In general, the ocean floor can be divided into four major divisions:
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.
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.
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.
VERTICAL DISTRIBUTION OF SALINITY
Salinity of ocean decreases of increases towards the bottom according to the nature of the water mass.