Oceans of the World
As seen from outer space, out planet is a brilliant blue sphere—a world of water. Indeed, it can be said that this planet is quite misnamed, for solid “earth” only covers a fraction of its surface. Oceans cover 71 percent of the globe, almost 140 million square miles (362 million square kilometers). Moreover, the tallest mountains found on land would easily disappear if dropped into the deepest ocean trench. It was not always this way.
The origin of oceans
Some 4 billion years ago, Earth’s surface was so hot that water evaporated on contact. Although the young planet’s surface was dry, its atmosphere was filled with volcanic steam and dust. This thick cloud cover shielded Earth from the blazing sun, and so helped cool it. As temperatures dropped, the atmospheric steam and dust began to condense into rain. The resulting downpour lasted for thousands of years.
When the clouds finally parted, Earth had been transformed into a sparkling blue orb. One vast ocean blanketed the entire planet. Above the ocean’s surface peeked the tips of the tallest volcanoes—the world’s first islands.
Earth’s primordial ocean was barely salty. Only later, after continents had formed, did waves wash salt and minerals from the land into the sea. The gradual leaching away of the continents increased the salt content until it reached the level that exists today.
After hundreds of millions of years, the rising continents took their present form, dividing Earth’s primordial sea into three major oceans: the Pacific, the Atlantic, and the Indian. The Arctic Ocean, considered by many oceanographers to be a marginal sea of the Atlantic, is nonetheless often elevated to ocean status.
Although the oceans are all distinct in character, all are interconnected. Each of the oceans contains many smaller sections, called seas, which are sometimes separated from the open ocean by a narrow opening called a strait. The Mediterranean and Caribbean are among the best known of the world’s seas, which total 70 in number. Gulfs and bays are small sections of ocean that border land.
What is seawater?
All the world’s oceans and seas share certain basic traits. Certainly all are salty. Common salt, or sodium chloride, is the most abundant, but by no means the only, salt found in seawater. Seawater also contains magnesium, calcium, and potassium salts. The overall salt content of seawater is about 3.5 percent. Of this fraction, 85 percent is sodium chloride.
The saltiness, or salinity, of seawater varies slightly from place to place. The least saline seawater is found in the Baltic Sea. Its salt content is just 1 percent. The saltiest seawater is found in parts of the Red Sea, where salinity soars to 27 percent.
The variation in salinity depends largely on climate. Abundant rainfall tends to decrease, or dilute, the salt content of seawater. Strong sunlight and high year-round temperatures increase salinity through evaporation.
The temperature of deep-ocean waters is more or less uniform around the world: just above freezing (except in the vicinity of thermal vents). But surface temperatures vary widely depending upon latitude, seafloor depth, and prevailing ocean currents.
As might well be expected, ocean surface temperatures are the highest in the tropics, where the water is continually warmed by strong sunlight. The highest ocean temperatures in the world, 96F (36 C), occur in the shallow waters of the Persian Gulf.
Surface waters are coldest near the poles, where the Sun stays low in the sky throughout the year. The coldest—several degrees below freezing—can be found in Russia’s White Sea. (The saltiness lowers seawater’s freezing point to below that of freshwater.)
Currents, which act like distinct rivers within the larger ocean, often contain unusually warm or cool waters. The Japan Current in the Pacific Ocean, for example, sweeps a “river” of warm water from the tropics north along the west coast of Alaska and Canada. As a result, the Pacific Ocean waters are noticeably warmer on the North American side than they are on the Russian side.
Sometimes, two currents of very different temperatures occur side by side. Off the coast of Newfoundland, Canada, where the warm Gulf Stream passes the cold Labrador Current, water temperatures can vary by 20F (11 C) over a distance of only a few feet.
Another important quality of seawater is its density, or specific gravity. Owing to its saltiness, seawater is denser or “heavier” than freshwater. The density of ocean water depends not only upon its salinity, but also on its temperature: warmer water tends to be less dense than cold water.
The greatest variance in ocean-water density occurs near the ocean surface, where water is subject to the influences of the weather. In general, density decreases in areas where freshwater is added to the ocean runoff from land, melting of ice, or precipitation. The density of seawater increases with cooling and when freshwater is removed through the process of evaporation.
Layers of less dense ocean water tend to float atop colder water layers. If the surface water becomes denser than the layer below it, the surface layer will sink until it reaches a layer of water with approximately the same density.
The density-related rising and sinking of water helps power the world’s ocean currents. The cold water near the poles, for example, sinks to great depths and from there, spreads out horizontally around the world. Similar but smaller cycles in each of the world’s oceans help maintain many of the dependable currents that mariners first detected and mapped centuries ago and still rely upon today.
