Oceanography Questions and Answers

Answer: B

Statements (i) and (ii) are correct, so the answer is (i) and (ii).

Under the UN Convention on the Law of the Sea (1982), measured from the baseline:

• the territorial sea extends up to 12 nautical miles;
• the contiguous zone up to 24 nautical miles (ii);
• the exclusive economic zone up to 200 nautical miles (i).

Statement (iii) gives 10 nautical miles for the territorial sea, which is wrong, so every option containing (iii) is wrong.

Answer: D

The answer is (1), (2), (3) and (4): all four factors.

Winds are the main driver of surface currents, dragging the water and setting up the great gyres. Differences in salinity and temperature change the density of seawater, and denser water sinks and flows, which drives the deep thermohaline circulation; seasonal changes in temperature alter these differences and the strength of currents through the year. Atmospheric pressure also matters: high pressure depresses the sea surface and low pressure lets it rise, creating slopes that water flows down.

Because every factor listed plays a part, each option that leaves one out is incomplete.

Answer: C

The wave is an Internal wave.

The pycnocline is the layer in which water density changes sharply with depth, separating lighter surface water from denser deep water. A disturbance there sets off internal waves that travel along this boundary below the surface. Because the density difference is small, they can reach heights of tens of metres while barely showing at the surface.

Swell is made of wind waves that have travelled out of the area where they were generated. A seiche is a standing oscillation of a lake or enclosed basin. Capillary waves are tiny ripples on the surface restored by surface tension. None of these forms at the pycnocline.

Answer: C

The answer is the Amphidromic point.

In each ocean basin the tide wave rotates around points where the tidal range is zero, called amphidromic points. Co-tidal lines (joining places with high tide at the same time) radiate out from them like spokes, and co-range lines (joining places with equal tidal range) form circles around them, with the range increasing away from the point.

A co-range line has a constant tidal range, not zero range, and a co-tidal line marks equal timing of high water. Average tide level is a reference datum, not a place that experiences no tidal change.

Answer: A

The wavelength of a tsunami is a few kilometers up to a few hundreds of kilometers.

A tsunami is generated when an earthquake, landslide or eruption displaces a whole column of seawater. In the open ocean its wavelength is commonly 100-500 km and its period 10 minutes to an hour, while its height may be under a metre, which is why ships at sea hardly notice it. It slows and grows in height as it reaches shallow water.

Wavelengths of metres to tens of metres, or up to one kilometre, belong to ordinary wind waves and swell, whose periods are only seconds. Those ranges are far too short for a tsunami.

Answer: A

The correct match is 1-c, 2-b, 3-a, 4-d.

• The Humboldt (Peru) current is a cold current along the western coast of South America (1-c).
• The Canaries current is a cold current along the western coast of north Africa (2-b).
• The Kuroshio is a warm current flowing north-east along the eastern coast of Japan (3-a).
• The Gulf Stream is the warm current of the North Atlantic Ocean (4-d).

Option B puts the Humboldt current off Japan and the Kuroshio off South America, and options C and D misplace two or more currents.

Answer: B

The constant is 1.80655.

Salinity is computed from chlorinity, the measured concentration of chloride (with other halides), because the major ions in seawater occur in nearly constant proportions. The internationally adopted relation is:

\[ S = 1.80655 \times Cl \]

with both salinity S and chlorinity Cl in parts per thousand.

It replaced Knudsen's older formula, S = 0.030 + 1.805 Cl. The other options carry the same digits with the decimal point shifted (18.0655, 180.665) or a digit dropped (18.655), and would give salinities ten or a hundred times too large.

Answer: D

The descending order is 1 - 4 - 3 - 2: Tonga, Japan, Peru-Chile, Aleutian.

Approximate maximum depths, as usually quoted:

• Tonga trench: about 10,800 m, the second deepest in the world after the Mariana trench.
• Japan trench: about 8,400 m.
• Peru-Chile (Atacama) trench: about 8,000 m.
• Aleutian trench: about 7,700 m.

Exact figures vary between sources, but the order is the same. Option A puts the Peru-Chile trench ahead of the Japan trench, and options B and C put the shallow Aleutian or the Japan trench ahead of the Tonga trench, which is far deeper than all three.

Answer: D

The most abundant salt is Sodium chloride.

Of the salts dissolved in seawater, sodium chloride (common salt) makes up about 78 per cent by the classic analysis of Dittmar. It is followed by magnesium chloride (about 11 per cent), magnesium sulphate (about 5 per cent), calcium sulphate and potassium sulphate.

Magnesium chloride is the second most abundant, which makes it the trap. Sodium sulphate is not one of the main salts in that list, and calcium carbonate is only a fraction of a per cent of the total, because marine organisms use it to build shells.

Answer: D

The key is 2000 metres.

The continental shelf ends at about 200 m and the continental slope falls away beyond it. The ocean basin proper, the deep floor with its abyssal plains, ridges and trenches, begins beyond the slope. Of the four values, 500, 800 and 1000 m are all still on the shelf or upper slope, while 2000 m is the only one that can mark the start of the deep floor.

The figure of 2000 m itself could not be confirmed in a reference. Books give different depths for where the basin begins, and the question's wording follows one textbook line. Answer D by elimination and by the key.