Mentors Comment:

• Provide a brief introduction about oceanic and continental relief.
• Discuss the reasons for formation of various oceanic reliefs.
• Discuss the significance of various oceanic reliefs.

Answer:
The lithosphere (crust and upper mantle) has two broad relief features — continents and ocean basins, which are created by the movement of plates along plate-tectonic boundaries, and consequential volcanic and depositional processes. The relief features of continents may be seen to consist of active mountain- making belts (narrow zones along the margins of lithospheric plates, like the Alps and Himalayas) and inactive regions of old, stable rock (continental shields and mountain roots).

Oceans have a more diverse relief than the continents. Much of the oceanic crust is less than 60 million years old, while the great bulk of the continental crust is over 1 billion years old. The floors of the oceans are rugged with the world’s largest mountain ranges, deepest trenches and largest plains. It is classified into major and minor relief features. Continental shelf, continental slope, oceanic deeps and deep sea plain are major while hills, sea mounts, guyots, trenches and canyons are minor.

Formation of main relief features of the Ocean and their significance
Continental margin: it is subdivided into continental shelf and continental slope, with the former simply being a submerged part of the continent. Passive continental margins such as most of the Atlantic coasts have wide and shallow shelves, made of thick sedimentary wedges derived from long erosion of a neighboring continent. Active continental margins have narrow, relatively steep shelves, due to frequent earthquakes that move sediment to the deep sea. Continental shelves are a rich source of Petroleum reserves e.g. Bombay High, Persian Gulf. Other resources available in shelves are sulphur, monazite sand, calcium and pearls. Presence of ample sunlight, optimum depth and nutrients deposited from rivers and waves make them flourishing habitat for organisms. Thus they are potential fishing grounds.
Submarine Canyons – It is another prominent feature which is cut into continental slope and usually found near the mouth of rivers. It is formed due to erosion of continental slope by turbidity currents or mass wasting. They act as preferential particle-transport routes from coastal zone down continental slopes to deep seafloor, enhance carbon sequestration, provide nursery and refuge sites for marine life and they can also be a rich source of genetic resources and chemical compounds.
Deep sea plains – It consists older parts of oceanic crust that are smoothened due to sediment deposition. It has deposits from continents (terrigenous), marine life (biogenous) and salts and mineral (inorganic). Abyssal plains of Southern Indian Ocean and Eastern Pacific Ocean are rich sources of Poly Metallic Nodules.
Mid-Oceanic ridge – These are the youngest portions of the ocean basins where new ocean crust is generated through mantle upwelling and plate divergence. Similarity of constituents, age and magnetic properties of rocks on either side of the ridge helped in understanding sea-floor spreading. Water from hydrothermal vents (along the mid-ocean ridges) is rich in dissolved minerals and supports organisms like chemoautotrophic bacteria.
Oceanic deeps – These are the deepest parts of ocean basins formed due to subduction of oceanic crust under continental crust. These play significant role in the study of plate movements.
Seamounts/Guyots – these are submarine volcanic cones. Seamounts and the water column above them serve as important habitats, feeding grounds and sites of reproduction for many open-ocean and deep-sea species.