Icebergs, formed by the breaking off, or calving, of blocks from a valley glacier or tongue of an icecap, may be as thick as several hundred feet. Being only slightly less dense than sea water, the iceberg floats very low in the water, about five sixths of its bulk lying below water level. The ice is fresh, of course, since it is formed of compacted and recrystallized snow.
In the northern hemisphere, icebergs are derived largely from glacier tongues of the Greenland icecap. They drift slowly south with the Labrador and Greenland currents and may find their way into the North Atlantic in the vicinity of' the Grand Banks of Newfoundland. Icebergs of the Antarctic are distinctly different. Whereas those of the North Atlantic are irregular in shape and therefore present rather peaked outlines above water, the Antarctic icebergs are commonly tabular in forms, with flat tops and steep cliff like sides. This is because tabular bergs are parts of ice shelves, the great, floating platelike extensions of the continental icecap. In dimensions, a large tabular berg of the Antarctic may be tens of miles broad and over 2000 ft (600 m) thick, with an ice wall rising 200 to 300 ft (60 to 90 m) above sea level.
Somewhat relative in origin to the tabular bergs of the Antarctic are ice islands of the North Polar Sea. These huge plates of floating ice may be 20 mi (32 km) across and have an area of 300 sq mi (800 sq km). The bordering ice cliff, 20 to 30 ft (6 to 10 m) above the surrounding pack ice, indicates an ice thickness of 200 ft (60m) or more. The few ice islands known are probably derived from a shelf of land-fast glacial ice attached to Ellesmere Island, about 83° N lat. The ice islands move slowly with the water drift of the Arctic Ocean and a charting of their tracks reveals much about circulation in that ocean. As permanent and sturdy platforms, ice islands serve as bases of scientific researches from which observations of oceanography, meteorology, and geophysics can be carried out over long periods.
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