Oceans – Bottom Relief
Four major divisions can easily be identified on the ocean floor:
- The continental shelf,
- the continental slope,
- the continental rise,
- the abyssal plain.
Besides these, there are many associated features—ridges, hills, seamounts, guyots, trenches, canyons, sleeps, fracture zones, island arcs, atolls, coral reefs, submerged volcanoes and sea-scarps.
This great variety of relief is largely due to interaction of tectonic, volcanic, erosional and depositional processes. At greater depths, the tectonic and volcanic phenomena are more significant processes.
Continental Shelf: This is a gentle seaward sloping surface extending from the coasts toward s the open sea. In all, about 7.5% of the total area of the oceans is covered by the continental shelves. The shelf is formed by the drowning of a part of a continent with a relative rise in sea level or marine deposition beneath the water.
The average width of the continental shelf is about 70 km and mean slope is less than one degree, but the width shows great variety from location to location. For instance, it is almost absent in the eastern Pacific, especially off South America and is upto 120 km wide along the eastern coast of USA. The seaward edge of the shelf is usually 150-200 metres deep.
The continental shelves are mostly covered by sediments of terrestrial origin. There are various types of shelves—glaciated shelf, coral reef shelf, shelf of a large river, shelf with dendritic valleys and the shelf along young mountain ranges.
As the continental shelf nears its seaward edge, the gradient becomes steeper—two to five degrees. This is the site of the continental slope which descends to a depth of 3,500 metres and joins the shelf to the deep ocean floor. The site of the slope also indicates the end of the continental block. The slopes may be furrowed by canyons and trenches. Continental Rise
The continental slope gradually loses its steepness with depth. When the slope reaches a level of between 0.5° and 1°, it is referred to as the continental rise. With increasing depth the rise becomes virtually flat and merges with the abyssal plain.
Beyond the continental rise, at depths from 3,000 m to 6,000 m, lie the deep sea plains, called abyssal plains or abyssal floors. Covering nearly 40% of the ocean floor, the abyssal plains are present in all major oceans and several seas of the world. They are uniquely flat with a gradient of less than 10,000. The large supply of terrigenous and shallow water sediments buries the irregular topography to form a generally flat relief.
Submarine ridges are mountain ranges, a few hundred kilometres wide and hundreds and often thousands of kilometres in length on the floors of oceans. Running for a total length of 75,000 km, these ridges form the largest mountain systems on earth.
These ridges are either broad, like a plateau, gently sloping or in the form of steep-sided narrow mountains. These oceanic ridge systems are of tectonic origin and provide evidence in support of the theory of Plate Tectonics.
These are elevated features of volcanic origin. A submarine mountain or peak rising more than 1,000 metres above the ocean floor is known as a seamount. The flat topped mountains are known as guyots.
Seamounts and guyots are very common in the Pacific Ocean where they are estimated to number around 10,000.
Submarine Trenches or Deeps: These are the deepest parts of the oceans with their bottoms far below the average level of the ocean floors. A trench is a long, narrow and steep-sided depression on the ocean bottom, which is usually 5,500 metres in depth. The trenches lie along the fringes of the deep-sea plain and run parallel to the bordering fold mountains or the island chains.
They are believed to have resulted from down faulting or down folding of the earth’s crust and are, therefore, of tectonic origin. The trenches are very common in the Pacific Ocean and form an almost continuous ring along the western and eastern margins of the Pacific. The Mariana Trench off the Guam Islands in the Pacific Ocean is the deepest trench with a depth of more than 11 kilometres.
These are steep valleys, forming deep gorges on the ocean floor. They are mainly restricted to the continental shelf, slope and rise. Broadly, there are three types of submarine canyons:
- Small gorges which begin at the edge of the continental shelf and extend down the slope to very great depths, e.g., Oceanographer Canyons near New England.
- Those which begin at the mouth of a river and extend over the shelf, such as the Zaire, the Mississippi and the Indus canyons.
- Those which have a dendritic appearance and are deeply cut into the edge of the shelf and the slope, like the canyons off the coast of southern California. The Hudson Canyon is the best known canyon in the world. The largest canyons in the world occur in the Bering Sea off Alaska. They are the Bering, Pribilof and Zhemchung canyons.
Bank, Shoal and Reef
These marine features are formed as a result of erosional, depositional and biological activity. Also, these are produced upon features of diastrophic origin. Therefore, they are located on upper parts of elevations.
A bank is a flat topped elevation located in the continental margins. The depth of water here is shallow but enough for navigational purposes. The Dogger Bank in the North Sea and Grand Bank in the north-western Atlantic off Newfoundland are famous examples. The banks are sites of some of the most productive fisheries of the world.
A shoal is a detached elevation with shallow depths, since they project out of water with moderate heights, they are dangerous for navigation.
A reef is a predominantly organic deposit made by living or dead organisms that forms a mound or rocky elevation like a ridge. Coral reefs are a characteristic feature of the Pacific Ocean where they are associated with seamounts and guyots. The largest reef in the world is found off the Queensland coast of Australia . Since the reefs may extend above the surface, they are generality dangerous for navigation.