Oceans were divided into regions by oceanographers depending on physical and biological conditions of these areas. The Ocean Floor, Continental Shelf, Continental Slope, Continental Rise and Abyssal of the Deep Sea Plain are discussed in here.
The oceans are included among the first order land-forms on the earth surface. They occupy the deep hollows on the earth. However, the floor of the oceans is not flat or uniform. In fact it is as irregular as the surface of the continents. Parts of the ocean floor are occupied by the deep trenches and parts are covered by submarine ridges of mountainous size. On the basis of the surface configuration, the ocean floor can be divided into four major units.
This is the coastal part of the oceans which is generally not very deep and the slope of the bottom in this part is very gentle. This part extends up to a depth of about 100 fathoms and structurally it is more like the continents rather than like the ocean bottom. It can thus be considered the low lying part of the continent which has been occupied by the sea. The breadth of the continental shelf is a function of whether the coastline is a coastline of submergence or emergence. Generally the shelf is broader along the coastlines of emergence and narrower along the coastlines of submergence. In those regions where the mountains extend along the coast, the shelf is narrower. In case of India, the breadth of the shelf is more along the Bay of Bengal coast than along the Arabian Sea coast. Similarly the western coast of South America is bordered by a rather narrow continental shelf while the western European region has a comparatively broader continental shelf.
This is the outermost part of the sea basin proper and this part extends seaward from the continental shelf. The boundary between the continental slope and the shelf is called the andesite line, so named after the andesite rock which is a volcanic rock forming the bottom of the seas and oceans and usually not occurring on the continents. The slope of the bottom in this part of the sea is comparatively steeper and generally this part does not have thick deposits of the terrigenous material brought by the streams and the glaciers etc.
At the foot of the continental slope is found an area slightly rising due to the accumulation of the debris transported over the continental slope. This part of the sea bottom having a rough convex slope is called the continental rise. Here are found large accumulations of the deposited rock material and usually this part is rich in minerals found in the sedimentary rocks. Some of the oil deposits of the oceanic areas occur in this zone.
Abyssal of the Deep Sea Plain
This is the deepest and the most extensive part of the sea bottom and this part accounts for the highest proportion of the total area of the oceans. This is somewhat like the vast plains on the continents with the difference that its surface is irregular unlike the surface of the plains on the continents. Parts of the abyssal plain are occupied by the raised ridges or submarine mountains and parts by the very deep trenches or the canyons. A number of volcanic mountains are also found on this part. The raised areas are generally called the ridges and the low lying deeper parts are called the trenches. A very well known example of such ridges is the Mid-Atlantic ridge extending north to south through the central part of the Atlantic Ocean. Indian Ocean has the famous 90 degree E. Ridge. Some parts of the ridges or the volcanic peaks may reach the surface of the oceans and they may thus form the islands. Hawaiian Islands are good examples of such volcanic islands. Some of these peaks fail to reach the surface of the oceans and their top parts are eroded through the wave action to make them almost flat-topped. Such rather flat-topped peaks in the oceans are called guyots and Pacific Ocean is specially known for these where they occur in hundreds. Some of the ridges on the ocean floors may be a result of folding activity also.
The trenches or canyons on the ocean floor are sometimes narrow and extremely deep. Their depth may be more than even the height of the highest mountains on the continents. Mariana Trench in the western Pacific (near Japan) is more than 12 km. deep. Some of the other important trenches include the Tuscarora Deep (off Japan), Kuril Trench (near Kuril Islands), Mindanao Deep and Philippine Trench. Romanche Deep in the Atlantic Ocean marks a break in the continuity of the Mid-Atlantic Ridge almost at the equator. Most of the trenches run parallel to the fold mountains on the coast or along the island chains. According to the theory of the plate tectonics the trenches mark the zones of subduction of the oceanic plates in the zones of plate convergence.