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Two main types of ice occur in the sea: pack ice, which is formed by the freezing of sea water, and icebergs, which break off from glaciers or other accumulations of snow on land. Pure water freezes at 0°C, but sea water does not freeze until the temperature drops to about -9°C because of the dissolved salts in it. In Polar Regions cold winds from the ice caps cool the ocean until the top layer reaches freezing point; at this stage, ice crystals start to form on the surface of the sea. Ice requires a larger volume than a similar mass of water. It is therefore less dense than water and so floats on the surface of the sea. Salts from the seawater are not included in the hexagonal ice crystals but are trapped in the liquid between them. The salt solution moves slowly downwards out of the ice, so that five or six years after its formation the ice looks like a honeycomb. Newly formed ice has a salinity of anything between five and ten parts per thousand, whereas old ice has a salinity of less than two parts per thousand. Ice packs develop from the accretion of small ice crystals. To begin with the crystals coalesce into small thin platelets, known as frazil ice. As the ice thickens, the frazil ice platelets freeze together into a continuous skin of ice which, in a rough sea, may break up into individual discs a half to a meter across. The plates collide with each other, which often causes their edges to turn up and form what is known as pancake ice. As the sea freezes still further, the ice thickens and the pancake ice joins together to form an ice floe which may be from 10m to as much as 8km across. The seawater underneath the ice is insulated by it against the cold air, so the ice floe thickens only slowly, reaching a thickness of about two meters in the Arctic and three meters in the Antarctic at the end of the winter of the first year. In the summer the Sun melts the surface of the ice floe, which forms pools of fresh water. The ice melts through completely in places, but if the floe survives the summer, an even thicker floe is formed when the water refreezes. The wind can change smooth ice floes into bumpy packs of ice by moving them together, so that they crush and deform. In addition, it can cause the ice to crack along lines of weakness. The movement of ice packs can also cause cracks in the ice which open into long, narrow channels or into patches of open water, called polynyas. The water in polynyas usually freezes quickly, forming a thin sheet of ice which may be crushed if the sides of the polynya start to close together. The Arctic Ocean has a large area of permanent pack ice which is even more extensive in winter, when it reaches Alaska, Russia, northern Canada and Greenland. Ordinary ships cannot cross the Arctic because of this ice, and even ice-breakers cannot force their way through the thicker areas. But submarines can travel below the ice pack, using sonar to detect the thickness of the ice above. The Antarctic has a smaller area of pack ice which consists of large floating ice shelves that join onto the landmass.
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