Pariksha SaathiLesson 7 of 10 · Physical and World Geography
The Oceans: Sea-Floor Relief, Trenches, Currents, Tides, Salinity and Coral Reefs
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The world ocean
About 71 per cent of the Earth's surface is water, and about 97 per cent of all the Earth's water is salt water in the oceans. The oceans in order of size are the Pacific, Atlantic, Indian, Southern and Arctic. The Pacific is the largest and deepest, bigger than all the land put together. The S-shaped Atlantic is split down the middle by the Mid-Atlantic Ridge. The Indian Ocean is the only ocean named after a country, and the Arctic is the smallest and shallowest.
Relief of the ocean floor
The continental shelf is the gently sloping, shallow, submerged edge of a continent, usually less than about 200 m deep; it holds rich fishing grounds such as the Dogger Bank in the North Sea and oil fields such as Mumbai High. It ends at the steeper continental slope, cut by submarine canyons, which leads down to the abyssal plains, the flattest and most extensive parts of the ocean floor, at about 3,000 to 6,000 m. Rising from the floor are mid-ocean ridges, together the longest mountain system on Earth, seamounts (underwater volcanic peaks) and guyots (flat-topped seamounts).
Trenches
Trenches are long, narrow, steep-sided depressions formed where one plate subducts under another, so most of them fringe the Pacific. The Mariana Trench in the western Pacific, near Guam, is the deepest; its Challenger Deep is nearly 11,000 m deep, so Mount Everest (8,849 m) placed in it would still have more than 2 km of water above its summit. The Puerto Rico Trench is the deepest in the Atlantic and the Sunda or Java Trench the deepest in the Indian Ocean.
Ocean currents
Currents are steady flows of surface water driven mainly by the planetary winds and steered by the Coriolis force and the shape of the coasts. They form large loops called gyres, clockwise in the Northern Hemisphere and anticlockwise in the Southern. In low and middle latitudes, warm currents flow from the Equator towards the poles mostly along the eastern coasts of continents, and cold currents flow towards the Equator along western coasts. The warm Gulf Stream leaves the Gulf of Mexico and crosses the Atlantic as the North Atlantic Drift, which keeps the ports of Norway free of ice in winter; where it meets the cold Labrador Current near Newfoundland, thick fog and the rich fisheries of the Grand Banks result. The Canary, Benguela and Falkland currents of the Atlantic are cold and the Brazil Current warm; in the Pacific the Kuroshio and East Australian currents are warm and the Oyashio, California and Peru (Humboldt) currents cold; in the Indian Ocean the Agulhas and Mozambique currents are warm and the West Australian cold, and in its north the currents reverse with the monsoon. Cold currents along tropical western coasts chill the air and stop rain, which is why the Atacama, Namib and Sahara deserts reach those coasts.
Tides
Tides are the regular rise and fall of sea level caused mainly by the Moon's gravitational pull and partly by the Sun's, whose tide-raising force is less than half the Moon's. Most coasts get two high and two low tides in about 24 hours 50 minutes, so high tide comes about 50 minutes later each day. Spring tides, with the largest range, occur on new moon and full moon days, when the Sun, Moon and Earth are in a line; neap tides, with the smallest range, come about 7 days later at the quarter moons, when the Sun and Moon pull at right angles. The Bay of Fundy in Canada has the highest tidal range in the world, and in India the Gulf of Khambhat has the highest, making it, with the Gulf of Kachchh, the best site for tidal power.
Salinity
Salinity is the salt dissolved in sea water, expressed in parts per thousand; the ocean average is 35, meaning 35 grams of salt in 1,000 grams of sea water. Evaporation raises salinity, while rain, river water and melting ice lower it, so salinity is highest in the subtropics, lower at the rainy Equator and lowest near the poles. Enclosed seas in hot, dry regions, such as the Red Sea and the Persian Gulf, are very salty, while the Baltic Sea, fed by many rivers, is among the least salty. Lake Van in Turkey, the Dead Sea and the Great Salt Lake in the USA are far saltier than the ocean.
Coral reefs
Corals are tiny marine animals called polyps that build skeletons of calcium carbonate, and algae living inside them give them food and colour. Tropical coral reefs grow only in warm (above about 20 degrees Celsius), clear, shallow, sunlit and salty water, so they lie mostly within 30 degrees of the Equator. Fringing reefs grow close to the shore; barrier reefs lie farther out behind a lagoon, like the Great Barrier Reef off Queensland, Australia, the largest reef system in the world; atolls are ring-shaped reefs around a lagoon, as in Lakshadweep and the Maldives. Water that is too warm makes corals expel their algae and turn white, which is coral bleaching.
Key facts
- Oceans by size: Pacific, Atlantic, Indian, Southern, Arctic. The Pacific is larger than all the land put together; the Arctic is the smallest and shallowest.
- Ocean floor: continental shelf (usually under 200 m, fisheries and oil such as Mumbai High), continental slope, abyssal plain, trench; a guyot is a flat-topped seamount.
- Deepest trenches: Mariana (Pacific, Challenger Deep, nearly 11,000 m), Puerto Rico (Atlantic), Sunda or Java (Indian Ocean).
- Warm currents: Gulf Stream, North Atlantic Drift, Brazil, Kuroshio, East Australian, Agulhas, Mozambique.
- Cold currents: Labrador, Canary, Benguela, Falkland, Oyashio, California, Peru (Humboldt), West Australian.
- Gulf Stream meets the Labrador Current near Newfoundland: fog and the rich Grand Banks fishery. The North Atlantic Drift keeps Norway's ports ice-free.
- Cold currents beside deserts: Peru Current and Atacama, Benguela and Namib, Canary and the Sahara coast.
- Spring tides: new and full moon, Sun, Moon and Earth in a line, largest range. Neap tides: quarter moons, at right angles, smallest range. High tide comes about 50 minutes later each day.
- Average ocean salinity 35 parts per thousand; highest in the subtropics; Red Sea very salty, Baltic Sea among the least; Lake Van, Dead Sea, Great Salt Lake far saltier.
- Coral reefs: fringing, barrier (Great Barrier Reef, Queensland, the largest), atoll (Lakshadweep, Maldives); need water above about 20 degrees C; bleaching when too warm.
Quick revision
71% of the surface is water, 97% of water is in the oceans; Pacific largest and deepest; continental shelf usually under 200 m, abyssal plain 3,000-6,000 m; Mariana Trench nearly 11,000 m; Puerto Rico Trench deepest in the Atlantic, Java Trench in the Indian Ocean; gyres clockwise in the north; Gulf Stream, Kuroshio, Brazil, Agulhas warm; Labrador, Canary, Benguela, Peru, California cold; spring tides at new and full moon; Bay of Fundy highest tides, Khambhat in India; salinity 35 per thousand; Great Barrier Reef largest.