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 ICEBERGS 



Icebergs are large masses of floating (or stranded) ice derived from 

 the fronts of glaciers, from glacier ice tongues, or from the shelf ice 

 of the Antarctic. They are products of the land, and not of the sea. 

 Their structure, and to some extent their appearance, depend upon 

 the source from which they are derived. 



Arctic bergs originate mainly in the glaciers of Greenland, which 

 has 90 percent of the land ice of the north polar region. Svalbard, 

 Novaya Zemlya, and Ellesmere Land also produce a few bergs. Arctic 

 bergs are irregular in form and take many varied shapes. Most com- 

 mon are the irregular dome-shaped bergs, produced by glaciers that 

 have plowed across the uneven foreland on their way to tidewater 

 which differ entirely from the flat-topped, straight-sided bergs orig- 

 inating where the ice sheet itself is thrust directly out into the sea. 



In color, bergs are an opaque flat white, with soft iridescent hues of 

 blue or green. Many show veins of soil or rock debris; others may 

 have yellowish or brownish stains, probably due to diatom films. Un- 

 der certain conditions of illumination, an iceberg will appear dark in 

 contrast with the sky or with other bergs in the direct sunlight, and 

 this phenomenon has often led mariners to report islands where none 

 exist. 



The higest berg yet measured in the Arctic stood 447 feet out of 

 water; 230 feet is a common height for a large berg. These figures 

 refer to bergs soon after calving ; the highest so far observed to the 

 southward of Newfoundland was 262 feet. The longest iceberg meas- 

 ured in those waters was 1,696 feet long, although one several miles 

 long was reported in 1928. 



The ratio of the mass of the submerged portion of a berg to its total 

 mass is equal to the ratio of the specific gravity of the berg to that of 

 the water in which it is floating. On account of the origin of glacial 

 ice in compacted snow, berg ice contains up to perhaps 10 percent of 

 trapped air and is therefore somewhat less dense than ordinary ice. 

 Measurements of the specific gravity of ice in Greenland bergs have 

 given values close to 0.90, while the cold sea water in which they float 

 has a specific gravity of about 1.027, so that about seven-eighths of the 

 mass is submerged. It is often erroneously assumed that a berg with 

 one-eighth above water and seven-eighths submerged should be float- 

 ing with a draft seven times its height above water ; but these ratios 

 hold good only for mass, and not for linear dimensions. Actual 

 measurements on Arctic bergs show that the draft is seldom more than 



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