Ocean Basins and Divisions of Ocean Floor in this very first and important topic of oceanography we will study here about divisions of ocean floor which includes Continental shelf, continental slope, continental rise, abyssal plain, submarine ridges, abyssal hills, submarine trenches or deeps, submarines canyons, bank, shoal and reef along with special features of each division…
Oceanography: Ocean Basins and Divisions of Ocean Floor
Table of Content
- Ocean Basins
- Continental Shelf
- Continental Slope
- Continental Rise
- Abyssal Plain
- Submarine Ridges
- Abyssal Hills
- Submarine Trenches or Deeps
- Submarine Canyon
- Bank, Shoal and Reef
The ocean floors can generally be categorised into four main divisions:
(i) Continental Shelf
(ii) Continental Slope
(iii) Continental Rise
(iv) Abyssal Plains.
Actually the border parts of continents submerged in ocean water are called Continental Shelf. From geological point of view these are like nearby continents.
(i) It extends between the Coast line and the continental slope.
(ii) Its width can be between few kilometres to 1300 km. In India the continental shelf is widest in Gujarat and spread to more than 100 km from the border line of the coast. On the coasts of Andhra Pradesh and Tamil Nadu its width is just 5 km.
(iii) The average width of continental shelf is 70 km.
(iv) The continental shelves form about 7.5 per cent of total area of the oceans.
(v) The total area of the continental shelves of the world is more than the area of the moon.
Depth:The continental shelf is generally between 150 metres 200 metres deep.
Gradient:The continental shelf has a gentle slope towards the ocean floor which on an average is 1º. In other words, with every 1 km width there is an increase of 2 metres in depth.
Structure:The sediments washed away from the land are deposited on the continental shelf. The foundation rock of the shelf can be igneous, sedimentary or metamorphic. From the point of view of its structure the continental shelf may be made by a glacier or be of coral origin like a reef or other such types.
Economic Importance:The continental shelf is of great importance to man. They can provide the richest ground for fishing. Many types of mineral ores like iron, gold, diamond tin, building materials such as sand, gravel, mineral oils and natural gas and hundreds of other chemicals are found hidden under their bed. The continental shelves together are a store house of 90% of all marine life.
Between the continental shelf and the continental rise lies the Continental Slope. The slope becomes suddenly steep seaward.
Extension:The continental slope forms about 8.5% of the total area of the ocean floor or the basin.
From geomorphological point of view continental slope forms a part of the Continental crust. It is considered the seaward edge of the continents.
Gradient:The slope is very steep in this part. At some places it stands like a wall Average gradient: 20-50
Depth:The depth of continental slope can be between 200 metres to 3660 metres.
Structure:On the continental slope, canyon like deep trenches are found. On the mouths of Indus and Ganga similar canyons are found. At the end of these trenches are found alluvial depositions in the shape of layers.
Types:Continental slope is of five types: (i) Steep Slope (ii) Gentle Slope (iii) Faulted Slope (iv) Terraced Slope (v) The slopes with sea mounts.
After the continental slope towards the ocean begins the Continental Rise. This is an area of gentle slope between 0.5° to 1°. The depth goes on increasing gradually and finally it merges and becomes flat with the abyssal plain.
Beyond the continental rise there is almost flat deep sea plain called the Abyssal Plain.
Extension:Abyssal Plain covers almost 40% of the entire oceanic area.
Gradient:These are almost flat. No sharp physical or relief features are found here. The slope is less than 1:100.
Depth:The depth of this part of the ocean is from 3000 to 6000 metres. On account of this deep depth the temperature of water in this part remains less than 4°C.
Deposition:There are different types of sedimentary layers on abyssal plains. Some sediments belong to land and some are the remains of marine life. On account of these sediments the abyssal plains have become almost flat.
On ocean floor are found mountain ranges like those on the land. These submerged mountain ranges are called Submarine Ridges.
(i) These are the longest mountain ranges of the world. The Himalayan mountain ranges look a triple in comparison to these.
(ii) The total length of the ridges is more than 75000 km.
