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.
Icebergs
Icebergs are chunks of ice that have broken from an ice cap on land and they therefore contain no salt. In the north Polar Regions, glaciers flow down to the sea from the ice caps.
When a glacier reaches the water, the front slides in and breaks into large pieces of ice - a process which is called calving. The icebergs then float in the sea and are carried away from the land by currents, or they remain frozen in the pack ice for a year or so.
Arctic icebergs drift southwards along the western side of the North Atlantic Ocean and travel as far south as Newfoundland before melting completely. Once in the open ocean they last for less than three months, but they can become a hazard to shipping because of their size.
Antarctic bergs differ from those of the Arctic in that they are formed from the ice shelf that surrounds Antarctica. Every year the shelf moves northwards about 100m and during this movement ice breaks off. The bergs may drift as far north as latitude 40°S before melting. Arctic bergs may reach up to 100m out of the water and extend to depths of 400m below the surface, whereas the longer, tabular Antarctic bergs rise to only about 50m above the water and reach only 150m below it, but they can be as large as 330km long and 100km wide.
Several thousand icebergs break off from Greenland and Antarctica each year - Greenland alone is the source of about 12,000 a year.
Icebergs have caused ships to sink, the best known example being the Titanic which sank in 1912 after hitting a relatively small iceberg.
In North Atlantic shipping lanes icebergs are now tracked by the International Ice Patrol, which has been in operation since 1914 in response to the Titanic disaster and now used airborne patrols to spot icebergs. Icebergs do not show up well on radar, but they drift quite slowly and if the location of one is known, ships can alter course around it.
Icebergs are an even greater danger to oil rigs at sea. Oil rigs cannot be moved, but could be severely damaged or destroyed if an iceberg drifted into one. Icebergs have been towed away from rigs to prevent collisions, but this is often a difficult operation because of the size of the icebergs.
Icebergs could, however, be put to use. They are composed of fresh water, and so they would be a useful source of water if it were possible to tow them to arid areas of the world (such as the Middle East) without melting. Unfortunately they can be towed only very slowly and it takes several months to reach a dry area - during which time much of the ice would melt.