The vast body of water that separates Europe and Africa from North and South America is the Atlantic Ocean. Its name, which comes from the Greek, may refer to Atlantis, the legendary island continent said to have been the site of a great civilization before the island sank. Extending from the Arctic Ocean in the north to Antarctica in the south, the Atlantic Ocean is divided by the equator into two parts, the North Atlantic and the South Atlantic. Its area is about 31,831,000 square miles (82,442,000 square kilometers), which is about one sixth of the Earth's total area. Although the South Atlantic is bigger than the North Atlantic, the North Atlantic has a much longer coastline, and large marginal seas and bays adjoin it. On the American side the largest bodies of water are the Gulf of Mexico, the Caribbean Sea, and Hudson Bay. On the European side the largest are the Mediterranean, North, and Baltic seas. More than half of the total land area of the world is drained by rivers that flow into the Atlantic Ocean and its adjoining seas. The large quantities of dissolved minerals carried from this huge area make the Atlantic the saltiest of the world's oceans. The Atlantic is also the youngest ocean. Its origin and development can be accounted for by the theory of continental drift (see continent, “Continental Drift”). This theory supposes that about 200 million years ago a mammoth ancient continent, called Pangaea by today's geologists, broke up into large pieces, which slowly separated to form the present landmasses of the Eastern and Western hemispheres and Antarctica. The coastline of North and South America as observed on a map appears to “fit into” the coastline of Europe and Africa, and there are similarities in rock structures on both sides of the Atlantic. Seafloor spreading, which also supports the theory, continues today in the Mid-Atlantic Ridge, which has been estimated to move at the rate of about 1/2 to 4 inches (1 to 10 centimeters) each year. (See also Earth.) The outstanding feature of the Atlantic's floor, the Mid-Atlantic Ridge is an immense, volcanic mountain range that stretches its entire length. It covers the central third of the ocean bed and is up to 1,000 miles (1,600 kilometers) wide. On either side of the Mid-Atlantic Ridge are broad mud- and clay-covered abyssal plains and basins. An underwater extension of a continental landmass is called a continental shelf. In some places the Mid-Atlantic Ridge protrudes to create islands such as the Azores and Iceland. Most islands, however, are isolated portions of the continental shelf, such as Great Britain, Newfoundland, and the Falkland Islands. The deepest point on the Atlantic's ocean floor—27,498 feet (8,381 meters)—is in the Puerto Rico Trench located about 90 miles (145 kilometers) northwest of Puerto Rico. Climate and CurrentsWeather over the ocean is largely determined by the prevailing winds and ocean currents. In the North Atlantic, especially during winter, frequent storms occur along fronts, or zones of great temperature contrast. The prevailing westerly winds of the middle latitudes carry cold air masses from Canada over the ocean, where they are met by warm air masses originating over the Gulf of Mexico or associated with a current known as the Gulf Stream. An area of low atmospheric pressure develops where the warm air is forced to rise over the cold air. Storms quickly develop around such a frontal zone. (See also storm; weather.) Hurricanes originate in the tropical latitudes of the North Atlantic. During the hottest part of the year large amounts of water vapor enter the atmosphere from the ocean. When the vapor condenses as rain, heat energy is released. Massive hurricanes can develop around large concentrations of such heat. In the South Atlantic the weather patterns are more regular than in the North Atlantic because there are no adjacent, large land areas in the middle latitudes over which cold or hot air masses can form. However, the prevailing westerly winds frequently carry storms across the South Atlantic creating dangerous gales and heavy winds that are a notorious shipping hazard. Circulation of the main water currents of the Atlantic Ocean is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. In the tropical latitudes the currents of both hemispheres flow generally from east to west, and their temperature becomes relatively warm. In the North Atlantic the warm current known as the Gulf Stream flows northward along the coast of North America. It continues northward and eastward across the ocean as the somewhat cooler North Atlantic Current and warms the climate of northwestern Europe. After turning southward along the coast of Europe and northwestern Africa, it becomes the relatively cold Canary Current. In the South Atlantic the southward flowing warm current along the east coast of South America is called the Brazil Current, and the northward flowing cold current along the west coast of southern Africa is the Benguela Current. Marine LifeThe Atlantic Ocean can be divided into five water layers, or zones, each of which supports a different type of marine life. From top to bottom these zones are the littoral, euphotic, mesopelagic, bathypelagic, and benthic. The littoral, or tidal, zone is the area along the shore between the high-tide and low-tide