The ocean occupies over 70 percent or 360 million square kilometers of the earth’s total area of 510 million square kilometers.

The oceanographers often study the oceans, the ocean water and the movement of the water under the course of study known as Oceanography.

The ocean basins of the world are large depressions on the earth’s surface which are filled by great mass of salt water. The Ocean basin is not flat. The depth of the ocean is measured by a technique called Echo Sound technique but this has been replaced by the radar sound technique.



There are four major oceans of the world and they are as follows:

  • Pacific Ocean which occupies 165 million sq. km (is the largest of all the oceans).

Pacific Ocean which separate Asia from the Americas. The Pacific Ocean is the biggest of the Earth’s oceanic divisions. It extends from Arctic in the north to Antarctica in the south, bounded by Americas in the east and Asia and Australia in the west.


  • Atlantic Ocean (82 million sq. km)

Atlantic Ocean which separate the Americas from Europe and AfricaThe Atlantic Ocean is the second largest ocean in the worldThe Kiel Canal (Germany), Oresund (Denmark-Sweden), Bosporus (Turkey), Strait of Gibraltar (Morocco-Spain), and the Saint Lawrence Seaway (Canada-US) are main strategic access waterways.


  • Indian Ocean (74 million sq. km)

Indian Ocean which separate India from Africa. The Indian Ocean is the third largest ocean in the world. Four critically important access waterways are the Suez Canal (Egypt), Bab el Mandeb (Djibouti-Yemen), Strait of Hormuz (Iran-Oman), and Strait of Malacca (Indonesia-Malaysia).


  • Arctic Ocean (14 million sq. km)

Arctic Ocean which border the northern continents and encompasses the far North including the North pole. The Arctic Ocean is the smallest ocean in the world. The Northwest Passage (US and Canada) and the Northern Sea Route (Norway and Russia) are two important seasonal waterways.



The ocean floor is made up of five major relief features which are:

  1. The Continental shelf
  2. The Continental slope
  3. The Ocean ridge
  4. The deep Sea plain
  5. The Ocean deeps


  • Continental Shelf
  1. It is a shallow water zone surrounding the continent.
  2. It is a continuation into the sea of the coastal plain.
  3. It forms a gentle sloping flat form extending from the sea-level to an average depth of about 200m.
  4. It is of varying widths which contain shallow water.
  5. It forms 7 percent of the total area of the ocean.
  6. It is found in Labrador.


Economic importance of Continental Shelf to man.

  1. Their shallowness enables sunlight to penetrate through the water. This aids the growth of fish food called Planktons. Hence, it serves as a good fishing ground.
  2. Areas with Continental shelf offshore experience high tide which favours port development e.g of seaport incude Southampton, London, Hamburg, Hong Kong, Singapore, e.t.c.
  3. Since the Continental Shelf is shallow, it eases the exploration of crude oil.
  4. Owing to its shallowness and peculiar position, they therefore facilitate tourist activities.


  • Continental Slope
  1. The Continental Slope links the Continental shelf with the deep sea/plain or ocean floor.
  2. It is narrow and steep.
  3. It drops in depth to about 400m.


  • Ocean Ridge
  1. Ocean ridge takes the form of either a ridge or a plateau.
  2. It rises up from the deep sea plain.
  3. Some rise above the surface to form oceanic islands.


  • The Deep Sea Plain
  1. This is a wide gently, undulating or fairly level surface.
  2. It has a depth of between 2000m and 3000m.
  3. It occupies 2/3 of the ocean floor.
  4. It is called the Abyssal Plain.


  • Ocean Deep
  1. These are the long, narrow trenches that plung as great oceans deeps.
  2. They are located close to the continental and are about 9000m in depth e.g Mariana trench.
  3. Materials, mainly deposited on the ocean floor include: Oozes, Clay and Muds.



Salinity simply refers to the degree of saltiness or concentration of the salt solution in the oceans.

The degree of saltiness varies from ocean to ocean and it is often measured in parts per thousand grammes . The average salinity of the ocean is 35.2 grammes for every 1000 grammes of water.

On maps, lines drawn to join all places in the ocean having equal degree of salinity are called Isohalines.



