Ocean Floor Topography Features - UPSC

• In our solar system, Earth is the only planet where availability of water is abundance; hence, it is called as 'Blue planet/Water planet'.

• Approximately 71% of the earth's surface covered by water and 29% land surface.

Ocean floor is not a plain area as believing earlier rather they have revealed a complex and varied relief feature, which is rival the relief feature on the land.

Various Relief Features of Ocean Floor Topography :

• Primary Features

i. Continental Shelf

ii. Continental Slope

iii. Continental Rise

iv. Abyssal Plain

• Secondary Features 

i. Ridges

ii. Trenches/Deeps

iii. Basins

iv. Banks

• Tertiary Features

i. Sea Canyons

ii. Coral Reefs

iii. Central Rift Valley

iv. Sea-Mounts 

v. Guyots

vi. Knots, Sea-hills etc.

Continental Shelf :

* It is place between land and continental Slope.

* Continental mass submerged under water.

* Depth - 200m/100 fathoms

* Slope angle - 1 degree or less

* Average width - 70km-120km

* Terrigenous deposits and marine organic deposits.

* Petroleum and minerals found.

* Either ware-cut or ware built platform (erosional or depositional)

* Very narrow along subduction zones where where parallel young fold mountains.

* Wider along stable coast line as it allows sea-wares to erode and deposit continental rocks.

* It covers 7.6% of the total sea area of Oceans. In Atlantic - 13.3%, Pacific - 5.7%, and Indian Ocean - 4.2%.

* Pacific has narrow due to subducted coast while Atlantic has wider due to stable coastline and wide opening at North. It is associated with Pleistocene.

* Shelves surrounding the glaciated regions have gentle slope in comparison to the coast where greater tectonic tension prevails or where high land areas are adjacent to the sea coast.

Continental Shelves in India :
* Demarcated by 100 fathom contour.
* Along the West coast, it is 150 Kms wide formed due to faulting and subsequent submergence.
Along the West coast, it is widdest in North (345 Kms off Daman), narrow down as we move South (120 Kms off Goa and 60 Kms of Kochi).
* Along Eastern Coast average width average width 50 Kms while 30-35 Kms off the mouth of the Ganga, the Mahanadi, the Godavari, the Krishna and the Cauvery.
* Along the mouth of these rivers, shelves have been formed through delta formation. Shelves from Midinapur to Madurai are the result of sedimentation and consequent subsidence while shelves of Andaman & Nicobar, Lakshadweep, Gulf of Mannar are Originated due to Coral reef.

Benefits of Continental Shelf :

* It is richest fishing grounds in the world due to presence of sunlight and growth of plankton e.g. the Grand Banks off Newfoundland, the North Sea and the Sunda Shelf.

* It is also richest resources of petroleum products.

* It support to develop natural harbour/sea port.

Continental Slope :

* It join Continental Shelf and Continental Rise.

* Slope angle - 4 to 5 degree.

* Depth extends from 200 m to 2000 m (sometimes upto 3000 m also).

* It covers 8.5% of total area of oceans. In Atlantic (7.6%), Pacific (5.6%) and in the Indian Ocean (5.8%).

* It is devoid of deposits due to its steep slope.

* Most significant features over slope are submarine canyons and trenches which are generally transverse to the coast.

Continental Rise : 

* It is place between continental slope and Abyssal Plain/Deep Sea plain.

* It occurs when continental slope loss it's steepness.

* Slope angle - 0.5 to 1 degree.

* The continental rise further loses steepness with depth and ultimately merged with ocean floor or deep sea plain.

Deep Sea Plains / Abyssal Plain :

* Largest covering area of the earth. It covers around 50% of total ocean area, Atlantic Ocean has less area under Deep Sea plain due to extensive shelf and slope area.

* Depth - 2000-6000m

* Flat rolling plain.

* Various secondary and tertiary features like trenches, sea mounts, sea hills, guyots etc.

* It is not complete flat area.

* Characterized by pelagic deposits of plant , marine animals, and siliceous remains but there is absence of erosional debris of terrigenous origin. 

Ocean Deeps / Submarine Trenches :

* Ocean deeps formed through subduction of plate. 

* Deposit zones of the ocean basin (Aphotic zone).

* It grouped into two categories - 

i. Deep - very deep but less expensive

ii. Trenches - long and narrow linear depression.

* Very steep Slope, sometime vertical.

* Pacific has 32 deeps, Atlantic has 19 while Indian ocean has 6 deeps.

* All the 57 deeps lie parallel to the recent folded mountains of the earth's crust and visually form steep vertical descent of great magnitude from land to the sea bottom.

* The Mariana trench in the Pacific Ocean is the deepest known part of the world.

* It has red clay deposits.

* Most of the earthquakes and tsunamis  are born in them.

Abyssal Hills : 

They are elevated features of volcanic origin. A submarine mountain or peak rising more than 1000m above the ocean floor is known as Sea Mount. A flat top Sea Mount is known as Guyots.

Sub-marine Ridges : 

It is mountain like structure on the floor which extends for 1000 km in length and several kilometres in width. e.g. Mid-Atlantic Ridges.

Submarine Canyons :

* Long, narrow and very deep valleys located on shelves and slope with vertical walls like Continental Canyons.

* Basis of Morphogenetic Process, It is classified into -

i. Glacially eroded Canyons - found along the Norwegian  Coast and also on the Arctic shelf. It is a wide trough like canyons, eroded eroded and carried by glaciers only.

ii. Non-glacial Canyons - More in number than glacial canyons and widly spread in all the oceans. These are actually described as submarine canyons.

* Except a few, all Canyons are found transverse to the coast and in front of the mouth of major rivers.

* According to Sheppard, the submarine canyons are similar to the youthful river valleys.

* Gradient decreases from upper to middle to lower segments of canyons.

* Depth vary from 610 to 915 m. A few places the depth has been noted upto 3048 m.

Banks : 

Bank is define as a flat top elevation in the the continental margin.  Here the depth of water is less but adequate enough for navigation. e.g. Grand Bank, George Bank in North Atlantic etc.

Shoal : 

It is a detached elevation having shallow depth. Since they project out of water moderately, therefore they are dangerous for navigation.

These great variety of relief is due to the interaction between tectonic force, erosional process, volcanic process as well as depositional process. At greater depth tectonic and volcanic process are more dominant. 

References
E-book
NCERT