Important Features Relating to Oceans and Seas


1. Continental shelf:

It is that part of the sea-floor adjoining a land mass over which the maximum depth of sea-water is 200 metres. It is a belt of shallow water and is mostly covered by sedi­ment. The average width is about 70 km and the mean slope is less than one degree. The continental shelves cover about 7’5 percent of the total area of the oceans and 18 percent of the land. About 20% of the world production of oil and gas comes from them.

2. Continental slope:


The continental slope joins the shelf to the deep ocean floor. The average gradient of the slope is about four degrees. It is a steeper zone that gives place to the continental rise which in its own turn is succeeded at far greater depths by the abyssal plain. The depth of sea-water in this zone ranges from 200 metres to 1000 metres. It is the margin of continental landmasses.

3. Continental rise:

It is a prism of thickened sediments located below the surface of the continental slope. The material of the rise has been derived from the shelf and slope. All the conti­nental rises are separated from the shelf by the continental slope.

4. Abyssal plains:


It is the zone of deep sea-floor and the depth of sea-water here ranges from 1,000 metres to 4,000 metres. It is characterised by a slimy, mud-like deposit called ‘ooze’ and Ted-clay.

An associated feature with the oceans or seas is:

(i) The coral-reef:

There are three types of coral-reef as


(a) Fringing reef:

Which are built as platforms attached to shore, i.e. close to the main islands or volcanic cones?

(b) Barrier reef:

Which are built away from the main land and separated from the main land by a lagoon in other words, they enclose a body of water between the mainland and themselves.


(c) Atoll:

These are more or less circular coral reefs enclosing a lagoon but without any land inside.

(ii) Spits:

Near about the shore the rock debris sometimes a heaped up in the form of ridge running more or less parallel to the shore. These ridges are in the long run visible above the sea-level and are known as spits.


(iii) Bars:

If the spits connect themselves with the coastal tract, they are described as ‘bars’.

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