Ocean Floor, Tides and Salinity: Complete Exam Notes

The correct answer is D, only II. Statement I is wrong because the equatorial belt receives very heavy rainfall and has high cloud cover, so the surface water there is diluted; the highest salinity in the open ocean is recorded in the subtropical high pressure belts, roughly between twenty and thirty degrees of latitude, where skies are clear, evaporation is strong and rainfall is slight. Statement II is correct, because the northern hemisphere, with its enclosed and partly landlocked seas and its narrower expanse of cold southern water, has a higher average surface salinity than the southern hemisphere, where the vast Southern Ocean is fed by melting ice. Option A is wrong because statement II is true. Option B is wrong because statement I is false. Option C is wrong for the same reason, since it accepts the equatorial claim.

The correct answer is B, the Mariana Trench, in the western Pacific Ocean, whose Challenger Deep reaches a depth of about eleven kilometres and is the deepest known point on the earth's surface. Option A, the Java or Sunda Trench, is wrong because it is the deepest trench of the Indian Ocean, not of the world. Option C, the Puerto Rico Trench, is wrong because it is the deepest part of the Atlantic Ocean. Option D, the Tonga Trench, is wrong although it is indeed one of the deepest trenches of the Pacific, second only to the Mariana. Trenches are the narrow, steep-sided deeps that lie beyond 6,000 metres, they form where one plate descends beneath another, and most of the world's trenches are found around the rim of the Pacific.

The correct answer is A, the continental shelf. It is the gently sloping, shallow margin of the continent, extending to a depth of about 200 metres with a gradient of one degree or less and an average width of about seventy to eighty kilometres, and it makes up roughly 7.5 per cent of the ocean area. Sunlight reaches its floor, so plankton grow in abundance and the great fisheries of the world lie over shelves; most offshore oil and gas also come from them. Option B, the continental slope, is wrong because it is steep and deep and marks the true edge of the continent. Option C, the abyssal plain, is wrong because it lies between 3,000 and 6,000 metres and yields only manganese nodules and fine oozes. Option D, the trench, is wrong because it is the deepest and least exploited part of the sea.

The correct answer is B, new moon and full moon. On these days the sun, the moon and the earth lie in a straight line, a position called syzygy, so the pulls of the sun and the moon act together and the tidal range is at its greatest; spring tides therefore occur twice in a lunar month and have nothing to do with the spring season. Option A is wrong because the first and third quarters give neap tides, when the sun and the moon pull at right angles to each other and partly cancel out, producing the smallest range of the month. Options C and D are wrong because they take only half of the correct answer; the alignment is equally effective whether the moon is between the earth and the sun or on the far side of the earth.

The correct answer is B, the Bay of Fundy, between Nova Scotia and New Brunswick in Canada, where the funnel shape of the bay and the length of the inlet make the incoming tide pile up to a range of the order of fifteen to sixteen metres. Option A, the Gulf of Kutch, is wrong as a world answer, although it does have one of the highest ranges in India along with the Gulf of Khambhat, which is why tidal power has long been proposed there. Option C, the Strait of Gibraltar, is wrong because the Mediterranean beyond it is almost tideless, its narrow entrance letting in very little tidal water. Option D, the Gulf of Mexico, is wrong because it has an unusually small range and chiefly diurnal tides.

The correct answer is B, 35 ppt and sodium chloride. Salinity is expressed in parts per thousand, so 35 ppt means 35 grams of dissolved salt in a kilogram of sea water. Of that dissolved matter sodium chloride, common salt, forms roughly 77 per cent, magnesium chloride about 11 per cent, and the sulphates of magnesium, calcium and potassium most of the remainder. Option A is wrong on both counts, since 24 ppt is far below the ocean average and magnesium chloride is only the second largest component. Option C is wrong because about 41 ppt is the salinity of the Red Sea, the most saline of the open seas, and not the ocean average. Option D names the right figure but the wrong salt, since calcium sulphate is a minor constituent.

The correct answer is C, Lake Van in Turkey, which records a salinity of about 330 parts per thousand, the highest figure given in standard textbooks. Option A, the Dead Sea, is wrong although it is the most famous example, with a salinity of about 238 ppt, which is why bathers float in it so easily. Option B, the Great Salt Lake of Utah, is wrong because its salinity is about 220 ppt. Option D, the Red Sea, is wrong because its 41 ppt is high only by the standard of open seas. All four are saline for the same reason: they are enclosed or nearly enclosed basins in hot or dry regions, where evaporation removes water and leaves the salt behind while little fresh water flows in.

The correct answer is B, guyots. A seamount is an isolated volcanic peak rising from the deep sea floor without reaching the surface; where such a peak once stood at sea level and had its top worn flat by waves before subsiding, it is called a guyot. Option A, atolls, is wrong because an atoll is a ring or horseshoe of coral enclosing a lagoon, built by coral polyps over a subsiding island. Option C, banks, is wrong because a bank is a flat-topped elevation on the continental shelf, shallow enough to be a great fishing ground, the Grand Banks off Newfoundland being the classic example. Option D, shoals, is wrong because a shoal is a detached elevation shallow enough to be a danger to shipping. The names are a favourite matching exercise in examinations.

The correct answer is B. The tide generating force of a body is not simply its gravitational pull but the difference in that pull across the earth, and that difference falls off as the cube of the distance rather than the square. The sun is enormously more massive than the moon, but it is about four hundred times farther away, and the cube of that distance outweighs the mass, so the moon's tide generating force is roughly twice the sun's. Option A is wrong because the moon is vastly less massive than the sun. Option C is wrong because the direction of the moon's revolution lengthens the tidal day to about 24 hours and 52 minutes but does not decide the strength of the force. Option D is wrong because an atmosphere has nothing to do with gravitation.

The correct answer is C, the Mid-Atlantic Ridge, a submarine mountain chain that runs down the middle of the Atlantic Ocean in the shape of an S, roughly parallel to the coasts on either side, and continues into the other oceans as part of a worldwide system of mid-oceanic ridges tens of thousands of kilometres long. It is a constructive plate margin, where molten rock rises and new ocean floor is created, and its highest peaks break the surface as islands such as Iceland and the Azores. Options A, B and D name great mountain chains of the land, the Andes of South America, the Himalayas of Asia and the Rockies of North America, but every one of them is shorter than the submerged ridge system, which is why the question is a favourite in examinations.