levels. It is the habitat for such sea life as clams, oysters, mussels, seaweed, sea moss, and brown algae. The euphotic zone is the ocean's surface layer into which sunlight can penetrate in sufficient amounts to support photosynthesis. It extends to a depth of about 600 feet (180 meters) and contains most of the sea's organisms. These range from microscopic sea animals (zooplankton) and plants (phytoplankton) to the largest fishes and whales. The mesopelagic, or midocean, zone extends from about 600 feet to 3,000 feet (180 meters to 900 meters). It contains organic debris from the euphotic zone above and supports such animals as the sperm whale and giant squid. The bathypelagic, or deep-ocean, zone extends from 3,000 feet (900 meters) to the bottom. About two thirds of the fishes that inhabit this zone are bioluminescent, or capable of producing their own light. The benthic zone occurs at the bottom of the ocean—from shallow waters to deep-sea trenches—and supports the greatest range of animal and plant life. At depths of 12,000 feet (3,700 meters) or more, fishes are not common on the bottom, but less developed life forms such as tube worms, crinoids (sea lilies), and glass sponges can exist. They feed on the organic matter that settles from the upper zones and, in very specialized deep-sea communities, on minerals that well up with hot water from midocean vents in the seafloor. (See also deep-sea life.) Economic Resources—Future ProspectsApproximately 40 percent of the world's fish catch comes from the Atlantic Ocean. In the northwestern Atlantic off Newfoundland is the Grand Banks, where herring, cod, and menhaden are plentiful. Traditional fishing areas in the northeastern Atlantic and the North Sea yield flounder, ocean perch, cod, hake, and herring, but they have been overfished. The warm waters of the Caribbean and Gulf of Mexico abound with shrimp and sponges. Since the 1950s fishing has greatly increased in the South Atlantic, where large quantities of tuna, hake, and herring are caught. Clams, oysters, crabs, and octopuses can be harvested along the coast of much of the Atlantic. The Sargasso Sea, in the center of the North Atlantic, is the breeding ground for the American and European species of migratory eels. The most important mineral resources derived from the Atlantic are petroleum and natural gas. Offshore fields lie in the North Sea, the Gulf of Mexico, the Caribbean, and along the coasts of Nigeria, Brazil, Canada, Argentina, and the United States. Other minerals being exploited include tin and coal from mines under the ocean floor off Great Britain. Large diamond deposits are found along the southwestern coast of Africa. Salt and bromine are extracted from sea water. Manganese nodules have been found on the deep ocean floor, but technology still needs to be developed to make them economically retrievable. History and TradeThough human beings have sailed on the waters of the Atlantic from ancient times, the Vikings were the first Europeans to document these voyages. During the 11th century they explored and charted the waters around Iceland, Greenland, and northeastern North America. In the 15th century the Portuguese discovered the Azores and Madeira and sailed along the entire western coast of Africa. Under the Spanish flag, in 1492, Christopher Columbus made the first of his trans-Atlantic crossings. Europeans explored and settled most of the Atlantic coast of the Americas during the 16th and 17th centuries. In the mid-1800s Lieut. Matthew Maury of the United States Navy made the first oceanographic study of the Atlantic. He compiled charts on winds and currents that established the first sea lanes, prepared a treatise on the Gulf Stream, and collected other data. The British Challenger expedition, made between 1872 and 1876, discovered the Mid-Atlantic Ridge. In more recent times, Jacques Piccard, the Swiss scientist, explored Atlantic waters in Trieste I, a deep-sea bathyscaphe. A wide range of other submarine devices have also been used to record new information about the depths of the Atlantic and other oceans. The submersible Alvin explored the wreck of the Titanic in 1986. Deep-sea drilling ships, such as the Glomar Challenger, have surveyed the bed of the Atlantic and discovered oil fields. The United States Navy established Sealab, an underwater habitat that was set up near Bermuda where aquanauts lived beneath the ocean and studied marine life for extended periods of time. (See also oceanography.) The North Atlantic, with its hazardous icebergs and winter storms, is the most heavily used and congested ocean trade route in the world. Shipping lanes were established and regulated to ensure the safety of all who sailed the Atlantic. Ice patrols maintain surveillance of icebergs and report on their locations to all ships and oil drilling platforms that could be in danger. Satellites are used to observe iceberg movements throughout the entire Atlantic. Among the Atlantic's many deep, natural harbors are those at New York City and Rio de Janeiro, Brazil. Other busy ports, such as Amsterdam, The Netherlands, are accessible to oceangoing vessels through dredged channels. Punta Arenas, Chile, is the southernmost port in the South Atlantic, and the northernmost port in the North Atlantic is in Russia, at Archangel. (See also ocean.) |