The relentless movement of the Antarctic ice towards the shores of the continent feeds large ice shelves which occupy the embayments on the coast of Antarctica. The largest of these is the Ross Ice Shelf (known by Scott and Shackleton as 'The Great Ice Barrier') which is about the size of France and consists of massive sheets of ice which can be as much as 700 metres thick. They tend to have flat surfaces and are, for the most part, floating on the ocean. At the seaward side, large chunks of this ice calve (break off) and float slowly away as icebergs. Icebergs formed in this way are known as 'tabular icebergs'. They have regular, even rectangular, dimensions with vertical sides and a flat, table like, top. They can be truly gigantic in size. The largest on record was about 12,000 square kilometres in area and about 500 metres thick.
Ice only looks white when it has a considerable amount of trapped air. As icebergs get older, their weight forces out the trapped air and they reveal their normal colour which is blue. As they float slowly North, they encounter warmer waters and start to melt, generally from the bottom. This sometimes makes them shift in shape and as they get older their shape becomes more irregular. Weak points are exploited by the strong tides and sometimes cause the formation of caves or tunnels. Sometimes it is possible still to detect the evidence of crevassing on the top of the icebergs created when they were in a glacial state on the icecap.
In some parts of Antarctica the ice streams or glaciers feed directly into the sea (without the intermediate stage of an ice shelf). This tends to create small icebergs with a more irregular shape. All icebergs are made of fresh water.
What would happen to the level of the sea worldwide if all the icebergs in Antarctica were suddenly to melt? The answer, perhaps surprisingly, is nothing. Because icebergs are floating, they cannot affect the level of the sea. The same applies to the ice shelves which are mostly afloat. Very different with the continental ice cap though. It does appear, though, as if the Antarctic icecap is not diminishing in size due to global warming although it its geography is shifting (reducing in the West and expanding in the East).