- 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 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 Godavari, the Krishna and the Cauvery rivers. These shelves 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°, it is 10° near Cape Comorin and only 1° near the Gulf of Cambay.
12.2.3 ECONOMIC SIGNIFICANCE –CONTINENTAL SHELF
- Most commercial exploitation from the sea, such as metallic-ore, non-metallic ore, and hydrocarbon extraction, takes place on the continental shelf.
- The shallowness enables sunlight to penetrate through the water, which encourages the growth of minute plants and other microscopic organisms – planktons (food for fishes). Thus continental shelves are the richest fishing grounds in the world. E.g. Grand Banks of Newfoundland, the North Sea and the Sunda shelf.
- Their limited depth and gentle slope increase the height of tides. Since ships can only enter and leave port on the tide, most of the World’s greatest seaports including Southampton, London, Hong Kong, Singapore and Rotterdam are located on Continental Shelves.
INTERNATIONAL CONVENTION ON CONTINENTAL SHELF
- Sovereign rights over their continental shelves up to a depth of 200 metres or to a distance where the depth of waters admitted of resource exploitation were claimed by the marine nations that signed the Convention on the Continental Shelf drawn up by the UN’s International Law Commission in 1958.
- This was partly superseded by the 1982, United Nations Convention on the Law of the Sea which created the 200 nautical mile-exclusive economic zone and extended continental shelf rights for states with physical continental shelves that extend beyond that distance.
- The legal definition of a continental shelf differs significantly from the geological definition. UNCLOS states that the shelf extends to the limit of the continental margin, upto 200 nautical miles from the baseline.
- Thus inhabited volcanic islands such as the Canaries, which have no actual continental shelf, have a legal continental shelf, whereas uninhabitable islands have no shelf.
12.3. 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.
- At the edge of the Continental Shelf, there is an abrupt change of gradient, forming the Continental Slope.
- The Continental Slope connects the continental shelf and the ocean basins.
- The most significant reliefs on the 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.
- It begins where the bottom of the continental shelf sharply drops off into a steep slope.
- The gradient of the slope region varies between 2-5°.
- The depth of the slope region varies between 200 and 3,000 m.
- The slope boundary indicates the end of the continents.
- Submarine Canyons and trenches are significant reliefs in this region, generally transverse to the continental shelves and the coasts.
12.4 DEEP SEA PLAIN
- Deep Sea Plain is the flat and rolling submarine plain lying two or three miles below sea level, and covering two-thirds of the ocean floor, generally termed as Abyssal Plains.
- These are gently sloping areas of the ocean basins cover 75% of the total area of the ocean to the other.
- These are the flattest and smoothest regions of the world. (Modern sounding services reveal that abyssal plain is not being level and it has extensive submarine plateaux ridges, trenches, guyots basins and oceanic islands)
- The depths vary between 3,000 and 6,000 m.
- These plains are covered with fine-grained sediments like clay and silt.
- The submarine ridges with steep side-slopes reach the sea level and even project above the water surface and appear as islands. E.g. Mid-Atlantic ridge.
12.5 OCEANIC DEEPS OR TRENCHES
- Ocean deeps represent depressions and trenches (reaches depth of 5,000 fathoms) on the ocean floors, are the deepest parts of the ocean basins.
- Ocean deeps are grouped into
- Deeps: very deep but less extensive depressions.
- Trenches: long and narrow linear depressions. (E.g. Mariana Trench located to the west of Philippines in the North Pacific Ocean is the
deepest trench (11,000 metres)).
- These are generally located parallel to the coasts facing mountains and along the islands. They are more often found close to the continents, particularly in the Pacific Ocean.
- The trenches are relatively steep sided, narrow basins. They are some 3-5 km deeper than the surrounding ocean floor.
- They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes. That is why they are very significant in the study of plate movements.
- As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean and 6 in the Indian Ocean.
12. 6 MINOR RELIEF FEATURES
Apart from the above mentioned major relief features of the ocean floor, some minor but significant features predominate in different parts of the oceans.
I. MID-OCEANIC RIDGES
- A mid-oceanic ridge is composed of two chains of mountains separated by a large depression.
- The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface.
- Iceland, a part of the mid- Atlantic Ridge, is an example.
- 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.
- The Emperor seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is a good example.
III. SUBMARINE CANYONS
❖ These are long, narrow and very deep valleys located on the continental shelves and slopes with vertical walls resembling the continental canyons are called submarine canyons.
❖ They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers.
❖ Submarine canyons are classified on the morphogenesis as
❖ Glacially eroded canyons
❖ Non-glacial canyons
❖ The Hudson Canyon is the best known canyon in the world.
- It is a flat topped seamount.
- They show evidences of gradual subsidence through stages to become flat topped submerged mountains.
- It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.
- 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.
