Oceanography Questions and Answers

Answer: A

Wave refraction disperses wave energy.

As waves approach an irregular coast, the part of each crest in shallow water off a headland slows down first, while the part over deeper water in a bay keeps moving faster. The crests bend to follow the shape of the sea floor. The wave rays converge on headlands, concentrating energy and erosion there, and diverge in bays, spreading the energy out so that bays receive gentle waves and become places of deposition.

Sediment, plankton and sea water are moved by currents and by the waves themselves; refraction is about the distribution of wave energy along the coast.

Answer: C

The circular flow is called a gyre.

A gyre is a large, roughly circular system of ocean currents driven by the trade winds and westerlies around a subtropical high. Deflected by the Coriolis effect, the subtropical gyres turn clockwise in the northern hemisphere, as in the North Atlantic with the Gulf Stream, Canary Current and North Equatorial Current.

A geostrophic current is any current in balance between pressure gradient and Coriolis force, not specifically a circular one. Upwelling is the vertical rise of cold deep water, and a littoral current runs along the shore.

Answer: B

The key gives between 20° and 30° N.

Surface salinity is highest not at the equator but in the subtropics, under the descending dry air of the subtropical highs, where evaporation greatly exceeds rainfall. The standard account places the maximum in the northern subtropical belt near the Tropic of Cancer: the North Atlantic there exceeds 37 parts per thousand, and the enclosed seas of the same belt, the Red Sea and the Persian Gulf, reach about 40 or more.

The 10° to 20° N belt and the 0° to 23½° S belt include the equatorial rain belt and river outflow, which lower salinity. The southern subtropics (20° to 30° S) are also saline, but have no enclosed seas comparable to the Red Sea, so the textbook maximum is the northern belt.

Answer: A

Gravel and sand are deposited in the littoral zone.

The littoral zone is the shore and shallow nearshore area, where waves and currents have enough energy to move coarse material. Heavy particles such as gravel and sand settle there quickly, forming beaches, bars and spits.

Only fine material, clay and silt, together with biogenic oozes, travels out to the bathyal zone of the continental slope and the deep sea. A rift valley is a tectonic feature on land or on the mid-ocean ridge, not a depositional zone of the shore, so it is not where wave-sorted gravel and sand collect.

Answer: C

Continental shelves are mainly covered by terrigenous sediments.

Terrigenous (lithogenous) sediments are particles eroded from the land, gravel, sand, silt and clay, carried to the sea by rivers, glaciers and wind. Most of this load is dropped close to the land, so it blankets the continental shelves.

Biogenic sediments (shells and skeletons of marine organisms) dominate the deep-sea oozes far from land. Hydrogenous sediments, such as manganese nodules, precipitate from sea water and are typical of slowly accumulating deep basins. Cosmogenous sediment from space is only a tiny fraction everywhere.

Answer: B

The boundary zone is the halocline.

A halocline is a layer in which salinity changes rapidly with depth, separating water masses of different salinity, for example fresher surface water over saltier deep water at the mouth of a large river or in high latitudes.

A thermocline is the corresponding layer of rapid temperature change. Thermohaline refers to circulation driven by differences in temperature and salinity together, not to a boundary layer. An isohaline is a line joining points of equal salinity, which is the opposite of a steep gradient.

Answer: D

The key gives 200 m.

Sunlight is absorbed rapidly in sea water: red light is gone within a few metres, and most of the remaining light is absorbed in the upper tens of metres. The photic (euphotic) zone, where enough light remains for photosynthesis, ends at about 200 m even in the clearest open ocean, and below that depth the solar radiation that penetrates is insignificant.

Depths of 15, 20 and 50 m still receive useful light in clear water, so the depth beyond which a significant proportion of solar rays cannot reach is the 200 m limit of the photic zone.

Answer: B

The true statements are (i) and (ii).

The ocean is layered by temperature and density. The surface mixed layer is a thin upper layer, typically tens to a few hundred metres deep, of warm water stirred by wind and waves (i). Beneath the thermocline lies the thick mass of deep, cold water that makes up most of the ocean's volume and moves only slowly (ii).

Statements (iii) and (iv) reverse the thicknesses and the stirring: the warm layer is not thick, and the deep cold water is not thin or well stirred. So options containing (iii) or (iv) are wrong.

Answer: C

Open ocean water holds about 30 g of dissolved salt per kg of water.

The average salinity of sea water is about 35 parts per thousand, that is 35 g of dissolved salts per kilogram, of which sodium chloride makes up the large majority. In terms of the options, about 30 g of salt per kg is the closest value.

10 and 20 g/kg are typical of brackish water in estuaries or semi-enclosed seas. More than 100 g/kg is found only in hypersaline lakes and lagoons such as the Dead Sea.

Answer: A

A seamount must rise Minimum 1000 m above the sea floor.

By the conventional definition, a seamount is an isolated submarine volcanic mountain rising at least 1000 m above the surrounding ocean floor without reaching the surface. Flat-topped seamounts, eroded at sea level and then submerged, are called guyots.

Smaller submarine volcanic hills, between a few hundred metres and 1000 m high, are usually called knolls or seaknolls, and lower ones abyssal hills, so the other height ranges do not define a seamount.