In general, the ocean floor can be divided into four major divisions: 1. Continental Shelf 2. Continental Slope 3. Continental Rise 4. Abyssal plain.
Division # 1. Continental Shelf:
Continental shelf is the shallow portion of the ocean which lies close to the continental margin. It is actually a part of the continent sloping gently towards the sea floor. The average depth is not more than 150-200 metres. The continental shelf may be formed either by the submergence of land or by the change in the sea level.
The width may vary from a few kilometres to more than 1000 km and has average slope of less than one degree. Nearly 7.5 per cent of the total area of the sea floor is covered by the continental shelves.
The continental shelves are full of sediments carried from the land surfaces. But igneous and metamorphic rocks are also found towards the seaward side. Continental shelves are very important from economic point of view.
Nearly 20 per cent of the total petroleum and natural gas, almost entire quantity of marine food and various important economic minerals come from continental shelves. The continental shelves are also major fishing grounds of the world.
Division # 2. Continental Slope:
Continental slope is beyond the continental shelf, where the sea floor slopes downwards. Its average slope is between 2°-5° and extends up to the depth of 3660 metres. The steep-sided continental slope separates continental shelf from deep sea floor. Continental slope has been considered as the area of termination of the continent. The slope area has many deep canyons and has fan-shaped deposits at the base.
Division # 3. Continental Rise:
Continental rise is the gently sloping area at the end of the continental slope. The average slope of this region is not more than one degree. It becomes very flat towards the deep sea floor and virtually merges with the abyssal plain.
Division # 4. Abyssal Plain:
At the end of the continental slope and rise, abyssal plain occurs at the deepest portion of the ocean. The average depth of the abyssal plain is 3000-6000 metres and it covers nearly 40 per cent of the ocean. It is completely plain and is covered by sediments of terrigenous and shallow water origin.
At the ocean floor, submarine ridges of a few hundred kilometres wide and a few thousands kilometres in length are also found. The total length of the submarine ridges is more than 75,000 km. They are longer than the longest mountain ranges on the continents. The ridges are either flat like plateau or steep-sided mountains.
The peaks of the ridges may rise above the sea level and they form islands. The submarine ridges are the evidences of the global tectonic process. The peak of submarine ridges when rises more than 1,000 metres is known as sea mounts. Flat topped sea mounts are called guyots.