13. RELIEFS OF THE OCEANS
13.1 BOTTOM RELIEFS OF ATLANTIC OCEAN
13.1.1 ATLANTIC OCEAN – ATLANTIC OCEAN
- The Atlantic Ocean located between North and South Americas in the West and Europe and Africa in the East covers an area of 82,000,000 km.
- It covers one-sixth of the geographical area of the earth.
- The ‘S’ shape of the ocean indicates the fact that landmasses (continents) on its either side were once a contiguous part.
- The ocean was formed due to drifting of North and South America to the west due to plate tectonics.
- The average depth of the ocean is less than the Pacific Ocean because of continental shelves and marginal and enclosed seas.
- The Atlantic Ocean widens to the south of equator. It narrows down towards the equator. It narrows down in the extreme north and contacts the Arctic Ocean.
- Currently, widening of the ocean (4 cm/year) is observed through seafloor spreading.
13.1.2 CONTINENTAL SHELF
- Continental Shelves have developed along both the coasts of Atlantic Ocean and with the width ranging from 2-4 km to 80 km.
- The width of continental shelves has been largely controlled by the reliefs of the coastal lands.
- Narrow shelves are observed when mountains and hills border the coasts.
E.g. the African shelves and Brazilian shelves between 5° and 10° S latitudes.
- Wider shelves (200-400 km) are observed along the north-eastern coast of North America and the north-western coast of Europe.
E.g. Extensive shelves are found around Newfoundland and British Islands. Very extensive shelves are found in South Atlantic Ocean, mainly between Antarctica and Argentina.
- Many marginal seas are located on the continental shelves in the North Atlantic, but absent in the South Atlantic.
- Continental shelf-seated seas significant are Hudson Bay, Baltic Sea, North Sea, Davis Strait, Denmark Strait, etc.
- Several islands are located on the shelves – Iceland, Newfoundland, Falkland, British Isles, West Indies, Canaries, St. Helena, etc.
13.1.3 MID-ATLANTIC RIDGE
- The mid-Atlantic ridge representing the zone of divergent or constructive plate margins (American plates moving westward and Eurasian and African plates moving to the east) is the most striking relief feature which having ‘S’ shape extending from Iceland(north) to Bouvet Island(south).
- This ridge is known as Dolphin rise (north of the equator) and
Challenger rise (south of the equator).
- Though major part of the mid Atlantic ridge is submerged under oceanic water but number of peaks and sea mounts project well above the water surface and forms islands. E.g. Pico island of Azores (8,200 m).
- The mid-Atlantic ridge has number of fracture zones. E.g. Gibbs fracture zone, Atlantis fracture zone, Oceanographic fracture zone, etc.
Important features of Mid Atlantic Ridge Wyville Thompson ridge – between Scotland & Ireland, Telegraphic plateau – between Greenland & Iceland (first cables laid down), Newfoundland rise, Azores rise, Sierra Leone rise, Para rise, Guinea ridge, Walvis ridge and Rio Grande rise are some of the important relief features in Mid Atlantic ridge.
13.1.4 OCEAN BASINS
- The mid-Atlantic Ridge divides the Atlantic Ocean into two major basins- East and West Atlantic Basins.
- Labrador Basin extends between the continental shelf of Greenland in the north and Newfoundland rise in the south covering latitudinal extent of 40°N to 50°N where the depth of basin ranges from 4,000 to 4,500 m.
- North American Basin is the most extensive basin of the Atlantic Ocean and extends between 12°N and 40°N latitudes.
- Brazilian Basin is confined between the Equator and 30°S latitude an East coast of Brazil in the west and Para Rise in the east. The depth is more than 4,000 m.
- Spanish Basin is located between the mid-Atlantic Ridge and Iberian Peninsula. It is bordered by Azores Rise in the south and extends upto 50°N latitude. The average depth is 5,000 m.
13.1.5 OCEAN DEEPS
- The number of deeps in the Atlantic Ocean is far less than in the Pacific Ocean because of the absence of effects of Tertiary orogenic movements along the Atlantic coasts.
- The Mediterranean Sea, Caribbean Sea and Gulf of Mexico are significant marginal seas in the Atlantic Ocean.
- The Mediterranean Sea is divided into two major basins (East & West Basins) by 4,000 m deep mid-sea ridge (which runs from Southern Italian coast and to the North American coast).
- The Gulf of Mexico and Caribbean Sea are separated by 1,600 m deep ridge running between Yucatan peninsula and Cuba Island.
- The prominent basins are Mexico basin and Caribbean basin.
13.2 BOTTOM RELIEFS OF THE PACIFIC OCEAN
13.2.1 PACIFIC OCEAN – INTRODUCTION
- The Pacific Ocean is the largest ocean in the Earth covering one-third area.
