The oceans are confined to the great depressions of the earth’s outer layer. In this section, we shall see the nature of the ocean basins of the earth and their topography. The oceans, unlike the continents, merge so naturally into one another that it is hard to demarcate them. The geographers have divided the oceanic part of the earth into five oceans, namely the Pacific, the Atlantic, the Indian, Southern ocean and the Arctic. The various seas, bays, gulfs and other inlets are parts of these four large oceans. The students have to understand that Oceona=ography is one of the most important subject in geography for both UPSC Prelims and UPSC Mains


  • Relief of Ocean Floors
  • Minor relief features


The ocean floors can be divided into four major divisions:

Continental Sheif

  • The continental shelf is the extended  margin of each continent occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean showing an average gradient of 1° or even less.

  • The shelf typically ends at a very steep slope, called the shelf break

  • The average width of continental shelves is about 80 km

  • The shelves are almost absent or very narrow along some of the margins like the coasts of Chile, the west coast of Sumatra,

  • the Siberian shelf in the Arctic Ocean, the largest in the world, stretches to 1,500 km in width.

  • The depth of the shelves also varies. It may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m

Continental Slope

  • The continental slope connects the continental shelf and the ocean basins.

  • 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.

  • Canyons and trenches are observed in this region.

Deep Sea Ptain

  • Deep sea plains are gently sloping areas of the ocean basins.

  • These are the flattest and smoothest regions of the world. The depths vary between 3,000 and 6,000m.

  • These plains are covered with fine-grained sediments like clay and silt.

Oceanic Deeps or Trenches

  • These areas are the deepest parts of the oceans.

  • 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.

  • 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.


Mid-Oceanic Ridges

A mid-oceanic ridge is composed of two chains of mountains separated by a large depression. The mountain ranges can have  peats 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.

Submarine Coupons

These are deep valleys, some comparable to the Grand Canyon of the Colorado river. They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers. The Hudson Canyon is the best known submarine 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


  • These marine features are formed as’ a result of erosional and depositional activity.

  • A bank is a flat topped elevation located in the continental margins.

  • The depth of water here is shallow but enough for navigational purposes.

  • The Dogger Bank in the North Sea and Grand Bank in the north-western Atlantic, Newfoundland are famous examples.

  • The banks are sites of some of the most productive fisheries of the world.


  • A shoal is a detached elevation with shallow depths. Since they project out of water with moderate heights, they are dangerous for navigation.


  • A reef is a predominantly organic deposit made by living or dead organisms that forms a mound or rocky elevation like a ridge.

  • Coral reefs are a characteristic feature of the Pacific Ocean where they are associated with seamounts and guyots.

  • The largest reef in the world is found off the Queensland coast of Australia. [We will study coral reefs in future posts]

  • Since the reefs may extend above the surface, they are generally dangerous for navigation.

Apart from the above mentioned major relief features of the ocean floor, there are many more features to the ocean, to read more click here