The Pacific Ocean
The Portuguese explorer Ferdinand Magellan named the world’s largest ocean “El Pacifico,” meaning “the peaceful one.” But Magellan’s first impression was a misleading one. Mariners would later discover that the Pacific gives birth to the world’s most powerful storms and highest waves.
The Pacific Ocean is by far the world’s largest and deepest body of water. Spanning nearly half the globe, it covers almost 70 million square miles (180 million square kilometers)—nearly the size of the Atlantic and Indian Oceans combined. The floor of the Pacific Ocean, with an average depth of 13,000 feet (4,000 meters), is ringed by trenches. The deepest, the Mariana Trench, descends 35,840 feet (10,924 meters). By comparison, Mount Everest, the world’s tallest mountain, stands just over 29,000 feet (8,845 meters) above sea level.
The Pacific Ocean floor can be divided into three main regions. The Eastern region extends along the west coast of the Americas, from Alaska to Tierra del Fuego. On this side of the Pacific, the shallow coastal floor or continental shelf is narrow and falls off steeply into a deep-sea trench.
The Central Pacific floor is a broad, flattened area of great depth, about 15,000 feet (4,600 meters). The Western Region of the Pacific is scarred by deep trenches and festooned with underwater volcanic mountains. Some of these mountains reach above the surface to form chains and clusters of islands.
Most of the Pacific is blessed with an even climate marked by pleasant, steady trade winds. But storms of great violence, known as typhoons, occur in the western Pacific. The western Pacific also experiences great monsoons, or rain-bearing winds. Weather in the north Pacific can be especially changeable and harsh. The worst is found off the coast of Siberia in Russia.
The Pacific Ocean is rich in natural resources. Its vast fisheries supply three-fifths of the world’s catch, with tuna and salmon especially abundant. Salt, magnesium, sand, and gravel are harvested from the Pacific’s coastal regions. The Pacific is also used as a dumping ground for vast amounts of waste. Although the Pacific as a whole has been able to absorb and dilute much of it, some of the smaller seas and other local areas have become seriously polluted. In places, the pollution has greatly reduced or contaminated once-valuable populations of fish and crustaceans.
The northernmost area of the Pacific is the Bering Sea. There the Pacific meets the Arctic Ocean through the Bering Strait, a narrow passageway where Alaska and Siberia are just a few miles apart. Although it is no father north than is England, the Bering Sea has a harsh climate and is infamously dangerous to sailors. The Bering Sea is rich in fish and other wildlife, although overfishing has drastically reduced fish populations. The islands of the Bering Sea are breeding grounds for seals and otters, and many species of whale feed there during summer.
Another important Pacific sea is the Sea of Japan. Its warm waters give Japan a moderate climate year-round and provide a rich supply of fish and minerals.
The Atlantic Ocean
The waters of the Pacific Ocean mingle with those of the Atlantic Ocean between Antarctica and the tip of South America. The Atlantic Ocean, narrow and curving, is less than half the size of the Pacific. It is just 1,770 miles (2,848 kilometers) wide at its narrowest, between Brazil and Liberia. At its broadest, south of Africa, it stretches about 3,000 miles (4,800 kilometers) across. However, its width is expanding by about 1 inch (2.5 centimeters) a year, owing to the phenomenon called continental drift.
The Atlantic has an average depth of 11,730 feet (3,500 meters), and plunges to a maximum depth of 27,498 feet (8,387 meters) in a trench north of Puerto Rico. The most dramatic feature of the Atlantic Ocean floor is the Mid-Atlantic Ridge, a broad, rugged region of mountains that extends from the Arctic to the Indian Ocean.
Although smaller than the Pacific, the Atlantic receives far more freshwater drainage. All the major continents slope toward the Atlantic and empty their rivers into its basin. The North Atlantic contains many islands, although not in the vast chains that occur in the Pacific.
Above 30 degrees north latitude, the Atlantic is buffeted by high winds and cooled by cold air masses from Canada and the Arctic. The lower latitudes of the North Atlantic tend to remain storm-free—except for hurricanes.
Hurricanes often develop near the equator in late summer and early fall. Like a gigantic traveling whirlwind, a hurricane may travel across the Atlantic for more than a week, deluging islands and coastal areas with heavy rain.
The south Atlantic is more less open ocean. Like the north Atlantic, it grows increasingly stormy at lower latitudes (nearer the poles).
Because of the Coriolis effect, prevailing currents tend to circle in a clockwise direction around the North Atlantic basin, and in a counterclockwise direction in the south Atlantic. The Gulf Stream, which runs up the East Coast of the United States, is one of the Atlantic’s best-known currents. The water in the warm Gulf Stream is so distinct that it appears as a blue river flowing through a green ocean when viewed from Space.
The Atlantic is rich in fish life. Large upwellings of cold, nutrient-rich water from the depths give rise to enormous blooms of plankton and seaweed that, in turn, feed large populations of fish and crustaceans. Historically, the Atlantic Ocean has been the world’s major fishing ground, but overfishing has taken its toll: the Atlantic now provides only one-third of the world’s sea catch.
