 10,000 seamounts and guyots are thought to be present in the Pacific Ocean alone.

(ix) Atoll

These tiny coral reef-encircled islands, which can be found in tropical seas, are low islands.
These could be a body of fresh, brackish, or extremely saline water, or sea (lagoon).

Temperature Of Ocean Waters

Ocean waters get heated up by the solar energy just as land.
The process of heating and cooling of the oceanic water is slower than land.

Factors Affecting Temperature Distribution

Latitude: The temperature of surface water decreases from the equator towards the poles because the amount of insolation decreases poleward.
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.
Prevailing wind: The upwelling of cold water from below is caused by warm surface water being driven away from the coast by winds flowing from the land towards the oceans. The temperature changes longitudinally as a result. In contrast, the onshore breezes cause warm water to collect close to the coast,
which raises the temperature.
Ocean currents: While cold ocean currents lower the temperature in warm ocean areas, warm ocean currents boost the temperature in cold areas. The Labrador current (cold current) cools the area near the north-east coast of North America while the Gulf stream (warm current) elevates the temperature close to the east coast of North America and the west coast of Europe.

Horizontal And Vertical Distribution Of Temperature

The ocean water's temperature-depth profile demonstrates how the temperature drops as depth increases.
A location where the ocean's surface waters and deeper layers divide may be seen in the profile.
The boundary often starts between 100 and 400 m below the surface of the water and descends several hundred metres.
The thermocline is the name given to this boundary zone where temperatures rapidly drop.
In the deep ocean, beneath the thermocline, there is about 90% of the total amount of water.
Temperatures here are close to 0°C. The temperature structure of oceans over middle and low latitudes can be described as a three-layer system from surface to the bottom.
The first layer indicates the top 500 m of warm ocean water, which has a temperature between 20°C and 25°C.
In the tropical zone, this layer is present all year long, whereas in midlatitudes, it only forms in the summer.
The second layer also referred to as the thermocline layer, is marked by a rapid drop in temperature with depth. The thermocline has a thickness of 500–1,000 m.
The third layer is extremely cold and reaches the deep ocean floor. The temperature difference with depth is
quite small in the Arctic and Antarctic circles because the surface water is near to 0°C. Only one layer of cold water is present here, and it stretches from the surface to the 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 oceans in the northern hemisphere record relatively higher temperature than in the southern hemisphere.
It results into decrease of temperature with the increasing depth, but the rate of decrease is not uniform throughout.

Salinity Of Ocean Waters

Salinity is the term used to define the total content of dissolved salts in sea water.
It is calculated as the amount of salt (in g) dissolved in 1,000 g (1 kg) of seawater.
It is usually expressed as parts per thousand (0 / 00) or ppt.
Salinity is an important property of sea water.
Salinity of 24.7ppt has been considered as the upper limit to demarcate ‘brackish water’.