Relief Zones of Ocean Basins | Oceanography | Geography

The ocean basins are characterized by four relief zones: 1. Continental Shelves 2. Continental Slopes 3. Deep Sea Plains 4. Ocean Deeps.

Zone # 1. Continental Shelf:

The continental marginal areas submerged un­der oceanic water with average water depth of 100 fathoms (one fathom = 6 feet) and gently sloping (1°- 3°) towards the sea or the oceans are called continental shelves.

The width of continental shelves largely de­pends on the nature of reliefs of the coastal land i.e.:

(1) the shelves are narrow where high mountains are very close and parallel to the coast (e.g., the Pacific conti­nental shelf along the western coast of S. America is narrow (16 km) because of the presence of the Andes mountain), and

(2) the shelves are wider where the coast lands are wide plains. Though the continental shelves are generally wider in front of river mouths but the shelf off the Mississippi mouth is exceptionally narrow.

On an average, the width of continental shelves is about 48 km though Sheppard has taken 67km (42 miles) as average width. The Pacific continental shelf of South America represents the example of narrow shelf (16 km), the Atlantic continental shelf off the east coast of North America represents the example of medium size shelves (96-120 km) arid extensive shelves having width of a few hundred kilometres are found off the coast of East Indes, in the Arctic Sea, China Sea, Adriatic Sea, Arafura Sea etc.

Continental shelves represent 8.6 per cent of the total area of the ocean basins. Regionally these cover 13.3 per cent, 5.7 per cent and 4.2 per cent of areal coverage of the Atlantic Ocean, the Pacific Ocean and the Indian Ocean respec­tively.

The maximum seaward limit of the continental shelves off the Indian coast is demarcated by 100 fathom contour. The continental shelves along the eastern and the western coasts of India are 50 km and 150 km wide respectively. The shelves are narrow (30-35 km) off the mouths of the Ganga, the Mahanadi, the Godawari, the Krishna and the Cauvery rivers but these are wider off the estuaries of the Narmada, the Tapi and the Mahi rivers. The average slope of the continental shelves off the eastern Indian coast is about 21° whereas it is 10° near Cape Comorin and only 10° near the Gulf of Combay.

Origin of Continental Shelves:

The Nature, composition, extension and depth of continental shelves are so varied that it becomes difficult to explain their exact mode of origin through a single mechanism and process.

The following differ­ent views have been expressed by several authorities to explain the complex origin of continental shelves:

(1) Continental shelves are basically the ex­tended form of continental platforms. Marine waves and currents erode the continental margins and thus form extensive platforms which receive deposits of sediments brought down by the rivers and sea waves. These sediments are continuously consolidated under sea water and ultimately extensive continental shelves are formed. Thus, the continental shelves are the result of marine erosion and fluvial deposits.

(2) Continental shelves are formed through pro- longed deposition of detritus (under sea water) brought by the rivers alone. Such type of continental shelves is formed only in those areas where sea conditions are calm so that prolonged sedimentation goes on uninter­ruptedly resulting into subsidence and thus allowing more and more sedimentation. Such continental shelves are constructional and are most extensive.

(3) Rising thermal convective currents from be­neath the continents and the ocean basins converge along the continent-ocean boundary and descend. The resultant compressive force causes subsidence of the continental margins and thus continental shelves are formed.

(4) Sometimes, parallel faults are created in the continental margins. This event causes subsidence of the marginal land areas and consequent submergence under sea water. Such submerged land areas become continental shelves, which are generally called as tectonically formed continental shelves.

(5) Continental shelves are formed through marine erosion of the continental margins when there is negative change in sea-level (fall in sea-level) either during ice ages or due to subsidence of oceanic floors. According to R. A. Daly the sea level fell by 38 fathoms during Pleistocene Ice Age, with the result the conti­nental margins which were previously submerged be­came free from sea water.

These exposed land areas were glacially eroded and extensive platforms were formed. Due to deglaciation the sea level rose again and these platforms were submerged under seawater and thus extensive continental shelves were formed. This concept of the origin of the continental shelves belongs to glacial control theory.

(6) The coastal lands are effectively eroded through abrasive work of strong sea waves and several sea cliffs are formed. These cliffs gradually but continuously recede towards the land due to basal erosion and conse­quent fall of their hanging crests and thus extensive wave-cut platforms are formed. These platforms are submerged under sea water to form continental shelves.

