1. With reference to ‘Continental Shelf’, consider the following statements:
(1) It is the shallowest part of the ocean.
(2) It provides rich sources of fisheries, petroleum and natural gas.
(3) It has deposited sediments known as oozes.
Which of the statements given above is/are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3
Answer: (b) Explanation: Continental shelf is one of the three sub-divisions of Continental Margin (the other two are Continental Slope and Continental Rise).
Statement 1 is correct. Continental shelf is the submerged part of the land adjoining the coast, shallow seas and gulfs. It is the shallowest part of the ocean showing an average gradient of 1 or even less. The width of the continental shelf varies from 60 km to as wide as 1300 kms.
Statement 2 is correct. Continental shelf’s shallowness enables sunlight to penetrate through the water, which encourages the growth of microscopic organisms such as planktons, on which millions of fish and polyps survive. The Continental shelves are therefore, the richest fishing grounds in the world e.g., Grand Bank, off Newfoundland in North America; and Dogger Bank in the North Sea. Apart from minerals, sand and gravels, continental shelves are great sources of petroleum and natural gas e.g., Bombay High, Persian Gulf, North Sea, Gulf of Florida, etc.
Statement 3 is incorrect. The sediments formed from the remains of living things are called oozes. They are pelagic sediments deposited in the deep sea/abyssal plains. They are divided into two groups on the basis of lime and silica contents - Calcareous oozes and Siliceous oozes.
2. In the context of oceanography, the term ‘Guyot’ refers to:
(a) A variety of sea-mount
(b) A kind of coral reef
(c) A type of marine sediment
(d) None of the above
Answer: (a) Explanation: A guyot is a flat-topped variety of a sea-mount occurring mainly in the Pacific Ocean. Their summits are almost entirely at depths of more than 1000 m from ocean water surface but rise up to 3 km from the ocean floor. The conical shape of the guyot has suggested to some scientists a volcanic origin, with the table- like summit having been planed down by marine erosion and finally submerged by a rise of sea level. Their summits are covered by sediments ranging from cretaceous sediments to volcanic materials of Tertiary age. Many guyots at the margin of oceans and continent are much deformed, tilted and broken at location of trenches/ocean margin. Their existence is one of the evidences of movement of plates (sea-floor spreading). The guyots must have been formed on the slope of the ridge but as the sea-floor spreads out, the guyots get located at the deep ocean floor.
3. Which of the following factors affect ‘Oceanic Salinity’?
(1) Influx of river water
(3) Rate of evaporation
(4) The degree of water mixing by ocean currents.
(5) Atmospheric pressure and wind direction
Select the correct answer using the code given below:
(a) 1, 2 and 3 only
(b) 1, 3 and 4 only
(c) 1, 3, 4 and 5 only
(d) 1, 2, 3, 4 and 5
Answer: (d) Explanation: Ocean salinity is the term used to define the total content of the dissolved salts in ocean water. It is calculated as the amount of salt (in gm) dissolved in 1000
gm of sea water. It is usually expressed as parts per thousand or ppt. The average salinity of oceans is 35 ppt but it varies spatially and temporally in different oceans, seas and lakes. Factors affecting oceanic salinity:
• Evaporation: There is direct positive relationship between the rate of evaporation and salinity e.g. greater the evaporation, higher the salinity and vice-versa. Salinity is higher near the tropics than at the equator. Both these regions, record high rate of evaporation but there is presence of dry air over the tropics of Cancer and Capricorn which helps in an efficient evaporation and subsequent increase in salinity, especially in the top layers of ocean water.
• Ice formation: Formation of ice in the high latitude regions of the ocean increases seawater salinity.
• Precipitation: It is inversely related to salinity e.g. higher the precipitation, lower the salinity and vice-versa. This is why the regions of high rainfall (equatorial zone) record comparatively lower salinity than the regions of low rainfall (sub-tropical high pressure belts).
• Influx of river water: Though the rivers bring salt from the land to the oceans, big and voluminous rivers pour down immense volume of water into the oceans and thus salinity is reduced at their mouths e.g. comparatively low salinity is found near the mouths of the Ganga, the Congo, the Niger, etc. There is seasonal variation of surface salinity with maximum and minimum run-off from the land.
• Atmospheric pressure and wind direction: Anti-cyclonic conditions with stable air and high temperature increase salinity of the surface water of the oceans. Sub-tropical high pressure belts represent such conditions to cause high salinity. Winds also help in the redistribution of salt in the oceans and the seas because winds drive away more saline water to less saline areas resulting into decrease of salinity in the former and increase in the latter.
• Circulation of ocean water: Ocean currents affect the spatial distribution of salinity by mixing sea waters. Equatorial warm current drive away salts from western coastal areas of the continents and accumulate them along the eastern coastal areas.
4. With reference to ‘Storm Surge’, consider the following statements:
(1) Storm surges can be caused due to gravitational pull of the Sun and the Moon.
(2) Coastal wetlands act as natural barrier in reducing the damage done by a storm surge.
Which of the statements given above is/are correct?
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2
Answer: (b) Explanation: A storm surge is a rise in sea level that occurs during tropical cyclones or intense storms. The storms produce strong winds that push the water into shore, which can lead to flooding. This makes storm surges very dangerous for coastal regions. Statement 1 is incorrect. Tides occur due to gravitational pull of the Sun and the Moon whereas a storm surge is primarily caused by the relationship between the winds and the ocean’s surface. The water level rises where the winds are strongest. Water is pushed in the direction in which the winds are blowing. The rotation of the Earth causes air to deflect toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere - a phenomenon known as the Coriolis Effect. If a cyclone develops in the Northern Hemisphere, the surge will be largest in the right-forward part of the storm. In the Southern Hemisphere, the surge will be largest in the left-forward part of the cyclone. Another factor contributing to storm surge is atmospheric pressure. The pressure is higher at the edges of a cyclone than it is at the centre. This pushes down the water in the outer parts of the storm, causing the water to bulge at the eye and eye wall—where the wind adds to the rise in sea level. Statement 2 is correct. Wetlands, such as swamps, estuaries, and mud flats, act as sponges for tropical cyclones. As the cyclone makes landfall, the marshy land and plants absorb the water and the energy of the storm surge. Silt and swamp vegetation prevent the most intense part of the storm surge from hitting homes and businesses. Coastal residents can reduce the damage done by a storm surge by protecting local wetlands.
5. Consider the following ocean currents of ‘North Pacific Gyre’:
(1) Kuro Shio current
(2) North Pacific current
(3) North Equatorial current
(4) California current
Select the correct sequence (clockwise) of the ocean currents using the code given below:
Answer: (b) Explanation: The North Pacific Gyre (NPG) is in the northern Pacific Ocean. It is located between the equator and 50° N latitude. The gyre has a clockwise circular pattern and is formed by four prevailing ocean currents:
• North Pacific Current to the north;
• California Current to the east;
• North Equatorial Current to the south; and
• Kuroshio Current to the west. Value addition: This site has unusual intense collection of man-induced marine debris known as the Great Pacific Garbage patch.