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iceberg, mass of ice that has become detached, or calved, from the edge of an ice sheet or glacier and is floating on the ocean. Because ice is slightly less dense than water about one ninth of the total mass of a berg projects above the water. Greenland and other N Atlantic icebergs are usually peaked and irregular in shape; Antarctic icebergs are tabular, with flat tops and steep sides, and are sometimes extremely large when calved from an ice shelf. Icebergs differ from other ocean ices: sea ice is formed directly from the freezing of ocean water; pack ice is tightly packed fragments of sea ice; ice floes are small, floating ice fragments that separate from pack ice; and fast ice is ice attached to a shore.
Greenland is the source of most of the icebergs in the N Atlantic, where the iceberg season lasts roughly from February to October. As a consequence of the loss of the Titanic through collision with an iceberg in 1912, a patrol of N Atlantic shipping channels was initiated in 1914 by the international agreement of 16 nations. Patrols use planes and surface vessels equipped with radar, loran, and underwater sound equipment. A constant census of bergs is maintained, and the location of an iceberg is reported to any ship in its vicinity. In May 2021, a large section of the Ronne Ice Shelf in Antarctica broke free to form the largest iceberg to date; known as A76, it measures approximately 1,668 square miles (4,320 square kilometers), outclassing the previous record iceberg, A23a, which formed in 1986.