The Sea Ice
The ocean surrounding the continent of Antarctica is subject to an annual freeze which extends to nearly 20,000,000 square kilometers at the coldest point (September). The salt water of the ocean has a salinity of about 35 (representing the number of grams of dissolved salts per 1kg of water). This has a freezing point of -1.8 C but as the water approaches this temperature, it becomes more dense which means that ice will not form until the top 3 metres of the water has been cooled to freezing point. This is a peculiarity of salt water. Fresh water on lakes will freeze more readily.
As the sea water freezes the salt content is forced out so sea ice is much less saline than the sea water. As the ice is less dense than the water, the sea ice naturally floats. At first small ice crystals form (called frazil ice). These expand until the surface of the sea is covered with a soft thin layer of translucent ice known as grease ice. As the ocean moves due to wind and wave pressure, the ice forms itself into small plates which collide forming what are known as ice pancakes. These grow until eventually the ocean is covered with a large sheets of ice which can be up to 2 meters thick. This is called pack ice. In common with icebergs most of the ice is below the surface of the water so there is generally not more than 20-30 centimetes above the water surface.
Normal ships (even if ice strengthened) will struggle to sail in the sea ice but there is a special class of vessel known as an icebreaker. An icebreaker has a double thickness hull without stabilisers and a rounded keel. They carry helicopters on board to assist with ice navigation. In the fierce Southern Ocean they roll violently in high seas. They are capable of forcing a route through sea ice up to 1.5 metres thick (a nuclear powered icebreaker can do even better than this). Though the ice, they function by first cutting into the sea ice which sometimes creates a split in the ice. Subsequently the whole weight of the ship is used to crush the ice. They may frequently be forced to reverse and retrace their route over the thickest ice, gradually advancing and forcing a way through.
The Icebreaker in some of the pictures in the gallery below is the Kapitan Khlebnikov. This is a Finnish built, Russian ship whose original purpose was for the clearing of trade routes through the Arctic Ocean but is now often used as a tourist expedition vessel.