The salinity of ocean is determined by the following factors:

  1. The amount of fresh water: Fresh water is derived from rain, rivers, streams and melting ice. The more the volume of water added to the ocean, the lower the degree of saltiness as these are experienced within the equatorial lands. On the other hand, the lower the amount of fresh water into the ocean, the higher the degree of saltiness.
  2. The degree of water mixing with ocean currents: Within the enclosed water bodies such as the Capsian and the Red Sea, the salinity is very high because of the absence of ocean current mixing with it, whereas, in seas that are not enclosed the degree of their mixing with ocean current is very high, hence, reduction in salinity.
  3. The rate of evaporation: In tropics, the higher the rate of evaporation in the ocean water the higher the salinity, whereas in the arctic, the lower the evaporation of the ocean water, the lower the salinity.



Ocean Currents refer to the regular movement of water from one part of the ocean to another. In other words, they are large masses of surface water that circulates in regular patterns around the oceans.


Ocean Currents are grouped into two main types which are, warm currents and cold currents.

  1. Warm Currents: Warm Currents are those that flow from the equatorial regions towards the poles and have a relatively low surface temperature.
  2. Cold Currents: Cold Currents are those that flow from the poles towards equatorial regions and have a relatively low surface temperature.



Ocean Currents are caused by the following factors:

  • Planetary wind
  1. Wind is one of the most important factors responsible for the movement of surface water in oceans.
  2. The trade winds move equatorial waters pole-wards and westwards and warm the eastern coasts of the continents.
  3. The North East trade winds move the North Equatorial Current to warm the Southern and Eastern Coast of U.S.A.
  4. Also, the South East trade winds drive the South Equatorial Current which warms the Eastern Coasts of Brazil e.g Brazilian Current.


  • Rotation of the Earth

The earth rotates from west to east and so causes oceans waters to deflect to the right. The deflection to the right makes the ocean waters flow in clockwise directions in the northern hemisphere and anticlockwise in the southern hemisphere.


  • Temperature Differences
  1. Ocean waters at the equator are more heated and therefore warmer and lighter than those at the poles.
  2. Thus, those at the equator flow on the ocean surface towards the poles.
  3. While those at the poles are cold and heavy, thus creeping below the bottom of the ocean towards the equator.


  • The shape of the Landmasses
  1. Warm and cold currents flow in obedience to the topography and configuration of the landmasses.
  2. The landmasses may divert the currents from the direction initially taken by the current to totally different directions.


  • Salinity
  1. Where the salinity of the ocean is high, the ocean water is dense or heavy, and creeps (or moves) at the bottom of the ocean to low salinity areas.
  2. While where the salinity is low, the ocean water is light and flow at the ocean surface towards a high salinity area.





Atlantic Ocean

  1. Cayene Current.
  2. Florida Current.
  3. Gulf stream Current.
  4. North Atlantic Drift.
  5. Guinea Current


Pacific Ocean

  1. Kuroshio Current.
  2. Oyashio-Bering Current.
  3. North Pacific Drift




Atlantic Ocean

  1. Brazilian Current.
  2. South Atlantic Drift.


Pacific Ocean

  1. East Australian Current.
  2. South Pacific Drift.


Indian Ocean

  1. Mozambique Current.
  2. South Indian Ocean Drift.




Atlantic Ocean

  1. Labrador Current.
  2. Irminger Current.
  3. Canaries Current.


Pacific Ocean

  1. California current.
  2. Okhotsk Current.




Atlantic Ocean

  1. Benguella Current.
  2. Falkland Current.


Indian Ocean

  1. West Australian Current


Pacific Ocean

  1. Peruvian Current



  1. It modifies the climate of an area by either, lowering the temperature if the current is cold or by raising it if it is warm.
  2. Cold currents contribute to aridity (deserts) on the landmasses they affect e.g Benguella current of west coast of South Africa causes Namid and Kalahari deserts. Also Canaries current of North-West Africa causes Sahara desert.
  3. Cold currents also cause coastal fogs instead of actual rain which is a sort of danger as it reduces visibility for airplanes and sailors.
  4. Cold currents bring regular and heavy rainfall to their coastland e.g. Mozambique current of South East Africa and Guinea current of West Africa brings rain to their coastlands.
  5. Coastal fogs caused by cold current reduce the temperature of the areas where they occur.
  6. Warm ocean currents, by warming the winds blowing over them raises the temperature of the adjacent lands.
  7. Warm ocean currents helps to keep ports in Polar Regions free from ice.
  8. The meeting of warm and cold current can be a blessing to the people of the place. Such meeting often brings about the release of plant nutrients and other micro-organisms which are food for fish e.g. the Grand Banks of Newfoundland noted as a great fishing ground.