Sea ice: cold polar seas

Sea ice has a completely different origin from glacier ice. Sea ice in fact forms due to direct freezing of sea water, when the temperature of the air remains below -1.8°C for a few days.
Its formation, which is seasonal, is spectacular: at first ice needles and thin plates form. These, floating, give the sea surface a particular “oily”  appearance, the so-called greasy ice. The crystals aggregate, originating slabs  that get increasingly thick, and due to the continuous collisions provoked by wave motion,  they take on a circular form with raised borders, which look like large white pancakes, from which the name pancake ice. As the low temperatures persist, the slabs join together thus forming a continuous sheet, the pack. The thickness varies between 1 to 5-7 metres, and increases continuously due to sea water that freezes at the base and supply of snow on the surface. Currents, winds, tempests keep the pack in constant movement, causing breakage of the big slabs, overlapping or piling up and collisions, creating a tormented landscape, made up of ridges jutting from the ice and large fractures, making exploration very difficult. It is a very dangerous area for navigation: even  very large tonnage ships have been trapped between drifting slabs and have been literally crushed by ice. The history of polar exploration is studded with adventures and tragedies connected with the dangers of the pack.
Unlike in the Antarctic, that is a continent, in the Arctic, the few lands above sea level are made of archipelagos of islands. Therefore there are no large glaciers or big ice-sheets, but only an enormous extension of sea ice floating on the Arctic  sea. However a vast area of sea ice also surrounds the Antarctic, and reaches its maximum extension in September, reaching a width of 2,000 km. Unlike glaciers and the ice-sheets, whose size remains practically unaltered during the course of the year, sea ice undergoes spectacular variations in its expanse, which can be appreciated particularly when observed from a satellite on areas that cover 15-20 million km2 of the Polar seas. The variations are particularly evident around the Antarctic , where sea currents tend to push away the fragments of the pack, dispersing them, when these are not so big, in the Arctic sea, where currents tend, on the contrary, to concentrate the large drifting ice floes around the North Pole. Their moving away is also hindered by the presence of the surrounding land above sea level. Unlike glacier ice, which may date back various thousands of years, the age of sea ice is rarely more than a year. Large ice floes that are many years old can be found only in the Arctic Sea.

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