It is well known that about 71 % of the surface of the earth is covered by the oceans and seas. The oceans and seas cover an area of about 361 million square kilometre out of 510 million square kilometre of the surface of the entire globe.
About 1.4 billion cubic kilometres of water is concentrated in oceans and seas. The greatest known depth in the ocean is 11022 metres at the Mariana Trench in the Pacific. Land is concentrated mainly in the northern hemisphere and the water bodies in the southern hemisphere.
Nearly 61 per cent of the area in the former and 81 per cent in the latter are covered by water. The four recognized oceans in the world are – the Pacific, the Atlantic, the Indian and the Arctic ocean. The Pacific Ocean covers about 49% of the earth surface, the Atlantic Ocean-26%, Indian ocean-21% and Arctic ocean – 4% of the world ocean.
The geological activity of seas and oceans, like other geological agents, comprises the processes of erosion, transportation and deposition, which depend on a large number of factors such as :
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(i) Relief of the floor.
(ii) Chemical composition of the sea water.
(iii) Temperature, pressure and density of sea water.
(iv) Gas regime of seas and oceans.
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(v) Movement of sea water.
vi) Work of sea organisms etc.
(i) Relief of the floor
It has been established that the floor of the oceans exhibits an uneven topography with prominent elevation and depressions.
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On the basis of available bathymetric maps, the ocean is divided into definite regions as indicated below:
(a) Continental shelf
The ocean floor gradually slopes down wards away from the shore. The shallow-water zone adjoining the land, with average depth down to 200 metres constitutes the continental shelf.
It varies in width from a few kilometres to several hundred kilometres. The continental shelves cover about 7.6 per cent of the total area of the oceans and 18 per cent of the land. About 20% of the world production of oil and gas comes from them.
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(b) Continental slope
From the edge of the continental shelf, the sea floor commonly descends to the ocean basin, with an average gradient of 3.5° to 7.5° and is known as continental slope.
Its depth ranges between 200-2500 metres and covers about 15% of the total area of the ocean. It has an average width of 16 to 32 Kilometres.
(c) Continental rise
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It extends from the bottom of the continental slope to the floor of the ocean basins. The rise has a slope of 1° to 6°. Its width varies from a few kilometres to a few hundred kilometres. The material of the rise has been derived from the shelf and slope.
(d) Ocean floor
It begins at a depth of 2000 metres and goes down to 6000 metres. It covers 76 per cent of the total area of the oceans and has a very gentle gradient, being measured in minutes.
It contains a number of distinctive topographic units such as abyssal plains, seamounts and guyots, mid-ocean canyons, and hills and rises that project somewhat above the general level of the ocean basins.
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Apart from these features, the most significant feature of the ocean floor is the occurrence of mid-oceanic ridges and deep-oceanic trenches.
(ii) Chemical composition of sea water
The oceanic water contains a large number of dissolved salts and have almost a uniform composition. These salts result in the property of salinity. The average salinity of sea water is 35 parts per thousand i.e. one litre of sea water contains 35 grams of various dissolved salt.
But the value is small where large rivers meet the sea and the value is higher within the zone of hot and dry climate. In the Mediterranean Sea, for example, the sea level. is lowered by evaporation and the salinity as well as the density of the water increases.
Sea water of normal salinity contains mostly chlorides which aggregate above 88% followed by sulphates more than 10% and small amounts of carbonates and other compounds.
Sodium chloride constitute the bulk of the dissolved salts in sea water, followed by Magnesium-chloride, Magnesium sulphate, Calcium sulphate, Potassium-sulphate.
Apart from these salts, there are also elements like iodine, fluorine, zinc, lead, phosphorous etc. in sea water.
Salinity determines features like compressibility, thermal expansion, temperature, density, absorption of insolation, evaporation, humidity etc. It also affects the movements of the ocean waters.
(iii) Temperature, pressure and density of sea water
The temperature of the oceanic waters plays a significant role in the movement of large masses of oceanic waters and distribution of organisms at various depths. The temperature of the oceans is not uniform.
The temperature of the water on the surface of oceans and seas is determined by the climatic conditions. In tropical zones it is usually higher than in polar regions. Besides the temperature also varies with the depth of the sea water.
There are two main processes of the heating of oceanic waters, viz. absorption of radiation from the sun and convection; whereas cooling is caused by back-radiation of heat from the sea surface, convection and evaporation. The interplay of heating and cooling results in the characteristics of temperature.
