Water (Oceans) - Geography Notes for UPSC

Geography is an important part of the UPSC syllabus, both for the prelims as well as the mains exams. In this article, you can read all about the oceans of the earth, the relief of the ocean floor, characteristics of the oceans such as temperature, salinity, etc. This is an important topic for the UPSC IAS exam. You can also download the PDF below.

Water – Oceans

Water is an important constituent of all life forms on the earth. Hydrosphere is one among the four spheres (other three being-lithosphere, atmosphere and biosphere) of the earth. The hydrosphere includes the water on the surface of the earth (oceans, rivers, lakes and glaciers), the water below the surface called groundwater and the water in the atmosphere above earth’s surface.

• Hydrological cycle, also known as water cycle is a continuous exchange of water between the atmosphere, the oceans and the continents through the processes of evaporation, transpiration, condensation and precipitation.
• About 70.8 % of the area of the earth is covered by water and 29.2% of its area is covered by land. About 96.5% of water is salty found in seas and oceans. Fresh water occupies only 2.5%. Saline ground water and saline lakes together form 1% .

Oceans

The term ocean is derived from the Greek word “Oceanus” which means enormous river encircling the earth. There are five major oceans in the world-  Pacific ocean, Atlantic Ocean, Indian Ocean, Arctic Ocean and Southern Ocean.

1. Pacific Ocean –

• It is the world’s largest ocean. It is bigger than all the continents put together (covers one-third of the earth’s surface area). The Pacific ocean separates North America, South America from Asia and Australia.
• Portuguese explorer Ferdinand Magellan named the ocean, Pacific Ocean which means “peaceful”, because he felt the ocean to be calm.
• The International Date Line passes through the Pacific Ocean. The Pacific Ocean joins the Indian Ocean through the Indonesian Seaway (Torres strait and strait of Malacca).
• Average depth of this ocean is about 4280 m.
• The Mariana trench located in the western Pacific Ocean is the deepest oceanic trench in the world.

2. Atlantic Ocean –

• It is the second largest ocean of the world. It covers about 20% of the earth’s surface area.
• Its name is derived from the Greek mythology- Atlantis Thalassa, meaning Sea of Atlas.
• It is an elongated S- shaped water body between Africa, Europe, Arctic Ocean, Americas and Southern Ocean. Average depth of this ocean is about 3,646 m.
• Deepest trench in the Atlantic ocean is Puerto Rico Trench (〜8,376 m).
• The north Atlantic ocean was formed by the breakup of the supercontinent Pangaea and the south Atlantic ocean was formed when the Gondwanaland broke in the geological past.
• The largest island- Greenland is located in the Atlantic ocean.
• The Sargasso sea, the only sea in the world without any shore, is present in the Atlantic ocean.
• In the south-east, the Atlantic ocean merges into the Indian ocean.

3. Indian Ocean –

• It is the largest ocean in the world. It is named after India which makes up much of its northern border.
• It lies in the eastern hemisphere. It covers about 19.8 % of water on the earth’s surface.
• It lies between Africa, Southern Ocean, Australia and Asia.
• It is landlocked in the northern hemisphere (closed ocean) and is the warmest ocean in the world.
• The largest island in the Indian ocean is Madagascar (fourth largest island in the world).
• Sundra trench/ Java trench (approx. 7450 m) is the deepest trench in the Indian ocean.
• Marginal seas in the Indian Ocean are extremely important for trade, providing major sea routes that connect the Middle East, East Asia, Africa, Europe and the Americas. A marginal sea is a section of an ocean partially enclosed by an island, a peninsula, or an archipelago (group of islands). They are shallower than the ocean and have close proximity to land.

4. Southern Ocean-

• Also known as the Antarctic Ocean or Austral Ocean, comprises the southernmost waters of the world ocean.
• It is the fourth largest ocean/ second smallest ocean in the world, located south of 60°S latitude and encircling Antarctica.
• It shares its northern boundary (largest boundary) with the Atlantic, Indian and Pacific Ocean. This ocean is known for its turbulent waters.
• The Drake Passage is the body of water between South America’s Cape Horn (Chile) and the South Shetland Islands of Antarctica. It is known for its violent storms. It connects the southwestern part of the Atlantic ocean (Scotia sea) with the southeastern part of the Pacific ocean and extends into the Southern Ocean.