(iii) Most of the ridges are found in the centre of the oceans. One such ridge extends from north to south in the centre of the Atlantic Ocean known as the Mid Atlantic Ridge.
(iv) Somewhere these ridges are like gently sloping plateaus and at other place they are steep sided mountain ranges.
(v) The tops of these ridges at many places are found in the form of islands above sea level. (vi) These ridges are representative of tectonic activities. At some places the ridges are found along the edges of continental plates.
There are many solo hills, sea mounts and guyots on the ocean floor
(1) Sea Mounts:A submarine mountain or peak rising to 1000 metres height above the ocean floor are called sea mounts. Their tops are cone shaped but remain submerged in water.
(2) Guyots:Flat topped submerged sea mounts are known as guyots. On account of ocean waves tops of sea mounts become flat and are called guyots. Sea mounts and guyots are found in large numbers on the ocean floor. According to one estimate their number is more than 10,000 in the Pacific Ocean alone. These are formed by volcanic eruptions on the ocean floor.
Submarine Trenches or Deeps
The long, narrow and steep sided depressions on the ocean floor are known as Submarine Trenches or Deeps.
(i) The trenches are the deepest part of the ocean.
(ii) Their length ranges between 300 km to 5000 km and width between 30 and 100 km.
(iii) These trenches are more than 5,500 metres deep.
(iv) They are located very close to fold mountain ranges near the sea coast or near borders of island chains.
(v) They are formed on account of tectonic activity.
(vi) Sometimes they may even be sloped like a standing wall.
(vii) The Peru-Chile Trench is the worlds largest trench. It is 5,900 km long.
(viii) Largest number of trenches are found in the Pacific Ocean. Among these Mariana Trench is the world’s deepest (11034 m). If Mt. Everest were to be submerged in this trench there will still be 2186 metre deep ocean water above it.
The deep valleys (gorges) or steep sided depressions on the ocean floor are called Submarine Canyons.
(i) They are generally confined to continental margins or between continental shelf and continental rise.
(ii) These are V-Shaped deep valleys. They have associated canyons also.
(iii) Their depth ranges between 610 metres and 915 metres. At places the depth may go down to 3048 metres.
Types:Submarine canyons are of three types:
(1) Small gorges:They originate from the edge of continental shelf and extend down upto the end of continental slope. For the sake of example Oceangrapher Canyon of New England (U.S.A) can be cited.
(2) Canyons at the mouths of rivers: These begin from the mouths of rivers and extend over the continental shelf and across the slope. Such canyons are found at the mouths of rivers Congo, Mississippi, Indus and Ganga. The famous Hudson Canyon beginning from the mouth of Hudson river goes down to the Atlantic Ocean floor.
(3) Dendritic Submarine Canyons:
There is a network of main canyons and associated canyons which cut deep on continental shelf and slope. They make the shape of a branched out tree. An example of this type of canyon is found in the canyons off the coast of southern California in the U.S.A
The world’s largest canyon occurs in Sea off Alaska. They are named Berring, Pribil of and Zhem Chug. On the submerged edges of some canyons alluvial fans have also been formed.
Bank, Shoal and Reef
(1) Bank:These are located close to continents. They are flat topped raised land. They are of very shallow depth but enough for passage of ships. The banks are famous for growing fish. The Grand Bank on Atlantic Ocean is located near Newfoundland (North America).
Dagger Bank:It lies in the east of U.K in the North Sea.
George Bank:It is situated in the north-east of New York Port in Atlantic Ocean. These banks were formed during ancient period by subsidence of ocean floor which were later on eroded or material deposited on them by glaciers.
(2) Shoals:Some raised spots on banks are called shoal. Due to very shallow waters they present a danger to shipping. On the Dagger Bank there are 18 such raised shoals.
(3) Reefs: These are masses of rock made by corals and other marine organisms. The reefs are of several types like fringing reef, the barrier reef and coral reef. Most of the coral reefs are found in the Pacific Ocean. The Great Barrier Reef off the Queensland Coast in Australia is the largest barrier reef in the world. The oceans are vast reservoirs of our future needs, conveniences and food. Therefore knowledge about ocean floor profile is very useful and important to us.