Oceans extend over 70.8 per cent part of the earth's surface.
About 60.7 per cent part of the Northern Hemisphere is covered by oceans while 80.9 per cent surface area of the Southern Hemisphere is occupied by the oceans.
Total area of the earth is about 510 million km2 of which about 360 million km2 (70.8 per cent) is represented by the seas and oceans.
Oceans with their vast extent have a profound impact on the climatic phenomenon of the world. They work as the repositories of the solar energy.
Oceanic currents are one of the chief means of the distribution of tempera-hire on the earth. Almost all of rainfall on the continent is caused by the moisture drawn from the oceans.
The elevation of the landmasses and the depth z&nes of oceans are represented by the Hysographic or Hypsometric curve which shows these regions as percentage of the area of the globe.
Ocean Bottom Relief
The study of the ocean bottom relief features has shed immense light on the origin and evolution of earth's crust and the theory of plate tectonics.
The mean elevation of the land surface is 840 metres while the average depth of oceans is more than 4.5 times, i.e. 3,800 metres.
The shallow submerged extention of the continent is called the continental shelf.
Average depth —100 fathoms (200 metres); average slope — 17 feet/mile or about 1°.
Average width — 70 km;
Continental shelf covers 7.5 per cent area of the oceans. It extends over 13.3 per cent part of the Atlantic Ocean, 5.7 per cent part of Pacific Ocean and 4.2 per cent part of Indian Ocean.
Narrow Shelves occur where mountains are found along the coasts e.g. along the Andean coasts.
Continental shelf of the western coast of India is wider than that of eastern coast, mainly because of the subsidence of the western continental part.
Continental shelves represent the rich fishing grounds of the world and they also contain other marine foods, metallic nodules, petroleum and natural gas etc.
Sea–ward part adjacent to continental shelf having steep slope.
Average slope – 20 to 50 degrees.
Depth-200-2,000 fathoms (3,660 metre).
An important feature of continental slope is the existence of steep subma-rine canyons on them.
Deep Sea/Abyssal Plains
Average depth — 3,000 to 6,000 metres
Very low slope gradient (1 : 100)
Covers 75.9 per cent of total oceanic area including 80.3 per cent of Pacific, 80.1 per cent of Indian Ocean & only 54.9 per cent of Atlantic.
The main reason for lesser extent of abyssal plains in the Atlantic is the existence of wide continental shelves and slopes.
Deepest part/feature of the oceanic floor.
Usually parallel to the coasts and island arcs and not in middle of the oceans.
Tectonic in origin, represents the site of sub ducting plate boundary.
Mariana trench (Challenger deep) is the deepest trench in the world situated in the N.W. Pacific oceans.
It is thousands of km long and hundreds of km wide mountain range on the oceanic floor.
These ridges have been formed by the volcanic activity along the spreading boundary of plates.
Their summits may rise above the sea level in the form of islands e.g. Iceland, Azores island, etc.
Sea Mounts and Gyots
Sea mounts are submarine hills which rise above the oceanic floor upto 1000 metres.
Flat topped sea mounts are called gyots.
Both have been formed by the volcanic activity.
Largest number of sea mounts and gyots are found in the Pacific.
Deep gorge like features on the ocean floor arc called submarine can-yons.
They are deep valley having very steep slopes, confining of the continental shelf, slope and rise.
Many submarine canyons arc found along the mouths of major rivers, e.g. - Hudson canyon.
Bank, Shoal, and Reef
Flat-topped elevations located in the shelf and slope area are known as banks. Adequate water depth exists for navigation. They represent world's famous fishing grounds such as Grand Bank (Atlantic), Dogger Bank (North Sea), etc.
Detached elavations with shallow depth are called shoals which are dan-gerous for navigation.
Coral reefs are formed by the skeletal remains of the coral organisms. These organic deposits are rich in calcareous matter. Many islands are formed by the coral deposits. 'Great Barrier Reef' off the N.E. coast of Australia is the largest one.
Usually the temperature of oceanic water ranges from —5°C to 33°C.
Mean diurnal range of temperature of oceanic surface water is almost negligible (about 1°C).
The maximum annual temperature of Northern Hemisphere is recorded in August and lowest in February.
High annual range of temperature is found in land-locked seas e.g. 200° F in Mediterranean Sea and 40°F in Baltic sea.
Atlantic Ocean exhibit more annual range of temperature than Pacific because of its smaller size. > The warm Gulf Stream current does not allow the Norwegian coast to freeze even in winter.
Salinity is defined as the total amount of salt particles in grams contained in one kg of sea water and is expressed as part per thousand (%).
Salinity affects the physical properties of the oceans such as temperature, density, pressure, currents, etc.
More saline water freezes slowly while the boiling point of saline water is higher than the fresh water. Evaporation is lower over more saline water. Salinity also increases the density of water.
The rise of sea water and its movement towards the coast is called tide and the resultant high water level is known as high tide water.
The fall of sea water and its movement towards the sea is called ebb and the resultant low water level is called low tide water.
Tides vary from place to place because of : (i) The change in the position of the sun and the moon in relation to earth. (ii) Uneven distribution of water over the globe. (iii) Irregularities in the configuration of oceans.
The moon though a smaller heavenly body, exerts a greater influence on tides because of its lesser distance from the earth than that of the sun.
The sun, the moon and the earth come in a straight line on the full moon and the new moon and therefore the gravitational forces of the sun and the moon work together producing very high tide, called spring tide.
The position of the sun, the moon and the earth in a straight line is called synergy.
When the sun and the moon arc in a straight line, the position is called conjunction. When the position of the earth is in between the sun and the moon, it is called opposition. The position of conjunction and opposition take place during new moon and full moon respectively.
The sun, the earth and the moon come in the position of a right angle (called quadraturc) on seventh or eighth day of every fortnight of a month and thus the tide producing forces of the sun and the moon work in opposite directions; as a result low tide is caused. Such tides are called neap tides.
Time of Tides
Generally tides occur twice a day. But Southampton, along the southern coast of England experiences tides four times a day because the tidal water comes through the English Channel and through the North Sea at different intervals.
Ocean currents are large masses of surface water that circular in regular patterns around the oceans.
The planetary winds have probably the most dominant influence on the flow of ocean currents.
Between the equator and the tropics blow the trade winds which move equatorial waters westwards and polewards and warm the eastern coast of continents.
In the temperate latitudes blow the westerlies which result in a north easterly flow of water in the northern hemisphere and to the left in the southern hemisphere.