5. Arctic Ocean-

• The Arctic Ocean is shallower and smaller than the other four oceans. It lies entirely within the Arctic Circle.
• It covers about 1.3% of the earth’s water surface area.
• It is surrounded by Eurasia and North America. It is completely covered by ice in winter.
• Its salinity is the lowest on average of the five major oceans.
• The Bering Strait connects the Arctic Ocean with the Pacific Ocean while the Greenland Sea, Labrador Sea and Norwegian Sea connect it with the Atlantic Ocean.
• The Molloy Deep, 5557 m (Molloy Hole) in the Fram Strait (within the Greenland sea) is the deepest point in the Arctic Ocean.

Relief of the Ocean floor

The ocean floors (land under the waters of the ocean) are rugged with the world’s largest mountain ranges, deepest trenches and the largest plains. These varied features (like those of the continents) are formed by volcanic, tectonic and depositional activities.

The ocean floor can be divided into four major divisions-

1. The continental shelf.
2. The continental slope.
3. The deep sea plain, and
4. The oceanic deeps.

Besides the above features, there are other relief features in the ocean floors like hills, guyots, ridges, sea mounts, trenches, canyons etc.

Continental Shelf-

• It is the shallowest part of the ocean with an average gradient of 1°or even less. The continental shelf is the seaward extension of land that lies under the sea water. The shelf usually ends at the shelf-break- a very steep slope.
• The average width of the continental shelf is about 80 km. The width varies according to the nature of the rock beneath the crust. The world’s widest continental shelf is located along the coast of Siberia (approx. 1500 km wide). The continental shelves are almost absent or very narrow along some of the margins like the coasts of Chile, the west coast of Sumatra etc.
• The depth of the continental shelf varies between 30 m – 600 m.
• Sedimentary deposits received by the shelves over a period of time become the source of fossil fuels. The continental shelves are rich sources of oil, mineral deposits, natural gas and coral reefs.

Continental Slope-

• The continental slope connects the continental shelf with the ocean basins. The bottom of the continental shelf sharply drops off into a deep slope called the continental slope.
• The gradient of the slope varies between 2°-5°.
• The depth of the continental slope ranges between 200 m- 3000 m.
• In this region of the oceans, landslides, turbid currents, underwater canyons, gorges, trenches occur.

Deep Sea Plain-

• It is the vast area of flat terrain in the bottom of the oceans and is the largest part of the ocean relief. These are the flattest and smoothest regions of the world.
• The depth ranges between 3000 m – 6000 m.
• Very fine sediments like clay, silt etc accumulate on the floor. These are inlayers and are used to trace geological events in the past.

Oceanic Deeps or Trenches-

• Oceanic trenches are the deepest parts of the oceans. They are prominent long, narrow topographic depressions of the ocean floor. They occur at the base of the continental slope and along island arcs. They are associated with active volcanoes and strong earthquakes.
• Oceanic trenches typically extend 3-4 km below the level of the surrounding oceanic floor.
• A trench forms a convergent boundary where one plate subducts below the other.
• The Challenger Deep in the Mariana Trench (about 11,034 m) in the western Pacific Ocean is the deepest part of the world. Horizon Deep (10,800±10 m) in the Tonga trench (south-western Pacific Ocean) is considered to be the second deepest point on the earth and the deepest in the southern hemisphere.
• As many as 57 deeps have been explored so far; of which 32 are in the Pacific ocean, 19 in the Atlantic Ocean and 6 in the Indian Ocean (NCERT).