(7) The submergence of continental margins due to tilting of land towards the sea results into the formation of continental shelves. This process also leads to the extension of existing continental shelves.

The continental shelves of India have been formed differently. The continental shelves off the Ganga, the Godawari, the Krishna and the Cauvery mouths have been formed through delta formation. The continental shelves from Midinapur to Madura are the result of sedimentation and consequent subsidence while the shelves of Andman Nicobar, Lakshadweep, Gulf of Manar (between India and Sri Lanka) are originated due to coral reefs. The continental shelves of western coast are due to faulting and consequent submergence.

Zone # 2. Continental Slope:

The zone of steep slope extending from the continental shelf to the deep sea plains is called conti­nental slope which varies from 5° to more than 60° at different places e.g. 40° near St. Helena, 30° off Spanish coast, 62° near St. Paul, 5° to 15° near Calicut coast (India) etc. The depth of water over continental slope varies from 200m to 2,000m. Continental slopes occupy only 8.5 percent of the total area of the ocean basins but it varies from one ocean to the other e.g., 12.4 per cent in the Atlantic Ocean, 7 per cent in the Pacific Ocean and 6.5 percent in the Indian Ocean.

The most extensive continental slopes are found between 20°N and 50°N latitudes and on 80°N and 70°S latitudes. Generally, the steep gradient of the continental slopes does not allow any marine deposits because the mate­rials coming down from the continental shelves are immediately removed downward but in some cases a thin veneer of deposits does exist. The most significant reliefs on the continental slopes are submarine canyons and trenches which are generally transverse to the continental shelves and the coasts.

The origin of continental slopes has been re­lated by various authorities to erosional, tectonic and aggradational processes. The erosion theory of the origin of continental slopes is based on the presence of submarine canyons. According to this theory slopes are formed due to erosion by marine processes mainly sea waves.

According to tectonic theory faulting is held responsible for the origin of continental slopes. Some exponents believe that the continental slopes are formed due to bending and warping of continental shelves followed by sedimentation.

Zone # 3. Deep Sea Plains:

Deep sea plain characterized by flat and rolling submarine plain is the most extensive relief zone of the ocean basins. These deep-seated plains having the depth from 3000m to 6000m cover 75.9 per cent of the total area of the ocean basins but this areal coverage varies from one ocean to the other (80.3 per cent in the Pacific Ocean, 80.1 per cent in the Indian Ocean and 54.9 per cent in the Atlantic Ocean).

Remarkably low areal coverage of deep sea plains in the Atlantic Ocean in comparison to the Pacific and Indian Oceans is attributed to larger extent of continental shelves in the former. Though vast and extensive deep sea plains are generally featureless but a few long, narrow and elon­gated ridges, guyots etc. are significant reliefs.

The submarine ridges with steep side-slopes sometimes reach the sea-level and even project above the water surface and appear as islands. Mid-Atlantic ridge, East Pacific Rise and mid-Indian Ocean ridge are typical examples. Deep sea plains are characterized by pelagic deposits of plant, marine animal and siliceous remains but there is absence of erosional debris of terrigenous origin. Volcanic deposits have been reported at few places in different oceans.

Zone # 4. Ocean Deeps:

Ocean deeps representing depressions and trenches on the ocean floors are the deepest zones of the ocean basins. These are generally located parallel to the coasts facing mountains and along the islands.

Ocean deeps are grouped into two categories accord­ing to size viz.:

(1) Very deep but less extensive depres­sions are called deeps while.

(2) Long and narrow linear depressions are called trenches.

These deeps and trenches are characterized by very steep slopes. Sometimes, these rise almost to verticality.

These deeps and trenches have been usually named after the explorers and their geographical locations e.g. Murray Deep (after J. Murrary), Japan and Sunda Trenches (after geographi­cal location). Out of the explored and surveyed 57 deeps, the Pacific Ocean, the Atlantic Ocean and the Indian Ocean account for 32, 19, and 6 deeps respec­tively. Mariana Trench located to the west of Philip­pines in the North Pacific Ocean is the deepest (11.02 km deep) of all the ocean deeps.