Prelude:

Oceans represent a lion’s share of our hydrosphere of the earth’s surface. The ocean is a sea in expanded form. There are four such oceanic realms such as the Pacific (49%), the Atlantic (26%), the Indian (21%), the

Arctic (4%) in order of their decreasing sizes. The oceanic areas inclusive of the seas, bays and gulfs cover about 510 million km2 of the earth’s surface. The marine areal coverage is more in the southern hemisphe than the northern counterpart. The highest point of the earth’s surface is the Mount Everest of the great Himalayan range which is 8848 metres (8.848 km) high above the mean sea level (MSL); where as the deepest part of the oceanic crust is that of Mariana Trench located in the Western Pacific ocean at a depth of about 11 km below the mean sea level.

The oceanic subdivisions based on depth: The oceanic realms of the earth could be classified into a number of schemes basing on various criteria of which the depth of ocean is considered as the most widely used and the fundamental parameter. The subdivisions made singularly on this depth basis are as follows.

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High tide limit

Low tide limit

A schematic section of oceanic depth zones

(i) Littoral (Foreshore) zone

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(ii) Sub-littoral or Continental shelf zone (Neritic zone)

(iii) Bathyal (Continental slope) zone

(iv) Continental rise zone

(v) Abyssal (Deep sea platform) zone

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(vi) Hadal (Restricted oceanic trench) zone

The major features / characteristics of these depth zones are comprehensively described in the following lines.

Littoral zone: It is the depth zone that lies between the levels of high tide and low tide. It is otherwise also known as foreshore zone or beach zone which gets regularly and alternately covered and uncovered by rise and fall of sea tide respectively. This zone enjoys the tidal wave dominated transition environment. It is characterized by beach sands, bar sands shore lines and tidal flats. The average width of this zone is 3 to 4 km sea-ward from the coast line.

Sub-littoral or continental shelf zone:It is the next shallow water depth zone. It is also termed as neritic depth zone. It is the part of the continental margin that lies between the littoral zone and the continental slope zone. It is the outer border of the continental platform that extends from the low tide line to the depth where ocean floor (bottom) shallow shows a marked increase in its slope. In other words, shelf is the under sea water borderland that is marked by a very gentle slope like that of a low terrace towards the oceanic end.

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The important characteristics of this shelf zone are as follows.

1) It is the shallow marine water zone that runs parallel to the sea coast.

2) Shelves of the world constitute about 18% of the earth’s total land area and about 7.5% of the total oceanic realm.

3) The maximum depth of the shelf is 200 metres (100 fathoms). Its average depth is taken as 133 m, below mean sea level.

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4) The width of the shelf varies appreciably from a few tens of kmto 1500 km the average being 78 km.

The floor of the shelf is a gently sloping surface that shows very low gradient. Its slope angle is averagely less than 0.1°.

Within shallow depths (45-60 m), the shelf floor is filled with coral reefs in form of ridges, barriers and islands in tropical and warm conditions.

It bears submarine canyons those deeply cut through the shelf floor.

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The shelf areas, in general, are built up of thick sedimentary strata which increase in thickness from near the shore line towards the oceanic part.

The sedimentary strata of the shelf area comprise of

i) Well sorted clastic and terrigenous (land derived) sedimen which are sands, silts and muds from near the shore towards the oceanic depth.

ii) Chemogenic sediments such as limestone and evaporites.

iii) Organogenic sediments such as phosphorites, coralline-algal limestone and variety of shell-limestones, composed of dead shells of benthic fauna such as corals, mollusks brachiopods.

10) Based on width, the continental shelves are of two principal types such as (a) Pacific type with narrow shelves and (b) Atlantic type with wide shelves.

11) Economic potentiality of the shelves comprises of (a) offshore oil and gas (b) monazite and limonite rich beach placers (c) phosphorites (d) store house of sea food.