Features of Ocean Bottom Relief (OBR)

Friday, July 24, 2020

The average depth of ocean is about 3800 metre from sea level while the average elevation of Lithosphere is only 840 metre from sea level. Ocean Bottom Relief features is of 5 types - 
  1. Continental Shelf 
  2. Continental Slope 
  3. Deep Sea Plains 
  4. Oceanic Deeps and Trenches
  5. Submarine Canyons
Hipsometric/Hypsographic Curve

1. Continental Shelves

These are extended continental margins which is submerged under Oceanic Water. 
It's slope is gentle and range between 1°-3° degree while average depth is 100 fathoms. 
The width of continental selves depend on two factors:-
  1. If there is a high mountain, closer to and parallel to the coast near the ocean than continental shelves will be narrow. Example- Pacific Continental Shelf(only 16 Kms) along Andes Mountain in South America.  
  2. When wide plains are along the coast than the shelves will be wider. Example - North America (East Coast) - 96-120Km


There are multiple theories for origin of Continental Shelves:- 
  1. Formed due to subsidence of continental Coasts due to rising thermal convective shells beneath the Earth. 
  2. Due to prolong Detritus deposition by rivers near the coast (generally formed in Calm Oceans/Seas). 
  3. Faulting of Continental blocks due to Tectonic activities. 
  4. Fall in sea level either due to glaciation or subsidence of oceanic floors.
  5. Bending/tilting of continental plate towards ocean due to Tectonic activities. 
  6. Marine Erosion by Ocean Currents and fluvial deposition brought by rivers. 
  7. Due to formation of wave cut platforms.

2. Continental Slopes

It extends from continental shelves to deep sea plains.
It's slope varies from 5° to 60°.  Most extensive slopes are generally found between 2 0° North to 50° North Latitudes and on 80° North and 70°  South Latitudes.
These generally lack any kind of deposition due to high gradient slope.
The most significant feature of Continental Slope can be formed in the form of Submarine Canyons and Trenches. 


  1. Formed due to faulting of Continental Shelves due to rising Convective Limbs. 
  2. Erosion by Marine Waves.  
  3. Bending and Warping of Continental shelves followed by sedimentation.

3. Deep Sea Plains 

These are characterized by flat and rolling Submarine plains. Their depth ranges from 300 metre to 6000 metre in sea. 
This is the most extensive relief of Oceanic Bottom. About 75.9% of Oceanic relief is Deep sea Plains, it's proportion in Pacific Ocean is  80.3%, while in Indian Ocean it is 80% but in Atlantic Ocean it is only 54.9%.
Very less extension of Deep sea Plains in Atlantic Ocean is due to it's more extensive Continental Shelves found. 
Deep Sea Plains comprises various ridges, some of them come out of the oceans.
Examples- Mid Atlantic Ridge, Indian Ocean Ridge, East Pacific Ridges etc.
Deep Sea Plains are also characterized by Pelagic deposits of plants, Marine animals and siliceous remains but there is no presence of terrigenous materials. Volcanic deposits are also found in some oceans but in limited proportion.

4. Oceanic Deep/Trenches 

These are the deepest part of Oceanic Bottom Relief generally located parallel to the coast(facing mountain) and along islands.
Basically these are of two types:-
  1. Deeps - very deep but narrow belt.
  2. Trenches - Long and extensive and parallel to the coast or Island. 
There total 57 deeps and trenches discovered till today Pacific Ocean (32), Atlantic Ocean (19) and Indian Ocean (6).
Example- Mariana Trench/West(11,022 m) of Philippines is the deepest trench of the Earth. 


It is well understood that these are formed due to subsidence of older/denser Oceanic Crust due to crustal movement.

5. Submarine Canyons 

It is similar to Continental Canyons in terms of appearance. These are found on Continental Shelves and Slopes. But these are more steeper than Continental Canyons. Their depth are generally 610m to 915m. These are found transverse to the coast and in front of the mouth of major rivers. They resembles youthful rivers on Continent, while some of them showing dendritic pattern.

These are classified into two categories:- 
  1. Glacialy eroded canyons (very few in number)
  2. Non-glacial canyons (widely spread)
Canyons located near islands are deep with steepest gradient (13.8%). 
According to a study by Seppard and Beard there are 102 Submarine Canyons are reported.
Here more coarser deposits are found than adjacent Continental Shelf and Slopes like, silts, sands, clay, grabble and pebbles.


  1. Disastrophic Theories (Tectonic activities).
  2. Erosion by river and later Sea level rose(Sub-arial Theory)
  3. Submarine Density Current Theory(Ocean Current formed due to difference in temperature and salinity).
  4. Turbidity current Theory.(Current with suspended sediments eroded the continental shelves).