Wednesday, October 28, 2009

Ocean and Oceanography

RELIEF FEATURES OF OCEANS
The floors of the ocean are rugged and complex with long mountain ranges, deep trenches and large plains. The development of the sonic depth recorder has made it possible to measure depth for mapping the ocean floor indirectly with the help of sound waves. The ocean floor can be divided into four major divisions.
(i) Continental Shelf The continental shelf is the gently sloping part of a continent that lies submerged below the sea. The shelf is generally formed due to a relative rise in sea level or marine deposition beneath the water. The average width of continental shelf is about 70 km, while the average depth is about 200 m. In all, about 7.5 per cent of the total area of the oceans is covered by the continental shelves. The North Sea and the Baltic Sea are examples of seas that lie on the continental shelf. Such seas are known as epicontinental or shelf seas.

About 20 per cent of the world production of petro­leum and gas comes from shelves.
(ii) Continental Slope The continental slope is the comparatively steep slope that descends from the edge of the continental shelf to the deep-sea platform. There are five type of slopes: (i) fairly steep with the surface dissected by canyons, (ii) gentle slope with elongated hills and basins, (iii) faulted slopes, (iv) slopes with terraces, and (v) slopes with sea-mounts.
(iji) Continental Rise The continental rise or apron is the gently inclined slope which leads up from the deep­sea floor (abyssal zone) to the foot of the continental slope. In other words, where the continental slope ends, the continental rise begins. The continental rise has an average slope of between 0.5° to 1° and its general relief is low.
(iv) Abyssal Plain The abyssal plains or abyssal floors are the areas of deep-ocean floor found at a depth of 3,000 to 6,000 metres.

They occupy about 40 per cent of the ocean floor and are present in all major oceans and several seas of the world. Besides, these four broad features, there are many associated features. Submarine ridges are the mountain ranges on the sea floor that rise above the general level of the abyssal plain. These ridges are either broad like a plateau, or gently sloping or steep-sided narrow mountains. The mid-ocean ridges are the largest. Areas of upland smaller than a mountain are found on the deep sea floor.

These upland areas are called abyssal hills. Seamount is an isolated mountain rising abruptly some 1,000 m from the ocean floor but. without extending above sea level. Seamounts are of volcanic origin, and are com­mon in the Pacific Ocean. Flat-topped sea-mounts are known as guyots. The summits of guyots are almost entirely at depths of more than 1,000 m but rise upto 3 km from the ocean floor. The conical shape of the guyot suggests a volcanic origin. Like seamounts, guyots are found in abundance in the Pacific Ocean. A long, narrow and steep-sided depression on the bottom of the ocean is called a trench or a deep. Trenches or deeps are the deepest parts of the oceans with their bottoms far below the average level of the ocean floors.

They are most common in the Pacific Ocean. Submarine canyons are deep gorges on the ocean floor. They occur around all the coasts of the world and are mainly restricted to the continental shelf, slope and rise. 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.

A bank is a more or less flat-topped elevation located in the continental margins. The depth of water over a bank is relatively small. The Dogger Bank in the North Sea and the Grand Bank in the Atlantic off Newfoundland are famous examples. A shoal is a detached elevation with shallow depths. The shoal is not composed of a rock or coral.

A reef is predominantly organic deposit made by living or dead organisms that forms a mound or a rocky elevation like a ridge. It may be permanently submerged, submerged at high tide, or normally just above the water. The word reef is most commonly used for coral reefs built up in tropical seas by the hard calcareous material secreted by minute sea animals. The largest reef in the world is found off the Queensland coast of Australia.

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