Ocean Relief (Image source – SCERT, Tamil Nadu)

Minor Relief Features–

Mid-Oceanic Ridges-

• A mid-oceanic ridge is a sea floor mountain system formed by plate tectonics.
• Mid-oceanic ridges are located on the divergent plate boundaries where magma flows through the fissure to form new oceanic crust.
• The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface. Iceland which is a part of the mid-Atlantic ridge is an example.

Seamount –

• It is a mountain with pointed summits rising from the sea floor. It does not reach the surface of the ocean.
• Height of these mountains varies between 3,000 m and 4,500 m.
• Seamounts are volcanic in origin.
• The Emperor seamount, an extension of the Hawaiian Islands in the Pacific ocean is an example.

Submarine Canyons –

• These are narrow steep sided valleys. They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers. Submarine canyons are so called because they resemble canyons made by rivers on land.
• The Hudson Canyon is the best known submarine canyon in the world.
• Submarine canyons act as conduits to bring sand sized sediments from the continental margins to the deep sea.

Guyots –

• The flat topped seamounts submerged under sea water are known as guyots.
• It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean.

Atoll –

• These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression.
• Atoll may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish or highly saline water.
• The Indian Ocean also contains a number of atolls e.g.,atolls are found in Maldives, the Chagos archipelago, the Cocos Islands (also known as the Keeling islands). Most of the atolls are found in the Pacific Ocean.

Relief features of the Ocean (Image source – SCERT, Tamil Nadu)

Temperature of ocean waters–

The Sun is the major source of heat energy for the oceans. Sea water has higher specific heat than land, therefore its heating and cooling processes are much slower than land. Oceans have a high storage capacity of heat.

Factors affecting temperature distribution –

1. Latitude – As the amount of insolation decreases from the equator towards the poles, therefore the temperature of surface water decreases in the same manner.
2. Unequal distribution of land and water – The oceans in the northern hemisphere receive more heat due to their contact with larger extent of land than their counterparts in the southern hemisphere. Thus, the temperature of surface water is comparatively higher in the northern hemisphere than the southern hemisphere.
3. Prevailing wind – The winds blowing from the land towards oceans/sea (offshore winds) drive warm surface water away from the coast resulting in upwelling of cold bottom water from below. This results in longitudinal variations in the temperature. Contrary to this, the onshore winds (blowing from ocean to land) pile up warm water near the coast and thus, raise the temperature.
4. Ocean currents – Warm currents raise the temperature of the affected areas whereas cool currents lower down the temperature. The Gulf stream (warm current) increases the temperature of the eastern part of North America and the west coast of Europe. Labrador cool current lowers down the temperature near the north-east coast of North America. Due to Kurile cool current the temperature of the eastern coast of Siberia becomes low.

There are also some minor factors like submarine ridges, local weather conditions like storms, hurricanes fog, cloudiness, evaporation and condensation which affect the surface temperature of the ocean water. The enclosed sea in the low latitudes record relatively higher temperature than the open seas whereas the enclosed seas have lower temperature than the open seas in the high latitudes.

Horizontal distribution of temperature in oceans–

• The diurnal range and the annual range of temperature of oceans is much less than the land.
• The average temperature of surface water of the oceans gradually decreases from the equator towards the poles. The rate of decrease of temperature with increasing latitude is generally 0.5℃ per latitude. The average temperature is about 22℃ at 22° latitude , 14℃ at 40°latitude and 0°C near poles.
• The highest temperature is not recorded at the equator but slightly towards north of it. The northern hemisphere oceans record comparatively higher temperatures than the southern hemisphere oceans. This is due to the unequal distribution of land and water in northern and southern hemispheres. For northern hemisphere oceans, the average annual temperature is around 19°C while for the southern hemisphere oceans it is around 16°C.

Vertical distribution of temperature in oceans-

The maximum temperature of the oceans is always at their surface because it directly receives the insolation and the heat is transmitted to the lower levels of the oceans through convection.