The coastal shelves of the Atlantic also contain a vast amount of oil and natural gas. Rich deposits are found in the Gulf of Mexico, off the east coast of Canada, in the North Sea, and off the west-central coast of Africa.
The major seas of the Atlantic include Europe’s Baltic Sea, which has the largest expanse of brackish, or somewhat salty, water in the world. The North Sea connects the Baltic to the Atlantic. The North Sea is one of the world’s most significant fishing grounds. To the south, the Mediterranean Sea, connected to the Atlantic by the Strait of Gibraltar, separates Europe from Africa. (The word “Mediterranean” comes from the Latin term for “the sea in the middle of Earth,” which the ancient Greeks and Romans believed it to be.) In the western hemisphere lies the Caribbean Sea and the West Indies. The North Atlantic contains many islands as well.
The Indian Ocean
The Indian Ocean is the third-largest body of water in the world, covering about 28 million square miles (74 million square kilometers) at an average depth of 12,760 feet (3,890 meters). To the west, the Indian Ocean merges with the Atlantic south of Africa, and, to the east, it mingles with the Pacific to the north and south of Australia.
At 36 million years old, the Indian Ocean is the youngest of the world’s oceans. Its present form came about by the final breakup of the supercontinent of Gondwanaland. Its seafloor is still expanding in many places.
The submarine geography of the Indian Ocean includes rugged, volcanically active ridges. These ridges form an upside-down Y, with the upper tip in the Arabian Sea, and the two branches extending to below Africa and Australia, respectively. The Indian Ocean contains many extinct submarine volcanoes, called seamounts, whose rounded peaks rise more than 3,000 feet (925 meters) above the ocean plain. A unique feature of this ocean’s underwater geography is the presence of huge submarine canyons in the continental shelf.
The Indian Ocean’s few islands include Madagascar, the fourth largest island in the world; Sri Lanka; and the Seychelles.
The prevailing currents of the Indian Ocean are complex and are greatly affected by infamous, rain-drenched monsoon winds. The monsoons—with wind speeds up to 28 miles (45 kilometers) per hour—occur in the north, in the Arabian Sea, and in the Bay of Bengal. From roughly April to October, the monsoons blow from the southwest—swirling waters of the Indian Ocean in a generally counterclockwise fashion. For the rest of the year, they blow from the northeast, and so reverse the current.
The northern portion of the Indian Ocean is also subject to cyclones—whirlwinds that arise over the open ocean and generally travel westward until they hit land with tremendous destructive force.
Steady southeasterly trade winds prevail in the central portion of the Indian Ocean. Farther south, the winds become light and variable. Gradually, nearer to the cold waters of the subantarctic, westerly winds often become gale force.
The Indian Ocean is of great economic importance. It includes the largest oil-producing region in the world—the Persian Gulf—as well as the Arabian Sea and Bay of Bengal, under which vast quantities of oil lie.
The Indian Ocean’s rich sea life is in part due to the great upwelling of nutrient-rich waters caused by the monsoon winds. The large number of fish and crustaceans draw fishing fleets from Japan, Russia, and elsewhere to the area to harvest shrimp, croakers, snappers, tuna, and a tuna-like species called the billfish.
In the past 50 years, humans have begun to threaten the health of the Indian Ocean. Cities and industries near the coast dump enormous amounts of waste into the water. Another serious source of pollution is the crude oil pumped from and transported across the ocean. Oil spills from tankers and drilling platforms occur with regularity, and sometimes have catastrophic effects on sea life.
The Arctic Ocean
As mentioned earlier, scholars are not in complete agreement as to the Arctic Ocean’s status. There is no question, however, that the Arctic Ocean is virtually unexplored and little understood, although this is changing rather rapidly as scientists come to appreciate the Arctic Ocean’s great importance. It now appears that cold waters flowing out of the Arctic greatly affect the climate of the entire world. With climate change, scientists are especially concerned about any melting of the Arctic Ocean’s ice cover. Such a melting could trigger even more dramatic climate changes by altering the cold currents flowing out of the Arctic and decreasing the amount of sunlight reflected away by the ice cap.
In physical size, the Arctic Ocean is just 5.44 million square miles (14 million square kilometers); by comparison, the Indian Ocean is approximately six times as large. Arctic waters average only 3,000 to 4,000 feet (900 to 1,200 meters) deep. The floor of the Arctic Ocean is quite complex, divided into two deep basins crisscrossed by underwater mountain ranges or ridges. Nearly one-third of the Arctic lies over the continental shelf. This shelf area is particularly broad and shallow—an extension of northern coasts of North America, Greenland, and Eurasia that encircle the ocean on all sides.