- It extends for 16,000 kms from the East coast of Asia (west) to the West coast of Americas (east). Its north-south extension is 15,000 kms from Bering Strait (north) to the north of Cape Adre, Antarctica (south).\
- The overall shape of the ocean is triangular and the average depth is 4,500 m.
- Both the coasts of the Pacific Ocean are paralleled by the chains of folded mountains, resulting in the steep abyssal plains.
- The ocean has the largest number of islands (>2000). It has numerous islands, island arcs and festoons in the western coast and only few islands in the eastern coast.
Classification of Pacific Islands:
- Continental Islands: Aleution island, Chilean island
- . Island Arcs and Festoons: Kurile, Japanese Archipelago, Philippines, Indonesian islands
- Scattered smaller Islands:
- i. Islands based on racial groups: Malanesia, Micronesia & Polynesia
- ii. Islands formed of volcanic materials & Coral reefs – Hawaii island, Fizi, Ellice, Coral Islands
13.3.2 Classification of Pacific Ocean
- . North Pacific: Deepest part (5000-6000 m), in contact with Arctic sea through Bering Strait.
- Central Pacific: Largest number of islands (volcanic & coral).
- South-West: number of Islands, Marginal seas, Continental shelves, Oceanic trenches.
- South-East: East Pacific Rise or Ridge (absence of marginal seas).
13.3.3 CONTINENTAL SHELF
- The Shelves are broad and extensive along the Eastern coasts of Australia and Asia where the width varies from 160 – 1600 km and the depth ranges between 1000 – 2000 m.
- Several Islands are seated on broad continental shelves. E.g. Kuril Islands, Japanese Islands, Philippines, Indonesia, New Zealand, etc.
- The continental shelves carry numeral marginal seas like Bering Sea, Okhotsk Sea, Japan Sea, Yellow Sea, China Sea, Java Sea, Coral Sea, Tasmanian Sea, Arafura Sea, etc.
- The continental shelves are less extensive along the western coast of Americas, because of presence of cordillerean chains of folded mountains to the coastal lands.
13.3.4 EAST PACIFIC RISE
- The Pacific Ocean does not have mid-oceanic ridge like Atlantic or Indian Ocean (only few scattered ridge of local importance).
- The East Pacific Rise or Ridge known as Albatross Plateau is 1600 km wide and it extends from north of New Zealand to the Californian coast.
- Other significant features in East Pacific Rise: New Zealand ridge, Fiji plateau, Hawaiian rise (most extensive ridge of the Pacific Ocean), New Guinea rise, etc.
- Fracture zones: Mendocino fracture zone, Murray fracture zone, Eastern Island fracture zone, Challenger fracture zone, etc.
13.3.5 OCEAN BASINS
- There are different basins of different shapes and sizes. These basins are separated by ridges and ‘rises’.
- Philippines Basin is located to the east of Philippines and extends from south of Japan to 5°N latitude. Kyushu – Paian Ridge runs through the middle of
the basin. Average depth ranges from 5000 m – 6000 m.
- Fiji Basin is located to the south of Fiji Island between 10° S and 32° S latitudes and the average depth is 4000 m. The basin to the north of 20° S is known as North Fiji Basin whereas the South Fiji basin between 20° and 32°S.
- East Australian Basin is situated between the east coast of Australia and New Zealand Ridge with average depth of more than 5000 m.
- South Australian Basin also known as Jeffreys Basin is located to the south-east of Australia having average depth of 5000 m.
- Peru Basin is located to the west of Peru coast between 5° S and 24° S latitudes extends upto 110° W longitude. The average depth of the basin is 4000 m.
13.3.6 OCEAN DEEPS
- There are several trenches and deeps in the Pacific Ocean. These depressions are located either along the island arcs or mountain chains.
- These trenches are found mainly in the western Pacific Ocean.
BOTTOM RELIEFS OF THE INDIAN OCEAN
INDIAN OCEAN – INTRODUCTION
- The Indian Ocean is the third largest ocean by area and it is bounded by Asia in the north and east, Africa in the west, Australia in the south-east and Antarctica in the south.
- The ocean has contact with the Pacific and Atlantic Ocean in the south near Antarctica and the average depth of the ocean is 4,000 m.
- Major parts of the coastal lands are formed by Block Mountains of Gondwana land.
- The marginal seas are less compared to the Pacific and the Atlantic Oceans. Some of the marginal seas are Mozambique Channel, Andaman Sea, Arabian Sea, Bay of Bengal, Persian Gulf, Red Sea, etc.
- Madagascar and Sri Lanka are big islands, whereas Laccadive, Andaman-Nicobar, Seychelles, St. Paul, Maldives, etc. are small islands.
- Indian subcontinent in the north divides the Indian Ocean into – Arabian Sea & Bay of Bengal in the north and the Indian Ocean widens in the south.