Test: Oceanography - 1

20 Questions MCQ Test Geography for UPSC (Civil Services) Prelims | Test: Oceanography - 1

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Water that rises to the surface as a result of Ocean Upwelling is typically

  • Winds blowing across the ocean surface push water away. Water then rises from beneath the surface to replace the water that was pushed away. This process is known as ‘upwelling’. Upwelling occurs in the open ocean and along coastlines.

  • Water that rises to the surface as a result of upwelling is typically colder and is rich in nutrients. These nutrients ‘fertilise’ surface waters, meaning that these surface waters often have high biological productivity. Therefore, good fishing grounds typically are found where upwelling is common.


Sea level rise at specific locations may be more or less than the global average due to many factors. Which of these factors can create differences between local sea level and global sea level?

1. Local land subsidence

2. Ocean currents

3. Variations in land height

Select the correct answer using the codes below:

  • Global sea level trends and relative sea-level trends are different measurements.

  • This is due to many local factors: subsidence, ocean currents, variations in land height, and whether the land is still rebounding from the compressive weight of Ice Age glaciers.

  • For example, land subsidence involves gradual settling or sudden sinking of the Earth’s surface. This can cause short-term changes in local sea levels.


In an open ocean, the higher the wind speed and the longer the distance of open water across which the wind blows and waves will be:

Solution: The wind not only produces currents, it creates waves.

As the wind blows across the smooth water surface, the friction or drag between the air and the water tends to stretch the surface. As waves form, the surface becomes rougher and it is easier for the wind to grip the water surface and intensify the waves.


Consider the following statements:

Assertion (A): Seamounts tend to be found on oceanic crust near mid-ocean ridges and island arcs.

Reason (R): Most seamounts are volcanic in origin.

In the context of the above, which of these is correct?

  • The uplifting of the ocean floor occurs when convection currents rise in the mantle beneath the oceanic crust and create magma where two tectonic plates meet at a divergent boundary.

  • Seamounts are found close to these ridges. Seamounts can be found in every ocean basin in the world, distributed extremely widely both in space and in age.

  • A seamount is technically defined as an isolated rise in elevation of 1,000 m or more from the surrounding seafloor, and with a limited summit area, of conical form.


Consider the following pairs:

Trench Location

1. Tonga Pacific Ocean

2. Java Indian Ocean

3. Mindanao Deep Pacific Ocean

4. South Sandwich Atlantic Ocean

Which of the pairs given above is/are correctly matched?

Solution: Tonga Trench: The Tonga Trench is an oceanic trench located in the south-west Pacific Ocean. It is the deepest trench of the Southern Hemisphere and the second deepest on Earth.

Mindanao Deep: The Philippine Trench (also Philippine Deep, Mindanao Trench, and Mindanao Deep) is a submarine trench to the east of the Philippines. The trench is located in the Philippine Sea of the western North Pacific Ocean. The trench reaches one of the greatest depths in the ocean, third only to the Mariana Trench and the Tonga trench.

Java Trench: Trench located south of Indonesia, between the Australian-Indian and the Eurasian Plates; it is the deepest point in the Indian Ocean

The South Sandwich Trench: Just southeast of the southern tip of South America, the British territories of South Georgia and the South Sandwich Islands provide a home for penguins and a few British administrative personnel. Just to the east, the ocean floor dips into the South Sandwich Trench, the second-deepest trench in the Atlantic Ocean after Puerto Rico Trench.


Mid-Oceanic Ridges form an interconnected chain of mountain system within the ocean. It is characterised by a central rift system at the crest. The rift system at the crest is the

Solution: It is the longest mountain-chain on the surface of the Earth through submerged under the oceanic waters. It is characterised by a central rift system at the crest, a fractionated plateau and flank zone all along its length. The rift system at the crest is the zone of intense volcanic activity. There is no such zone of Wegner’s oscillations.


Consider the following statements:

Assertion (A): The level of the sea remains almost the same everywhere.

Reason (R): All the oceans of the world are interconnected with each other.

In the context of the statements above, which of these is correct?

Solution: However, at the equator, the level of water is a little higher by 8 cm. This is because of greater solar insolation received at the equator. More heat causes expansion in the seawater volume.


Regarding continental shelf, consider the following statements:

1. In mountainous coasts, continental shelves are broad.

2. The continental shelf is an underwater landmass which extends from a continent.

3. The continental shelves are poor in fish resources as they are shallow.

Which of the above is/are incorrect?

  • The width of the continental shelf varies considerably - it is not uncommon for an area to have virtually no shelf at all, particularly where the forward edge of an advancing oceanic plate dives beneath continental crust in an offshore subduction zone such as off the coast of Chile or the west coast of Sumatra where they are mountainous.

  • The largest shelf - the Siberian Shelf in the Arctic Ocean - stretches to 1,500 km in width. Continental shelves are the richest fishing grounds.


Consider the following statements:

1. Continental shelf can be rich sources of fossil fuels because of the river sediment load it carries.

2. Oceanic deeps are important for the study of plate movement as they contain several active volcanoes.

Which of these is/are correct?

  • The continental shelves are shielded with sediments of variable thicknesses brought down from the land by rivers, glaciers, wind and distributed by waves and currents.

  • Massive sedimentary deposits that continental shelves have received over a long period of time serve as the source of fossil fuels. The deepest parts of the oceans are Oceanic Deeps or Trenches.


Guyots refer to

  • It is a flat-topped seamount. They show evidence of gradual subsidence through stages to become flat-topped submerged mountains. It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.


Ocean acts as a large carbon sink on Earth due to

  • Carbon dioxide readily dissolves in water and the oceans provide a huge reservoir of carbon.

