According to variations in topographic features with the depth the relief of ocean floor is divided into five zones.
(1) Continental Shelf. Oceans are full of water. Some of it spreads on to the neighboring land. The portions of the land, which are submerged under sea water, constitute continental shelf. The continental shelf is shallow and its depth is not more than 200 meters. Its slope from the land to the sea is about 2 meters per km. The breadth of the continental shelf is not the same everywhere. The maximum breadth is almost equal to 1,100 km. found between Northern Norway and Novaya Zemlya along Berints Sea. An extensive shelf along the Arctic Coast Ranges 480 km to 1280 km wide and 183 m deep. The deepest shelves surround the Antarctic continent. The continental shelves covers about 7.6% of the total area of the sea. The area of continental shelf is maximum in Asia, i.e., 9.38 x 106 sq. km. The second place is occupied by North America (6.74 x 10 2 sq. km). The smallest shelf i.e., 0.36 x 10 6' sq. km., is found in Antarctica.
These continental shelves are rising as also subsiding at different places. Sediments are deposited on them by river, wind and ocean currents and waves. Sunrays penetrate to some depth into the seawater and support animal and vegetation lives on sea floors. The world famous fisheries are situated in these areas.
(2) Continental Slope. At the outer edge of the continental shelf, the slope suddenly steepens. This is found to be 35 to 61 meters per km. The value of the slope is much more adjacent to the fold mountains than that near the coastal plains. One end of the slope connects it with continental shelf while the other one merges into the ocean floors. The area occupied by continental slope is 8.5% of the total ocean area.
The continental slope consists of light continental rocks, which overlie the dense rocks of ocean floors. It extends up to a considerable depth between 200-2000 m. The average slope is about 40 17'. It is devoid of sea deposit.
(3) Deep Ocean Basins or Floors. About 2/3 of ocean surface is occupied by deep ocean basins or floors. They are situated on the base of the continental slope. The depth varies from 3,000 to 6,000 meters. These basins are not as 1evelled and flat as they were considered by older scholars. Many long zigzag ridges, plateaus, volcanic summits, etc., exist on the basins. Many mountain tops rise steeply from the ocean floors and emerge out of sea as islands. Such mountain tops are found in Indonesia and in Pacific Ocean. Many pelagic deposits and oozes are found on the basins.
(4) The Ocean Deeps. A depression, long and narrow, existing on the deep ocean basins are called ocean deeps. The deep are commonly found in Caribbean Sea and Pacific Ocean. The depth of these deeps is found to range from 7,000 to 9,000 meters. The deeps are also called trenches or troughs. Sometimes their name suggests that the trenches have a steep slope. In fact, their slope is nowhere more than 7%. Due to the non-existence of sediments, the deeps are almost empty. The slopes of their walls is, however much more steeper than that of any other feature.
The origin of the ocean deeps is attributed to the endogenetic forces, which have produced the mountains.
(5) Submarine Canyons. These are depressions with wails of steep slopes and have V-shape. They exist on the continental slopes and the shelves. They are found to have a length of 16 km., at the maximum and have a dendritic pattern. The canyons are found to be close to river mouths such as those of the Congo, the Sindh, the Hudson, the Delaware, the Columbia, etc. The depth varies from 1,800 meters to 2,800 meters. Coarse deposits are found in the canyons.
These canyons are found In Asia in Aleutian Island, Japan, Philippines, etc., In Europe near the Mediterranean Sea, in North America at the coasts of Mexico and California, and along the eastern coasts from Cape Hatteras to the middle of Canada.
There are many mountains found on ocean floors. The mountains which are above 1 ,000 meters in height and have a conical summits are known as sea mounts but if the summits are flat, they are known as guyots.
Many sea mounts are found in the Gulf of Alaska. Patton, Pratt, Fails, etc., are well known sea mounts. Some of them attain a height of 3,000 meters. Though the ocean floors have not been studied with the detail with which land surface has been studied yet a lot of knowledge has been accumulating throwing light on the topography of ocean floors. 1.26 Pacific Ocean
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