Oceanography: Ocean Relief, Salinity

Oceans cover about 71 per cent of the Earth's surface and hold about 97 per cent of its water. They control the climate, support the food chain, give minerals and energy, and carry most of the world's trade. Oceanography is therefore an important part of geography for the UPSC. The Prelims asks about the relief of the ocean floor, salinity, tides, coral reefs and the law of the sea, and the Mains connects oceans with climate change, fisheries and the blue economy. This note explains the main topics in simple language, with tables and Mains points.

Relief of the Ocean Floor

The floor of the ocean is not flat. It has mountains, valleys and plains like the land. The main features are as follows.

The five oceans are the Pacific (the largest and deepest), the Atlantic (S-shaped, the second largest), the Indian (the only ocean named after a country), the Southern or Antarctic, and the Arctic (the smallest and shallowest).

Temperature of the Ocean

The surface temperature is highest near the equator and falls towards the poles. It also falls with depth. In the upper layer of a few hundred metres, the water is warm and well mixed by the winds and waves. Below it, there is a layer called the thermocline where the temperature drops quickly with depth. Below the thermocline, the water is very cold, close to freezing, in all the oceans. The temperature influences the evaporation, the marine life, the formation of cyclones and the density of the water, and it drives the movement of the currents.

Salinity of the Ocean

Salinity is the amount of dissolved salts in the water, expressed in parts per thousand (ppt). The average salinity of the ocean is about 35 ppt (35 grams of salt per kilogram of water). The main salt is common salt (sodium chloride). Lines joining places of equal salinity on a map are called isohalines.

Factors Affecting Salinity

• Evaporation: High evaporation increases the salinity; it is highest in the tropical zone (about 20 to 30 degrees north and south), not at the equator, where the rainfall is heavy.
• Precipitation and river water: These dilute the surface water and reduce the salinity, as at the mouths of large rivers.
• Melting ice: Reduces the salinity in the polar regions.
• Winds and currents: Move the water and mix saltier and fresher waters.
• Enclosed seas: Seas with little exchange have a different salinity.

Waves, Tides and Tsunamis

Waves are caused mostly by the wind blowing on the surface of the water. The highest point is the crest, the lowest is the trough, and the distance between two crests is the wavelength. A tsunami (a Japanese word for "harbour wave") is not caused by wind but by an undersea earthquake, a volcanic eruption or a landslide, and has a very long wavelength and a high speed in the deep sea; it becomes high when it reaches the shore.

Tides are the regular rise and fall of the sea level, caused mainly by the gravitational pull of the Moon and, to a lesser extent, the Sun. There are two high tides and two low tides in about every 24 hours and 50 minutes.

• Spring tides: Occur on the new moon and the full moon, when the Sun, Moon and Earth are in a straight line; the combined pull gives the highest high tides and the lowest low tides.
• Neap tides: Occur at the first and last quarter of the moon, when the Sun and Moon are at a right angle; the tidal range is the smallest.
• Perigee and apogee: When the Moon is nearest to the Earth (perigee), the tides are higher than normal; when it is farthest (apogee), they are lower.

The Bay of Fundy in Canada has the highest tidal range in the world. In India, the Gulf of Khambhat and the Gulf of Kachchh have high tidal ranges and are considered for tidal power. Tides help navigation, since ships can enter the harbours at high tide, and they can be used to produce electricity.

Marine Resources and the Law of the Sea

The oceans give fish, oil and gas, sand, salt and minerals. On the deep sea floor, there are polymetallic nodules rich in manganese, nickel, cobalt and copper, and India has been given an area in the Central Indian Ocean Basin for exploration by the International Seabed Authority (ISA), which is located at Kingston, Jamaica. The UN Convention on the Law of the Sea (UNCLOS, 1982) gives the coastal states a territorial sea of 12 nautical miles, a contiguous zone of 24 nautical miles, an Exclusive Economic Zone (EEZ) of 200 nautical miles and rights over the continental shelf (up to 350 nautical miles in some cases). The water beyond the national jurisdiction is the high seas, the common heritage of humankind.

