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Sea Ice - also called pack ice, is a whole other thing than iceberg. It forms from frozen seawater, as its name suggests, not compacted snow. Sea ice is dynamic: Like a plant or animal, it grows in stages, with different names for different stages. It changes with the seasons. And it is almost always on the move, impelled by currents and winds.
Sea ice forms when the temperature of the ocean surface falls below -1,89 Celsius. Unlike the calm surface of a lake, where ice forms in a gradually thickening sheet, the ocean surface is frequently stirred by waves. This leads to some interesting forms of ice. In turbulent water, tiny disc-shaped crystals of ice form into a substance called frazil. As they are stirred through the sea surface, the crystals give the water a greasy appearance, hence the name "grease ice." Another form that sea ice takes as it grows is shuga, composed of small chunks of ice that undulate on the surface of the water in a sheet. When the ice clumps together, it forms rounded sheets with upturned edges, called pancake ice. The pancakes damp down the waves somewhat, allowing pancakes to consolidate into larger pancakes. Eventually, the pancake ice freezes into floes, larger sheets of sea ice floating on the water's surface.
There are two types of pack ice: first-year ice and multi-year ice. First-year ice freezes in fall or early winter and melts in the summer. Typically, first-year ice is between a foot to six feet thick. Multi-year ice is simply pack ice that has survived at least one winter. A smaller fraction of the polar sea ice grows along shorelines and in enclosed bodies of water such as bays. This is called lastfast ice.
Sea ice is dynamic in another sense: It is alive. Far from being a frozen wasteland, floating pack ice is laced with creatures living in a complex food web.
(from:www.eastgreenland.com)