• An ocean (from Greek ” Oceanus”) is a major body of saline water and a principal component of the hydrosphere. Oceanography is a branch of science which deals with the physical – chemical characteristics of ocean water and its depth, temperature, salinity, ocean current, waves, tides, flora and fauna found at the bottom of the oceans. Oceans cover about 70% of the Earth’s surface and they contain roughly 97% of the Earth’s water.
• A large stretch of water covering a vast area is called an ocean. The Earth is nick named “Watery Planet”. It is believed that the life on the Earth first originated in the seas. The Pacific, Atlantic, Indian, Arctic and Antarctic are the five well known oceans and these are connected to one another by means of seas, gulfs, bays, straits and canals.
• The Pacific Ocean: The Pacific is the largest ocean. It is triangular in shape and covers 33% of the total area of the earth’s surface. Its average depth is 5000 meters. It is bounded by Asia and Australia on the west, North America and South America to the east and Antarctica to the south. The Challenger Deep (Mariana’s Trench) in the South Pacific is the deepest in the world with 11,033 meters. There are 20,000 islands in the Pacific Ocean. New Zealand, Indonesia, Japan and Hawaii are the well known islands.
• The Atlantic Ocean: The Atlantic Ocean is elongated ‘S’ in shape and it extends over 16.5% of the total area of the earth. The area of this ocean is about 50% of the Pacific Ocean. It is bounded on the west by North and South Americas, east by Europe and Africa. The important islands found in the Atlantic Ocean are Greenland, British Isles, Newfoundland, West Indies, Cape Verde and Canaries. The Atlantic trade route is considered as the world’s busiest trade route.
• The Indian Ocean: The Indian Ocean covers 20% of the total area of all oceans of the world. On the north, it is bounded by India, Pakistan and Iran; on the east by Australia, Sunda Islands and Malay and West by the Arabian Peninsula and Africa. On the southwest, it joins with the Atlantic Ocean near the southern tip of Africa. On the east and southeast, it meets the Pacific Ocean.
• The average depth of the ocean is 4,000 metres. Andaman, Nicobar, Maldives, Madagascar, Srilanka, Sumatra and Java are the well known islands of Indian Oceans. This is the only ocean named after a country since the sea route was familiar for the traders from time immemorial.
• Antarctic Ocean: This is the fourth largest, coldest and southern most oceans found around the continent of Antarctica. This ocean is often referred to as the “Southern Ocean”. Alexander Islands, Balleny islands and Ross islands are some of the islands found in the ocean. The average depth of this ocean is about 4,500 metres and its temperature varies from -2 to 10°C. In winter, more than half of its surface is covered with ice.
• The Arctic Ocean: The Arctic Ocean is almost circular in shape and it surrounds the North Pole of the earth. Its total area is about 14 million square kilometers. It has an “average depth of 4,000 metres. The main islands are Victoria islands, Elizabeth islands, Iceland, Spitsbergen and Novaga Somlya.
• World Seas: There are many inlets known as seas. Seas are often partly enclosed land. The South China Sea, the Caribbean Sea, Mediterranean Sea are some of the major seas of the World.   
• Topography of the Ocean Floor: The Topography of the Ocean floor consists of:

1. Continental shelf
2. Continental slope
3. Ocean floor (abyssal plain)

• The continental shelf is the shallow area found along the coast with depth of 100 meters. The continental shelf is a suitable place for coastal fishing since it has rich fish food known as plankton example Grand banks (Newfoundland), Dogger Bank (U.K). This is an area known for many off shore oil fields. Example: Mumbai High.  Continental slope is a rapid steep slope which connects the continental shelf and the ocean floor.
• The continental slope and continental shelf are together referred to as the “continental margin”. The average depth of the continental slope is about 155 meters. The irregular coasts and the deep continental slopes are essential for the formation of natural harbors. Example: Kochi and Mumbai.
• The continental slope is often marked with underwater canyons, cliffs and underwater mudslides. The abyssal plain is flat to low rolling hills of the ocean floor. The abyssal plain is covered with ooze which is rotten organic materials of dead marine organisms.
• The Mid oceanic ridge is the underwater mountain formed by the plate tectonic forces. Oceanic trenches are the deepest features in the ocean. Ocean trenches are created when two tectonic plates collide and the denser plate is pushed under the lighter plate.

Salinity:

• Salinity is the saltiness are dissolved salt content in water. The technical term for saltiness in the ocean is salinity. Salinity is generally reported in terms of parts per thousand the average ocean salinity is 35 grams per kilogram. The saltiest water is in the Dead Sea, Red Sea and in the Persian Gulf, which have a salinity of about 40 grams per kilogram (due to very high evaporation rates and low fresh water entry).
• Ocean Temperature: Temperature of the ocean water plays the most significant role in controlling its biological characteristics. The role of the sea water temperature is in causing the ocean currents and other movements of water. Since oceans have greater capacity for the storage of the solar energy, they play a major role in maintaining the equilibrium in the heat budget of the earth. Land surface gets heated and cooled quickly but water tends to heat up and cool down slowly. This differential heating of the land and water make distinct types of marine and continental types of climates found on the surface of the earth.
• Waves: The water in the oceans is always moving. This movement of water is called waves. Wind is the most important causes of generation. The waves travel in some definite pattern, but water does not travel with waves.
• Ocean currents: Ocean currents are the general movement of a mass of surface water in a fairly defined direction. In other words, an ocean current may be defined as any persistent, dominantly horizontal flow of the ocean water. The ocean currents like rivers flow with certain velocity along a certain path. There are two types of ocean currents: warm currents , cold currents
• Warm current originate from low latitude drifts towards poles; whereas cold currents originate from high latitudes and move towards equator.
• There are many factors that influence the generation of ocean currents are:

1. Differences in temperature
2. Density of ocean water (salinity)

• Winds and Atmospheric pressure

1. Coriollis force
2. Gravitational force
3. Precipitation and evaporation

• Melting of snow and ice.

Tides:

• The rise and fall of sea water is known as tides which occur twice a day due to the gravitational pull of the moon and sun on the earth at an interval of 6 hours. When the sea water rises, it is called high tide and the fall of sea water is called the ebb or low tide.
• Spring tide occurs during the full moon days and new moon days. During these days, Sun, Moon and Earth straight line. On such days, the gravitational pull of the moon and sun are combined. At these times, high tides are very high and low tides are very low.
• Neap tides occur during the third quarter phases of the moon. During this phase the moon, sun and earth are at right angles. As a result, the gravitational pull of the moon are perpendicular to one another. During this time, the high tides are very low and low tides are very high.

Dependence of Human Life on Oceans:

• Oceans are a most important part of man’s physical environment. Oceans influence man various ways, from the climate he experiences, the oxygen he breathes and the food he eats to the economic, social, political and military conditions under which he operates. Oceans are being looked to as a major source of protein rich fish food. Several valuable minerals are known to be abundant in the sea including gold, silver, manganese, petroleum and pearl. Apart from the other minerals found in seawater (magnesium, bromine and sodium chloride or common salt).
• The ocean floor yields sand, gravel, and oyster shells for construction purposes and small quantities of diamonds are found in some submarine gravel bars. Offshore oil and gas wells supply about 17 per cent of the world’s petroleum production.
• Oceans also hold potential as an important alternate source of energy. Thermal energy of the oceans resulting from absorption of solar heat and from ocean currents can be converted into electricity, a process known as ocean thermal energy conversion (OTEC).