Coral Reefs

Coral reefs are underwater structures built by tiny animals called coral polyps that make a hard calcium carbonate skeleton. Corals live in a partnership with the algae called zooxanthellae, which give them food and colour. They need warm (about 23 to 29 degrees Celsius), clear, shallow and salty water with sunlight, so they are found between about 30 degrees north and 30 degrees south, mostly on the eastern coasts of the continents where the cold currents are absent.

Charles Darwin's subsidence theory explains the formation of the atoll from a fringing reef around a volcanic island that slowly sinks. India's coral reefs are in the Gulf of Mannar, the Gulf of Kachchh, Lakshadweep and the Andaman and Nicobar Islands. Coral bleaching takes place when the warm water forces the corals to expel the algae, and the corals turn white and may die. Other threats are ocean acidification, pollution, destructive fishing and tourism.

Marine Pollution

The oceans are threatened by plastic waste (the Great Pacific Garbage Patch is a huge collection of floating plastic), oil spills, untreated sewage, chemical runoff from farms, and noise. Ocean acidification, caused by the absorption of carbon dioxide, harms shell-forming creatures and corals. Efforts to protect the oceans include the MARPOL convention against pollution from ships, the Sustainable Development Goal 14 on life below water, and the United Nations Decade of Ocean Science (2021-2030).

Mains Angle: Why Are Oceans Important for Climate and the Economy?

The oceans absorb heat and about a quarter of the carbon dioxide emitted by humans, so they reduce the speed of global warming. They also drive the weather through currents, El Nino and the monsoon. Economically, they support fisheries, trade, tourism, offshore energy and minerals, which together form the blue economy. For India, with a long coast and the EEZ, the sustainable use of the ocean is a major opportunity, as seen in the Deep Ocean Mission launched in 2021 to explore the deep sea resources and technology.

Practice Questions

• Q1. What is the average salinity of the ocean? Answer: About 35 ppt.
• Q2. Which tides occur on the new moon and the full moon? Answer: Spring tides.
• Q3. Which place has the highest tidal range? Answer: The Bay of Fundy in Canada.
• Q4. Which theory explains the formation of an atoll? Answer: Darwin's subsidence theory.

Read it with our notes on ocean currents and plate tectonics, earthquakes and volcanoes.

Quick Revision

• Shelf, slope, rise, abyssal plain, ridge, trench; Mariana Trench is the deepest.
• Average salinity 35 ppt; highest in the tropics, lowest in polar regions and river mouths.
• Spring tides: new and full moon; neap tides: quarters; the highest range: Bay of Fundy.
• Coral reefs: fringing, barrier and atoll; coral bleaching.
• UNCLOS: 12 NM, 24 NM, 200 NM EEZ.

Frequently Asked Questions (FAQ)

What is the continental shelf?

It is the gently sloping shallow edge of a continent under the sea, about 200 m deep, and is rich in fish and oil.

Which is the deepest point of the ocean?

The Challenger Deep in the Mariana Trench is the deepest point, at about 11 km.

What is the average salinity of the sea?

The average salinity of the ocean is about 35 parts per thousand.

Which sea has the highest salinity among the open seas?

The Red Sea has the highest salinity among the open seas.

What are isohalines?

Isohalines are lines on a map joining places of equal salinity.

What is the difference between spring and neap tides?

Spring tides occur at the new and full moon and have the highest range, while neap tides occur at the quarters and have the lowest range.

Which place has the highest tidal range in the world?

The Bay of Fundy in Canada has the highest tidal range.

What causes a tsunami?

A tsunami is caused mainly by undersea earthquakes, volcanic eruptions or landslides.

What are the three types of coral reefs?

They are fringing reefs, barrier reefs and atolls.

What is coral bleaching?

It is the loss of the colour of corals when they expel the symbiotic algae because of warm water or stress.

What is the EEZ?

The Exclusive Economic Zone extends up to 200 nautical miles from the coast, where a country has the right to use the resources.

What are polymetallic nodules?

They are lumps on the deep sea floor rich in manganese, nickel, cobalt and copper.