Vertically the oceans are divided into three layers from the surface to the bottom-

1. The upper layer/ first layer – It represents the top layer of warm oceanic water. It is about 500m thick and in this layer temperature ranges between 20°C and 25°C. This layer, within the tropics, is present throughout the year but in mid latitudes it develops only during summer.
2. Transitional zone/second layer/ thermocline layer – Thermocline layer lies beneath the upper layer. This layer is unique because the temperature decreases rapidly with increasing depth. This layer is 500 – 1000 m thick. About 90% of the total volume of water lies below the thermocline in the deep ocean. In this zone, temperature approaches 0°C.
3. The lower layer/ third layer – This layer of the ocean is very cold and extends up to the deep ocean floor. In the Arctic and Antarctic oceans, temperature change with depth is very slight because the surface water temperatures are close to 0°C.

Vertical distribution of temperature in oceans (Image source – NCERT)

Salinity of ocean waters–

• Salinity is an important property of seawater. It is defined as the amount of salt (in grams) dissolved per kilogram (1000 g) of sea water. It is usually expressed as parts per thousand, ppt or °/₀₀. For example, 30 ppt means 30g of salt in 1000 g of seawater. Salinity of 24.7 ppt has been considered as the upper limit to demarcate brackish water.
• The composition of seawater varies both spatially (varies from one area to another) and temporally (varies from one time span to the other time unit- seasonally, yearly).

Sources of ocean salinity –

• Most of the ocean salts are derived from weathering and erosion of the earth’s crust and their transport by the rivers to the oceans.
• Atmosphere and biological interactions.
• Some of the ocean salts are derived from rocks and sediments below the seafloor, while others have escaped from the earth’s crust through volcanic vents as solid and gaseous materials.

Factors controlling the salinity of ocean water–

1. Evaporation.
2. Precipitation.
3. Influx of river water.
4. Prevailing winds.
5. Melting of ice.
6. Ocean currents and sea waves.

• Evaporation and formation of ice increase the ocean salinity.
• Precipitation, river runoff and melting of ice decrease salinity.
• Salinity, temperature and density of ocean water are interrelated. Any change in the temperature or density influences the salinity of that area.

Horizontal distribution of salinity –

• On average, the salinity decreases from the equator towards the poles. The highest salinity is observed between 20° and 30° N and S latitudes because this zone is characterized by high temperature, high evaporation and but less rain than the equatorial region. Subpolar and polar zones record minimum salinity due to less evaporation.
• The North Sea, though in higher latitude, records higher salinity due to more saline water brought by the North Atlantic Drift.
• Baltic Sea records low salinity due to the influx of river waters in large quantities.
• The average salinity of Indian Ocean is 35°/₀₀.. The Bay of Bengal records low salinity due to influx of river water and the Arabian Sea has higher salinity due to high evaporation and low influx of fresh water.
• Highest salinity in water bodies – Lake Van in turkey (330 °/₀₀), Dead Sea (238 °/₀₀), Great Salt Lake, Utah USA (220°/₀₀).

Vertical distribution of salinity –

• There is a marked difference in the salinity between the surface zones and the deep zones of the oceans. Salinity at the surface increases by loss of water to ice or evaporation, or it decreases by the input of fresh waters. However, salinity at depth is very much fixed, because there is no way that water is lost or the salt is added.
• Although salinity generally increases with the depth, there is a distinct zone where the salinity increases sharply called the halocline.
• Highly saline sea water (dense) generally sinks below the lower saline sea water (less dense) which leads to stratification of seawater.

Density stratification of oceans

1. Surface layer of lowest density – Also called photic zone as this layer is directly penetrated by solar radiation. It has a thickness of 100 m- 200 m. It carries only 2% of the total volume of ocean water.
2. Pycnocline layer of sharp density gradient – (pycno means density). This layer is found between 300 m – 1000 m depth of ocean water. It carries about 18% of the total volume of ocean water. Pycnocline (sharp density gradient), thermocline (steep temperature gradient) and halocline (sharp salinity gradient) occupy almost the same depth zones of 300 m – 1000 m.
3. Bottom layer of highest but uniform density- It carries about 80% of the total volume of ocean water.

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