  • Across the world’s oceans, there is a continual cycle of equilibration of dissolved carbon dioxide in water with carbon dioxide in the atmosphere.

  • The difference in the partial pressure of the CO2, between seawater and air, facilitate gaseous exchange. This allows atmospheric CO2 to dissolve in seawater.

  • The carbon dioxide which dissolves in our oceans occurs in three main forms. Aside from the normal carbon dioxide form, it is also found as bicarbonate and carbonate ions.


Hydrothermal vents in oceans are closely associated with regions of


Hydrothermal vents are globally distributed, but their location is determined by tectonic conditions where seawater meets magma.

  • Where plates form, such as mid-oceanic ridges and in areas where there is volcanic activity, the Earth’s magma is close to the seafloor and can heat water which has seeped down.

  • The hydrothermal vents are therefore associated with regions of high tectonic activity and intersections of continental plates.

  • It is a seafloor fissure from which a spring of geothermally heated mineral and gas-rich seawater emerges. Hydrothermal vents are found on some oceanic ridges in zones of active seafloor spreading.

  • Fauna, which lives around these vents, are based on chemosynthetic food chains where the species at the lower end of the food chain, typically bacteria, synthesise energy from the chemicals in the water.


The thermocline layer represents vertical zones of oceanic water below the first layer and is characterised by a rapid rate of decrease of temperature with increasing depth. The largest number of such layers is usually found in

  • Oceans absorb more than 80% of the solar radiation and water which has the highest specific heat is the remarkable capacity of storing the heat.

  • The uppermost 10% of the oceans have more heat than the entire atmosphere of Earth.

  • Concerning temperature, there are three layers in the oceans from surface to the bottom in the tropics as follows: The first layer represents the top-layer of warm, oceanic water and temperature ranging between 20°C and 25°C. This layer is present within the tropics throughout the year but it develops in mid-latitudes only during summer.


A lot of interest has been recently shown in seafloor hydrothermal systems on account of their

1. High concentration of-base metals

2. High biodiversity

Which of the above is/are correct?

Solution: The discovery of some modem hydrothermal systems in the deep oceanic realm along the mid-ocean ridges has kindled a lot of interest primarily on account of the high concentration of base metals (Cu and Zn) and many noble metals (Au, Ag, Pd, Pt) in them.

Apart from their economic potential, seafloor hydrothermal vents are characterised by dense biologic communities. Over 700 species have been reported, making the vents among the most productive ecosystems on Earth.


Regarding ocean temperatures, consider the following statements:

1. Highest temperature is observed in open seas

2. The ocean temperature decreases with depth

3. The temperature of oceans constantly reduces near the polar regions in every part of the globe

Which of the above is/are incorrect?

Solution: Highest temperature is observed in closed seas such as the Red Sea. The temperature of the ocean varies even near-polar regions thanks to the presence of warm ocean currents.


Increased snow cover on a water body can lead to

1. Sudden spurt in phytoplankton population which are not dependent on photosynthesis

2. Improved oxygen exchange and nutrient recycling in the lake

3. A condition of winterkill causing large scale death of fishes and organisms

Select the correct answer using the codes below:

  • Phytoplankton float on the top surface of water bodies and require sunlight to thrive. Ice cover will reduce their breeding grounds and population.

  • The cover blocks exchange of nutrients and oxygen from the atmosphere, however, the same may continue within the water stream. But, it is worse than before.

  • Snow cover of ice on the water body can effectively cut off the light, plunging the waters into darkness.


The annual range of surface temperature is much greater in the North Atlantic and North Pacific oceans than in the southern oceans. This is due to

Solution: The high ranges in the northern oceans are associated with the character of the prevailing winds. In these parts of the oceans, cold winds blow from the continents towards the oceans and greatly reduce the winter temperatures. So, annual variations of up to 10°C are found sometimes in these oceans due to low winter temperatures and high summer temperatures.


Oceans distant from deserts or with limited accessibility to dust-carrying winds from deserts often have limited primary productivity. This is due to


Option (a): A recently discovered to play a significant role in oceanic primary production is the micronutrient iron.

• This is used as a cofactor in enzymes involved in processes such as nitrate reduction and nitrogen fixation.

• A major source of iron to the oceans is dust from the Earth’s deserts, picked up and delivered by the wind as aeolian dust.

• In regions of the ocean that are distant from deserts or that are not reached by dust-carrying winds (for example, the Southern and North Pacific oceans), the lack of iron can severely limit the amount of primary production that can occur.


Concerning ocean salinity, consider the following statements:

1. In oceans, sodium chloride alone constitutes 95% of ocean salts.

2. The proportion of ocean salts vary in different parts of the ocean.

3. The salinity is constant throughout the oceans.

Which of the above is/correct?

Solution: All are incorrect statements.


Which of the following statements about ocean salinity is/are correct?

1. The salinity of water in the surface layer of oceans depends mainly on evaporation and precipitation.

2. Surface salinity is greatly influenced in coastal regions by the freshwater flow from rivers, and in polar regions by the processes of freezing and thawing of ice.

3. Wind also influences the salinity of an area by transferring water to other areas.

4. The ocean currents have a negligible role in affecting ocean salinity.

Select the correct answer using the codes below:

Solution: Factors affecting ocean salinity are mentioned below

(i) The salinity of water in the surface layer of oceans depends mainly on evaporation and precipitation.

(ii) Surface salinity is greatly influenced in coastal regions by the freshwater flow from rivers, and in polar regions by the processes of freezing and thawing of ice.

(iii) Wind also influences the salinity of an area by transferring water to other areas.

(iv) The ocean currents contribute to the salinity variations. Salinity, temperature and density of water are interrelated. Hence, any change in the temperature or density influences the salinity of an area.

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