The broken-off pieces of glaciers that reach open sea are called icebergs. They form through calving, the process in which a glacier or ice shelf fractures and releases a mass of freshwater ice into the water.
Unlike ordinary sea ice, which freezes directly from ocean water, an iceberg began as snow that accumulated on land, compacted into glacier ice, and eventually flowed toward the coast. Once afloat, currents and winds can carry it far from its source. Because ice is less dense than seawater, roughly nine-tenths of a typical iceberg remains below the surface—the origin of the familiar phrase “tip of the iceberg.”
Not every floating fragment is technically an iceberg. Smaller pieces are often called bergy bits or growlers, depending on their size. Icebergs may be rounded, pinnacled, or flat-topped; large tabular icebergs are especially common where Antarctic ice shelves break apart. They are also significant navigation hazards because most of their mass is hidden underwater.