ICEBERG

floating mass of freshwater ice that has broken from the seaward end of a glacier or a polar ice sheet. Icebergs are typically found in open seas, especially around Greenland and Antarctica. They form mostly during the spring and summer when warmer weather increases the rate of calving (separation) of icebergs at the boundaries of the Greenland and Antarctic ice sheets and smaller outlying glaciers. In the Northern Hemisphere, for example, about 10,000 icebergs are produced each year from the west Greenland glaciers, and an average of 375 flow south of Newfoundland into the North Atlantic shipping lanes, where they are a hazard to navigation. At the point where glaciers or ice shelves meet the sea, water pressure beneath the ice shelf or glacier tongue interacts with the outward-moving glacier. Tides, which may have ranges up to 6 m (20 feet) in the Arctic, together with small sea level changes associated with wind and swells, produce an intermittent increase and decrease in force on the protruding end of the glacier or ice shelf, and this results in the birth of a large monolith of drifting ice. Another way in which an iceberg is formed, characteristic of southern Greenland glaciers, consists of a melting or evaporation of the surface portions of the glacier near its terminus at a greater rate than the water erosion on its underside. This results in an underwater shelf, and eventually, through continued erosion by water, natural buoyancy of submerged ice, and periodic tidal and other hydraulic forces, the shelf is broken off and an iceberg floats to the surface. Most Antarctic icebergs are formed through gradual break off from the Antarctic continental ice sheet as it thins toward the coast and exudes into the ocean as a great ice shelf with fronts hundreds of kilometres long. Icebergs are produced at a rate of approximately 280 cubic km per year (67 cubic miles per year) in the Arctic and 1,800 cubic km (430 cubic miles) per year in the Antarctic. Western Greenland is the major iceberg source area in the Northern Hemisphere. Eastern Greenland icebergs tend to move northward; they are small and few in number. There are also few icebergs in the Arctic Basin proper. The source area for most icebergs in the Barents Sea is Franz Josef Land. Icebergs are not found in the North Pacific except in sounds along the Alaskan-Canadian coast between latitudes 55 N and 60 N. In the Southern Hemisphere most icebergs are concentrated south of the Antarctic current convergence at about 60 S. Arctic icebergs vary in size from the size of a large piano, called growlers, to the dimensions of a ten-story building. Most icebergs have a specific gravity of 0.9, and thus six-sevenths of the mass is below the sea surface. Many icebergs in the Arctic are about 45 m tall and 180 m long. Icebergs of the Antarctic are tabular in shape and not only are far more abundant but are of enormous dimensions compared with those in the Arctic. Lengths up to 8 km (5 miles) are not unusual, with ice 45 m above water. From the known amount of precipitation over Greenland, and the assumption that the ice volume and precipitation rate now are approximately the same as they were many centuries ago, it can be estimated that the mean age of the ice in icebergs is 5,000 years. Arctic ice islands and giant Antarctic bergs last as long as 10 years at high latitude. Most icebergs from western Greenland melt within two years of calving from the parent glacier. Once an Arctic Ocean iceberg has been calved and moves out to the open sea, it usually transits Baffin Bay in from three months to two years, during which time it undergoes some disintegration through melting and calving of smaller chunks of ice from its perimeter. This results in a decrease in mass of about 90 percent by the time it reaches the coast of Newfoundland and the Grand Banks in the North Atlantic. When the iceberg enters the region of the Grand Banks, where the warm waters of the Gulf Stream meet the colder waters of the Labrador Current, it has only a few days of life remaining. In mild sea conditions an iceberg deteriorates at a rate of height decrease of about 2 m per day in 04 C (3240 F) water, and about 3 m per day in 410 C (4050 F) water. Although the bulk of the iceberg is below water, in many situations wind has a dominant influence on the movement. The wind intensity and direction over Baffin Bay in the spring of one year, for example, influences the number of icebergs that slip into the North Atlantic that year and the following year. The day-to-day movement of an iceberg is controlled by the size and shape of the iceberg, previous and present wind, surface wind current, and general ocean current. The most important factor in assessing wind drift of icebergs is size and shape. Winged icebergs, those with saillike pinnacles around the central mass, are very much influenced by the winds and move at speeds of one knot (44 kilometres, or 24 nautical miles) per day under the influence of steady winds of 30 knots. The momentum of icebergs is so great that once in motion they continue for hours after the wind has abated. In general, however, the drift of icebergs is the combined result of both the ocean current and the wind. When the winds are variable or less than 32 knots, and the current greater than 0.5 knot, the current predominates, but when steady 30-knot winds blow for more than 12 hours, the wind effect becomes important even in areas where the ocean current is one to two knots. Icebergs transport sediment in the form of pebbles, cobbles, boulders, and finer material, and even plant and animal life, thousands of kilometres from their source area. The distribution of icebergs 10,000 years ago, for example, can be inferred from sediments that are widely disseminated on the ocean floor of the North Pacific Ocean as far south as latitude 48 N and in the South Atlantic Ocean as far north as the latitude of the Cape of Good Hope in the Southern Hemisphere. Icebergs are coloured brown, black, and green by a combination of sediment and plankton deposits under the source area ice shelf, and glacial blue ice. In the open ocean most ice is seen by radar at ranges depending upon the size of its subaerial silhouette, but smaller icebergs or growlers can only be detected when the sea surface is calm, and then only at ranges of about 1.6 km. Sonar is effective in detecting icebergs; the range of detection, however, is frequently limited by the water conditions and speed of commercial vessels. The problem of iceberg protection in the Northern Hemisphere is one of tracking icebergs as they come down the Labrador Current, and reporting the whereabouts of these floating menaces to all North Atlantic shipping as often as twice daily. Additional reading The published literature on icebergs is found mainly in U.S. journals and in atlases prepared by the U.S. Coast Guard and U.S. Navy for Arctic and Antarctic military and scientific maritime resupply expeditions. A classic discussion of icebergs and sea ice in the Arctic Ocean is The Marion Expedition to Davis Strait and Baffin Bay, vol. 3 by E.H. Smith, Arctic Ice (1931). Also of interest is the treatment of the oceanography of the north polar basin by Fridtjof Nansen, The Norwegian North Polar Expedition, 18931896: Scientific Results, vol. 3 (1902, reprinted 1969), and Farthest North, 2 vol. (1897, reissued 1967). The importance of ice in the water budget of the planet may be studied by consulting James L. Dyson, The World of Ice (1962, reissued 1972). Robert C. Pritchard (ed.), Sea Ice Processes and Models (1980), a collection of proceedings papers, reports field observations and the development of models in an effort to establish usable flow laws for sea ice. Two reference works are United States Hydrographic Office, A Functional Glossary of Ice Terminology (1952); and World Meteorological Organization, Sea-Ice Nomenclature (1970).The age of icebergs can be understood by consulting W. Dansgaard et al., One Thousand Centuries of Climatic Record from Camp Century on Greenland Ice Sheet, Science, 166(3903):377381 (1969); and P.F. Scholander et al., Composition of Gas Bubbles in Greenland Icebergs, The Journal of Glaciology, 3:813822 (1961). A major reference work on the proposed use of icebergs as a freshwater source is the collected conference papers in A.A. Husseiny (ed.), Iceberg Utilization (1978). Thomas F. Budinger The Editors of the Encyclopdia Britannica