Ocean Relief and Salinity - Physical Geography, UPSC, IAS. | Geography (Prelims) by Valor Academy PDF Download

 Ocean Relief & Salinity

In general, the ocean floor can be divided into four major divisions:

1. The Continental Shelf.
2. The Continental Slope.
3. The Continental Rise.
4. The Abyssal Plain.

Besides, there are many associated features including ridges, hills, seamounts , guyots, trenches, canyons, deep and fracture zones.

1. The Continental Shelf :- The Continental Shelf is a gentle seaward sloping surface extending from the coasts towards the open sea. The shelf is generally formed by the drowning of part of a continent with a relative rises in the sea level or marine deposition beneath the water. The seaward edge of the continental shelf is usually 150-200 metres deep. The average a width of Continental Shelf is about 70 kilometres and mean slope is less than one degree.

The continental shelves are mostly covered by sediments derived from rocks on land. Marine food comes almost entirely from them. About 20 percent of the world production of petroleum and gas comes from shelves.

2. Continental Slope: - At the edge of the continental shelf, the seaward slope becomes considerably steep, the angle of slope varying from 2 to 5 degrees. This steep slope, which descends to a depth of about 3660 metres the mean sea level, is known as continental slope.

3. Continental Rise: - where the continental slope ends, the gently sloping continental rise begins. The continental rise has an average slope of between 0.5° to 1° and its general relief is low. With increasing depth the continental rise becomes virtually flat and it merges with the abyssal plain.

4. Abyssal Plain:-Beyond the continental risse lie the deep sea plains known as the abyssal plains or abyssal floors. They are areas of deep ocean floor found at a depth of 3000 to 6000 metres. They occupy about 40 percent of the ocean floor. Abyssal Plains are more common where land derived sediments are in great supply. The irregular topography is buried forming relatively flat areas due to the large supply of sediments.

Submarine Ridges :- submarine ridges are mountain ranges a few hundreds kilometre in length on the floors of oceans. The submarine ridges of high relief form the longest mountain system on the earth. Their summit may rise above sea level forming islands.

Abyssal Hills :- The deep sea floor also contains thousands isolated abyssal hills, seamounts and guyots. A submarine mountain or peak rising more 1000 metres above floor is known as a seamount. Flat topped seamounts as known as guyots . All these features are of volcanic origin.

Submarine Trenches and Deeps :- these are the deepest part of the oceans with their bottoms far below the average level of the ocean floors. A long, narrow and steep sided depression on the ocean bottom is called a Trench. They are usually 5,500 metres in depth and lie along the fringes of the deep sea plain. They are believed to have result from down faulting of down folding of the earth’s. crust and are, therefore, of tectonic origin. The Trenches generally run parallel to the bordering fold mountain or the island chains.

Submarine Canyons:- Submarine canyons are deep gorges on the ocean floor. They are strikingly deep valleys with steep slopes that form long, conclave profiles. They occur around all the coasts of the world and are mainly restricted to the continental shelf, slope and rise.

SALINITY
It is defined as the total amount of solid material in gram contained in one    kilogram of sea water and is expressed as part per thousand. It is measured by electric salinity meter.

• The average salinity in the oceans and sea is 35%. Very high salinity is recorded in inland seas and lakes. Lake Van in Turkey records the highest salinity.
• The origin of salinity is attributed to erosion of earth’s crust by dissolving action of running water, produced by rain, river, constant washing of sea shore and agencies which causes erosion in the oceanic crust and volcanic ash which contains minerals like Calcium, Boron, Iodine etc.
• Salinity is excessive in region of high temperature, strong winds and less rainy days. For example, it is comparatively lower in the equatorial region due to high relative humidity.
• Influx of fresh water by river reduces the salinity and so there is less salinity near the months of rivers.
• In spite of high temperature salinity is less in equatorial region because of high rainfall. Around the poles there is a belt of low salinity because of addition of fresh water in the form of iceberg and excessive snow falls.

HORIZONTAL DISTRIBUTION OF SALINITY

It decreases from Equator towards the Poles. In general there is low salinity in equatorial zone, high in tropical belt low in temperature zone and minimum in sub-polar zone.

• Amount of salinity varies from oceans to ocean, mainly due to supply of fresh water, rapidity of evaporation and water mixing tendency.
• The greatest evaporation of salt is found in two areas which lie about the Tropic of Cancer and the Tropic of Capricorn from these regions the salinity decreases both onwards equator and the poles.
• Salinity of the inland seas and lakes is very high because of the regular supply of salt by the rivers flowing into them and the evaporation makes continuously more and more saline.

VERTICAL DISTRIBUTION OF SALINITY

Salinity of ocean decreases of increases towards the bottom according to the nature of the water mass.

• In high latitude salinity increases with depth due to dense water found at the    bottom. In the middle latitude salinity increases with the depth up to 366 metres and then it starts decreasing..
• At equator surface salinity is low but just below it greater salinity is found which again decreases at the bottom due to presence of cold water mass.