The ocean basins are in many ways similar to the land surface.

There are submarine ridges, plateaux, canyons, plains and trenches. A section drawn across an ocean (Fig. 12.4) illustrates the typical submarine relief features.



1) The continental shelf: Surrounding the continent is a shallow water zone known as the continental shelf. It can be considered as a continuation into the sea of the coastal plain. It forms a gently sloping ledge, extending under the sea to an average depth of 180 metres (100 fathoms). The width of this shelf varies considerably. Off the west coast of the United States and along other coasts bordered by young fold mountains, it is less than two km wide or may be absent altogether. Off the Siberian coast the shelf attains its greatest width of 1200 km (about 750 miles). Continental shelves form 7 per cent of the total area of the oceans. A width of 32 to 160 kilometres is generally encountered. The angle of the slope is also variable and is normally smallest where the continental shelf is widest. A gradient of 1 in 500 is common to most continental shelves.

Many regard the continental shelf as part of the continent submerged due to a rise in sea-level, e.g. at the close of the Ice Age, when the ice in the temperate latitudes melted and raised the sea-level by several hundred metres. Some smaller continental shelves could have been caused by wave erosion where the land is being eroded by the sea, as shown in Fig. 12.5.

Conversely, such shelves might have been formed by the deposition of land-derived or river-borne materials on the off-shore terrace as in Fig. 12.6.

The continental shelves are of great geographical significance for the following reasons.

(a) Their shallowness enables sunlight to penetrate through the water, which encourages the growth of minute plants and other microscopic organisms. They are thus rich in plankton on which millions of surface and bottom-feeding fishes thrive. The continental shelves are therefore the richest fishing grounds in the world e.g. the Grand Banks off Newfoundland, the North Sea and the Sunda Shelf. The continental shelf of Mauritania, with plenty of plankton and cool waters (derived from the cold Canary Current), provides a good habitat for fish of various types. This has attracted both local and European fishermen who have set up fishing industries, with Port Etienne as the fishing port. The shelf extends to the neighbouring Senegal with similar conditions, where the Cayor deep, regarded as a drowned river valley, has become a major fishing ground.


(b) In addition, areas with continental shelves offshore experience higher tides than those without them. The higher tides allow ships to penetrate further inland and also assist in scouring river estuaries of silt deposits. Most of the world’s greatest seaports, including Southampton, London, Hamburg, Rotterdam, Hong Kong and Singapore, are located on continental shelves.


2) The continental slope: This links the continental shelf with the deep-sea plain or ocean floor. It displays a much steeper slope than the continental shelf, and extends to a depth of 3000 metres (about 10 000 feet) where the deep-sea plain begins.


3) The deep-sea plain: This is the undulating plain lying two or three kilometres below sea-level, and covering two-thirds of the ocean floor, generally termed the abyssal plain. It was once thought to be featureless, but modern sounding devices reveal that 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 oceans, e.g. the Azores and Ascension Island.


4) The ocean deeps: These are the long, narrow trenches that plunge as great ocean deeps to a depth of 5000 fathoms or 9000 m (30 000 feet)! Contrary to our expectations, most of the deepest trenches are not located in the midst of oceans. They are more often found close to the continents, particularly in the Pacific Ocean, where several deep trenches have been sounded. The greatest known ocean deep is the Mariana Trench near Guam Island, which is more than 10 900 m (36 000 feet) deep. We can see from this that ocean trenches are greater in magnitude than the highest mountains on land, for the highest peak, Mt. Everest, is only 8845 m (29 028 feet). Other notable ocean deeps include the Mindanao Deep (10 700 m/35 000 feet), the Tonga Trench (9450 m/31 000 feet) and the Japanese Trench (8500 m/28 000 feet), all in the Pacific Ocean.


The Deposits of the Ocean Floor

Materials eroded from the earth which are not deposited by rivers or at the coast are eventually dropped on the ocean floor. The dominant process is slow sedimentation where the eroded particles filter very slowly through the ocean water and settle upon one another in layers. The thickness of the layer of sediments is still unknown. Its rate of accumulation is equally uncertain. Generally speaking, we may classify all the oceanic deposits as either muds, oozes or clays.

  1. The muds: These are terrigenous deposits because they are derived from land and are mainly deposited on the continental shelves. The muds are referred to as blue, green or red muds; their colouring depends upon their chemical content.
  2. The oozes: These are pelagic deposits because they are derived from the oceans. They are made of the shelly and skeletal remains of marine micro-organisms with calcareous or siliceous parts. Oozes have a very fine, flour-like texture and either occur as accumulated deposits or float about in suspension.
  3. The clays: These occur mainly as red clays in the deeper parts of the ocean basins, and are particularly abundant in the Pacific Ocean. Red clay is believed to be an accumulation of volcanic dust blown out from volcanoes during volcanic eruptions.

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