Relevance: mains: G.S paper I: Geography: Oceanography
About 71 percent of Earth s Surface is covered by water and nearly 97% of all the water is in the oceans. The image below shows the distribution of water on the Earth s surface. This shows the importance of oceans to the planet Earth and also to living and non living beings existing on it.
Ocean Relief: Major Ocean Relief Features
Ocean relief is formed due to tectonic, volcanic, erosional, depositional processes and their interactions. The ocean basins have many features similar to the topography of the land surface.
Ocean relief can be broadly divided into major and minor relief features.
Major relief features
The major relief features of the oceans can be divided into four. The Continental Shelf The Continental Slope The Continental Rise The Deep Sea Plain or the Abyssal Plain
The Continental Shelf
It is the seaward extension of the continent from the shoreline to the continental edge. Shelves are formed due to the following reasons: Submergence of a part of a continent The relative rise in sea level Sedimentary deposits brought down by rivers Smaller continental shelves could have been caused by wave erosion where the land is eroded by the sea. It is gently sloping with a gradient of 1 in 500 for most continental shelves. Continental shelves of all oceans cover an area of 7.5% of the total area of the oceans and 18% of earth s dry land area. The average width of the shelf is 70 -80kms. The width of continental shelf varies greatly, from a few kilometres in the North Pacific off the coast of North America to a few hundred kilometres off Northwest Europe. The shelves are almost absent or very narrow along some of the margins like the coasts of Chile, the west coast of Sumatra, etc. Also, in some places where the coasts are extremely mountainous like the Rocky Mountains and Andean coasts, the shelves are almost absent while along the broad lowland coasts like the Arctic Siberia it stretches to around 1,500kms (largest in the world).
The significance of Continental Shelves
Continental Shelves have rich economical and ecological significance. It is the most accessible and the best-understood part of the ocean. Shelves encourage the growth of millions of plankton and microorganisms through penetration of sunlight which makes them excellent breeding ground for fish. Hence continental shelves are the richest fishing grounds in the world. Example: Great Banks off Newfoundland, Sunda shelf and the North Sea. Marine food almost comes entirely from continental shelves. Most commercial exploitation from the sea, such as metallic-ore, non-metallic ore, polymetallic nodules and hydrocarbon extraction, takes place on the continental shelf. 20% of the world s petroleum and gas comes from the shelves. Most of the world s greatest seaports including London, Southhampton, Rotenberg, Hong Kong and Singapore are located on continental shelves.
The Continental Slope
The continental slope connects the continental shelf and continental rise. Continental shelf ends where there is a sharp increase in the slope. From this point, continental slope starts. There is an abrupt change in the gradient to about 1 in 20. The gradient of the slope is highest off coasts with young mountain ranges and narrow continental shelves and lowest off stable coasts without major rivers Most Pacific slopes are steeper than Atlantic slopes. Gradients are flattest in the Indian Ocean. The depth of the slope region varies between 100 and 3,200 m. About one-half of all continental slopes descend into deep-sea trenches or shallower depressions.
The Continental Rise
The Continental Rise connects the Continental slope to the deep sea or abyssal plain. The steepness of continental rise is lower than that of the continental slope and it gradually merges into the deep sea plain. It is a major depositional regime in oceans made up of thick sequences of continental material that accumulate between the continental slope and the abyssal plain.
The Deep Sea plain or the Abyssal plain
This is an undulating plain generally two to three miles below the sea level. It covers around two-thirds of the ocean floor and is generally termed as the abyssal plain. The abyssal plain is far from being level. It has extensive submarine plateaux, ridges, trenches, basins and oceanic islands that rise above sea level in the midst of the oceans.
Minor Relief features
Ridges: Mid-oceanic ridges or submarine ridges are underwater mountain systems formed by plate tectonics (Divergent boundary).
Seamounts: It is a mountain with pointed summits, rising from the seafloor that does not reach the surface of the ocean. Seamounts are volcanic in origin. These can be 3,000-4,500 m tall Guyots: The flat-topped mountains (seamounts) are known as guyots
Trenches: They are relatively long, steep-sided, narrow basins (Depressions). These areas are the deepest parts of the oceans. They are of tectonic origin and are formed during ocean-ocean convergence and ocean-continent convergence. They are most often found close to the continents particularly in the Pacific Ocean where several trenches have been found. The greatest known ocean deep, the Mariana Trench which is nearly 36,000 feet deep is found near Guam Island in the Pacific Ocean.
Coral reefs: Coral reefs are built by colonies of tiny animals found in marine water that contain few nutrients. They are diverse underwater ecosystems held together by calcium carbonate structures secreted by marine invertebrates called corals.
Atolls: These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression. It may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline water. Example: Lakshadweep is formed on Atolls.