Oceanography Questions and Answers

Answer: B

The deciding characteristic is its depth and the vertical displacement of the sea floor.

A tsunami is generated when the sea floor is pushed suddenly up or down, lifting or dropping the whole column of water above it. That happens most with shallow-focus earthquakes on subduction thrust faults.

Magnitude alone is not enough: a large earthquake with mainly horizontal (strike-slip) movement, or one too deep to deform the sea floor, may produce no significant tsunami. Distance from land affects how soon and how hard a tsunami strikes, not whether it forms, and duration is not the deciding factor.

Answer: A

The Sargasso Sea is in the North Atlantic Ocean.

It lies within the North Atlantic subtropical gyre and is bounded not by land but by ocean currents: the Gulf Stream to the west, the North Atlantic Current to the north, the Canary Current to the east and the North Equatorial Current to the south. It is named after the floating Sargassum seaweed that collects in its calm water, and it is the breeding ground of European and American eels.

No sea of this kind lies in the South Atlantic, Indian or North Pacific Ocean, so the other options are wrong.

Answer: A

The correct matching is (a)-(ii) (b)-(i) (c)-(iii).

• Barrier reef: separated from the coast by a wide, deep lagoon, as in the Great Barrier Reef.
• Fringing reef: attached to the coast, growing directly from the shore.
• Atoll: a ring of reef enclosing a central lagoon, with no central island.

Darwin explained the three as a sequence around a subsiding volcanic island: fringing reef, then barrier reef, then atoll. Every other code makes either the fringing reef separated from the coast or the barrier reef attached to it.

Answer: B

Atlantic and Pacific types describe continental margins.

An Atlantic-type (passive) margin lies within a plate, far from a plate boundary: it has a broad shelf, slope and continental rise with thick sediment and little seismic or volcanic activity, as along the eastern Americas. A Pacific-type (active) margin coincides with a plate boundary, usually a subduction zone, with a narrow shelf, a deep trench offshore, earthquakes and volcanoes, as along western South America.

Abyssal plains and deep sea trenches are features found at or beyond such margins (trenches mainly at the Pacific type), but the Atlantic and Pacific classification is of the margins themselves, so 'None of the above' is also wrong.

Answer: D

The one not connected with hot spring vents is thick turbidite deposits of the continental rise.

Turbidites are laid down by turbidity currents that carry sediment down the continental slope and spread it over the continental rise and abyssal plain. They are a product of sediment transport at continental margins and have nothing to do with hydrothermal activity.

The other three are all linked to submarine hot springs at spreading ridges: black smokers are vents discharging hot, mineral-laden fluid; the metals precipitate as sulphide-rich sediments and mounds; and the vents support communities of tube worms, clams and bacteria that live on chemosynthesis instead of photosynthesis.

Answer: B

The tongue is Antarctic Intermediate Water.

Antarctic Intermediate Water forms near the Antarctic Polar Front at about 50-60°S, where cold, relatively fresh surface water sinks. It spreads northwards at depths around 500-1,500 m as a distinct salinity minimum that can be traced across the equator in the Atlantic.

Mediterranean Water is a warm, salty tongue in the North Atlantic, not a low-salinity water of the southern ocean. 'North Hemispheric Water' is not a named water mass, and Arctic Intermediate Water belongs to the Arctic, not the Southern Hemisphere.

Answer: C

It is the thermohaline circulation.

The thermohaline (meridional overturning) circulation is the global ocean circulation linked to differences in temperature and salinity: dense water sinks at high latitudes and spreads through the deep ocean, carrying heat, salt, freshwater and dissolved substances between basins. Studies such as Munk and Wunsch's have shown that it needs mechanical energy from winds and tides, through mixing, to return deep water to the surface, which is the 'mechanical stirring' of the question.

Monsoon, atmospheric and tropospheric circulations are motions of the air, not oceanic circulations.

Answer: C

Upwelling is in the North Pacific and downwelling in the North Atlantic.

In Broecker's 'great ocean conveyor' picture, cold salty water sinks in the northern North Atlantic (Greenland and Labrador Seas) to form North Atlantic Deep Water. It flows south, joins the circumpolar current and spreads into the Indian and Pacific Oceans, where it slowly rises; the oldest deep water, in the North Pacific, upwells there before returning as surface flow.

The South Atlantic, the Gulf of Oman and the Indian Ocean are not the conveyor's main sites in the pairing asked for, and the North Atlantic is the region of sinking, not upwelling.

Answer: C

The photic zone is the layer of ocean upto which sufficient light penetrates.

It is the sunlit surface layer, usually down to about 200 m in clear open water and much less in turbid coastal water, in which there is enough light for photosynthesis to exceed respiration. Phytoplankton, seaweeds and the food webs that depend directly on them are concentrated here.

The other options are wrong. Life is not confined to it: animals live throughout the dark water below, down to the deep-sea floor and the vent communities. Coral reefs occur in only a small part of the photic zone, in warm shallow tropical seas. And the layer in which photosynthesis is not possible is the opposite, the aphotic zone.

Answer: C

The important driving force is prevailing winds.

The major surface currents of the oceans, such as the Gulf Stream, the Kuroshio and the equatorial currents, are driven by the drag of the planetary winds on the sea surface. The trade winds push water westward near the equator and the westerlies push it eastward in middle latitudes, and with the deflection of the earth's rotation and the shape of the basins this produces the great circular gyres.

Density, temperature and salinity differences do move water, but they drive the slow, deep thermohaline circulation rather than the main surface currents. Each of the other three options names one ingredient of density-driven flow, which is secondary to wind for ocean current movement as usually described.