An iceberg is a large piece of ice from freshwater that has broken off from a snow-formed glacier or ice shelf and is floating in open water. It may subsequently become frozen into pack ice. Alternatively, it may come to rest on the seabed in shallower water, causing ice scour (also known as ice gouging) or becoming an ice island.
Because the density of pure ice is about 920 kg/m?, and that of sea water about 1025 kg/m?, typically only one-tenth of the volume of an iceberg is above water. The shape of the underwater portion can be difficult to judge by looking at the portion above the surface. This has led to the expression "tip of the iceberg", for a problem or difficulty that is only a small manifestation of a larger problem.
Icebergs generally range from 1 to 75 metres (3?250 ft) above sea level and weigh 100,000 to 200 000 tons. The tallest known iceberg in the North Atlantic was 168 metres (550 ft) above sea level, making it the height of a 55-storey building. Despite their size, the icebergs of Newfoundland move an average of 17 kilometres a day (10 mi). These icebergs originate from the glaciers of western Greenland, and may have an interior temperature of -15 to -20?C (5 to -4 ?F).
Though usually confined by winds and currents to move close to the coast, the largest icebergs recorded have been calved, or broken off, from the Ross Ice Shelf of Antarctica. Iceberg B-15, photographed by satellite in 2000, measured 295 km long and 37 km wide (183-23 mi), with a surface area of 11,000 km? (4,250 mi?). The mass was estimated around three billion tonnes.