The pressure in the oceans and seas varies vertically and increases with depth by 1 atmosphere for each 10 metres of the water column. It is highest in the oceanic trenches (between 800-1000 atmosphere). At great pressure the dissolving capacity of sea water increases.
The density of sea water varies with narrow limits between 1.0275 and 1.0220, due mainly to variation of temperature and salinity. It is highest in higher latitudes and lowest in the tropical areas.
(iv) The gas regime
The sea water contains mostly dissolved oxygen and carbon dioxide. Sea water derives oxygen from the air and also through photosynthesis by marine plants. Similarly the content of the carbon dioxide is mainly due to the atmosphere, river waters, the life activity of marine animals and volcanic eruptions.
It has been seen experimentally that at a temperature of 0°C, the sea water can absorb about 50 cubic centimetre of carbon dioxide and 8 cubic centimetre of oxygen. The content of oxygen and carbon dioxide are of much significance in the processes of marine sedimentation and dissolution of chemical compounds.
(v) Movement of sea water
The sea is a mobile mass of saline water and the movements of sea water are of great geological importance as they determine the intensity of destruction caused by the oceans and seas on the shore and the floor and also the distribution and differentiation of the sedimentary materials that enter the seas and oceans.
The waters of oceans and seas are subjected to the action of wind, the attraction of the sun and the moon, and to changes of temperature, salinity, density etc. All these factors give rise to three main types of movements. Such as Waves, Currents and Tides.
Waves
Waves are generated mainly by the wind blowing over the surface of the ocean and sea water. The friction of wind moving over the water surface causes the water particles to move along circular or near- circular orbits in a vertical plane parallel to the direction of wind.
There is almost no forward motion. Thus energy is transferred from the atmosphere to the water surface by a rather complex mechanism involving both the friction of the moving air and the direct wind pressure.
The ocean waves are oscillatory waves (or transverse waves) as they cause an oscillatory wave motion. The waves consist of alternating crests and troughs. Wave length is horizontal distance from crest to crest or trough to trough.
Wave height is the vertical distance between trough and crest. Wave period is the time taken by two consecutive crests to pass any reference point.
Wave velocity is the ratio between the Wave length and the Wave period.
The waves of oscillation are characteristic of deep water. As waves move into shallow water, they are slowed by the friction with the sea-floor and thus the wavelengths become shorter while the wave height increases, the paths become elliptical and the wave steepens.
Since the front of the wave is in shallower water than its rear part, there is an increase of the steepness of its frontal slope and the wave becomes highly unstable. At this stage, the wave is transformed into a breaker, which then collapses forward in a curling, frothing zone of surf.
As the wave breaks, its water suddenly becomes turbulent. The turbulent water mass then moves up the beach as the swash or uprush. Thus, both water and wave energy move forward against the shore and the wave is called a Wave of translation.
This energy causes erosion and transports material along the shore. The return flow which sweeps the sand and gravel sea ward is called backwash.
Currents
In the currents, there is an actual movement of the water over great distances, which may be caused by various factors, such as-the differences in temperature, salinity, action of steady and periodic winds etc.
Tides are periodic movements of the ocean waters due to the gravitational attraction of the sun and moon on the earth. Twice a day, about every 12 hours 26 minutes, the sea level rises and it also falls twice a day.
When the sea rises to its highest level, it is known as ‘high tide’ and similarly when it falls to the lowest level, it is called ‘low tide’.
There are two tides of a special occurrence viz. (a) the spring tide and (b) the neaptide. The spring tides occur twice every month at new moon and full moon, whereas in the first and third quarter the attraction of the sun and moom tends to balance each other and small tides, which are termed ‘neap tides’, occur.
The currents caused by high tides in the littoral zone are quite strong and can carry quite big fragments of rocks to the shore or along it, eroding the bottom.
(vi) Work of sea organisms Seas and oceans are inhabited by a large variety of animals and plants. Their development and distribution depends on the depth of the sea, its temperature, salinity, pressure, penetration of light and the dynamics of the sea water etc. Marine organisms are divided into three major groups such as benthos, plankton and nekton.
The benthos group includes organisms both mobile and sessile which inhabit the bottom of the sea. The plankton group includes organisms which are passively floating by the waves and currents.
Unicellular organisms (animals) like foraminifers and radiolarians, and, diatoms (plants) belong to this group. The nekton group includes all actively swimming animals which comprises all the sea vertebrates and invertebrate molluscs.
These marine animals are important in producing biogenic sediments.
The above factors together play significant roles in the erosion, transportation and deposition by the oceans and seas.