Ice

Ice, water in the solid state. (Frozen forms of other substances, such as carbon dioxide, are also known as ices.) Ice is colourless and transparent; it crystallizes in the hexagonal crystal system. Its melting point is 0° C (32° F); pure water also freezes at 0° C, but ice will only form at 0° C if the water is disturbed or contaminated with dust or other objects.

One important property of ice is that it expands upon freezing. At 0° C it has relative density 0.9168 as compared to relative density 0.9998 of water at the same temperature. As a result, ice floats in water. Because water expands when it freezes, an increase of pressure tends to change ice into water and therefore lowers the melting point of ice. This effect is not very marked for ordinary increases of pressure. For instance, at a pressure 100 times the normal atmospheric pressure, the melting point of ice is only about 1° C (1.8° F) less than at normal pressure. At higher pressures, however, several allotropic modifications, or allotropes (different forms of the same element that exist in the same physical state) of ice are formed. These are designated Ice II, Ice III, Ice V, Ice VI, and Ice VII. Ordinary ice is Ice I. These allotropes are denser than water and their melting points rise with increased pressure. At about 6,000 atmospheres the melting point is again 0° C and at a pressure of 20,000 atmospheres the melting point rises above 80° C (176° F).

The expansion of water when it freezes has important geological effects. Water that enters minute cracks in rocks on the surface of the Earth creates an enormous amount of pressure when it freezes, and splits or breaks the rocks. This action of ice plays a great part in erosion.

These properties of freezing water explain the way in which open bodies of water freeze. When the temperature of the surface of an open body of water is reduced towards the freezing point, the surface water becomes denser as it cools, and therefore sinks. It is replaced at the surface by warmer water from beneath. Eventually the entire body of water reaches a uniform temperature of 4.0° C (39.2° F), the point at which water has its maximum density. If the water is cooled further, its density decreases and finally ice is formed on the surface. Bodies of water freeze from the top down rather than from the bottom up because of these density differences.

In rivers, however, ice is sometimes formed beneath the surface. On cold winter nights the surface of a swiftly flowing stream may become cooled well below 0° C because of its contact with the air. Such “undercooled” water, mixing with the warmer layers beneath, produces a spongy mass of ice crystals known as frazil, which floats downstream. Sometimes masses of frazil lodging under surface ice in quieter water may dam a stream and cause floods. Another form of below-surface ice is anchor ice, which is formed around rocks on streambeds. During cold nights enough heat may be radiated from the rocks so that they become cool enough to freeze the water flowing around them. When the rocks are warmed by the sun in the daytime, masses of the anchor ice may detach and rise to the surface of the stream. See Snow.

Whenever glaciers or ice sheets reach the sea, the movement of the ice eventually pushes the end of the sheet into water which is deeper than the thickness of the glacier ice. Portions of the end of the glacier break off and form floating masses known as icebergs or bergs. Icebergs are often of enormous size and may reach a height of 90 to 150 m (about 300 to 500 ft) above the surface of the sea. Yet about 90 per cent of the mass of an iceberg is beneath the surface. Icebergs are common in both the Arctic and Antarctic regions and are often carried into lower latitudes by sea currents, particularly in the North Atlantic Ocean. North Atlantic icebergs all come from the Great Greenland ice sheet and have been observed as far as 3,200 km (about 2,000 mi) from their origin. After the Titanic disaster, 16 nations instituted an iceberg patrol of the North Atlantic. Now known as the International Ice Patrol, it tracks icebergs and reports their location to ships.