Avalanche, sudden flow of a large mass of snow or ice down a slope or cliff, sometimes at speeds exceeding 160 km/hr (100 mph). Such flows can be destructive of life and property. Avalanches are most common on slopes exceeding 30°, frequently when a deep snow falls suddenly and does not have a chance to cohere, or when a thaw undercuts a blanket of older snow. Pellet-like snow (graupnel) is also more prone to avalanche than a fall of ordinary snowflakes. Flows of wind-packed slabs of snow can be especially hazardous.Avalanches are set off by a combination of factors, including temperature, shearing of creeping snow masses, and sudden vibrations, including loud noises. Snow patrols in mountain areas reduce the hazard by detonating strategically placed explosives that cause smaller, less destructive flows. A landslide is a similar massive movement of rock and soil. Snow CrystalSnow is actually transparent, although reflection from the many sides of its crystals makes it appear to be white. A closeup of a snow crystal reveals its beautiful symmetry and hexagonal design. The crystals form when supercooled water vapour condenses or ice particles collect around a piece of dust. Partially melted crystals stick to each other to form snowflakes. All snow crystals have six sides, but each individual snowflake has a unique pattern.

Snow, transparent ice crystals formed around dust or other small particulates in the atmosphere when water vapour condenses at temperatures below freezing point. Partly melted crystals usually cling together to form snowflakes, which may in rare cases grow in size up to 7 to 10 cm (3 to 4 in) in diameter.Structurally, elemental crystals of snow occur in any of various hexagonal forms, depending upon exact atmospheric temperatures during formation. Among these six-sided, basically symmetrical shapes are needle, columnar or stud, platelike, and star-shaped crystalline types. Because of the infinite variability of weather conditions, every snow crystal is unique in its precise configuration, and it is the large number of reflecting surfaces of the crystal that make snow appear white. The longer rays that constitute the arms of the six-rayed stars are generally hollow tubes; they are evidently built up by additions to the edge of an original crystal.

Snowfall measurement is usually stated as depth in centimetres, or other unit, of newly fallen snow; it is also measured in terms of the depth of the layer of water that would result if the snow were melted in place; 25-30 cm (10-12 in) of snow melts to 2.5 cm (1 in) of water. Antarctic IcebergIcebergs are free-floating chunks of glaciers, particularly common in the polar regions. These often spectacular ice formations pose a twofold problem for navigators: 90 per cent of their bulk is hidden below the surface, and it is impossible to map their position because they are constantly moving.

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.Properties 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. Hubbard GlacierIn Alaska many alpine, or valley, glaciers flow down mountainous valleys to the sea. Here, for example, the Hubbard Glacier enters Glacier Bay near Yakutat, Alaska. Icebergs form when pieces break off the snout of the glacier and float around in the sea. This is called calving and is the primary process by which glacial ice is recycled into liquid form.

Glacier, large, usually moving mass of ice formed in high mountains or in high latitudes where the rate of snowfall is greater than the melting rate of snow. Glaciers can be divided into four well-defined types—alpine, piedmont, ice cap, and continental—according to the topography and climate of the region in which the glacier was formed.Alpine Glaciers

The snow that falls on the walls and floors of valleys in high mountain regions tends to accumulate to a great depth, because the rate of melting, particularly in wintertime, is far lower than the rate at which the snow falls. As a result, the earlier snows, compressed by later falls, are changed into a compact body of ice having a granular structure. In some areas, however, where the temperature rarely rises as high as the melting point, this accumulation of ice can be formed by the recurrent process of sublimation and recrystallization. (Sublimation is a change from the solid state into vapour without an intermediate liquid stage.)

When the depth of the glacier reaches approximately 30 m (100 ft) the whole mass begins to creep slowly down the valley. This flow continues as long as a superabundance of snow falls at the top of the glacier. As the glacier flows down the valley to a lower altitude where it is not replenished by snowfall, it melts or wastes away, the meltwater forming the source of streams and rivers.In cross section the structure of all glaciers is similar. At the top is a mantle of freshly fallen snow with a very low density of not more than 0.1. Below this is a layer in which the snowflakes have diminished in size to become granular snow, of which the density may be 0.3 or greater. This is caused either by the influence of moisture and the pressure exerted by accumulated snow, or by sublimation and recrystallization. Further recurrent action results in névé, or firn, which approaches a density of 0.5. At the base of the glacier is a layer of clear ice that may approach a density of 0.7 to 0.8 and flows like a viscous fluid.The lower glacial ice is under such great pressure that any cracks or separations occurring in this layer are quickly healed. The upper layers, however, may suffer tensions and strains from moving over underlying obstructions or from differential movement, in which the centre of the glacier moves more rapidly than its edges. These strains produce crevasses that may be many metres deep and are frequently covered by newly fallen snow. A large crevasse, known as the bergschrund, is usually formed in the shape of a semicircle at the head of the glacier—between the glacier itself and the headwall of the valley in which it lies.Glaciers are usually bordered at their sides by zones of rock debris that have fallen from the sidewalls of the valley as a result of frost-wedging action. These zones of rock fragments are called lateral moraines. At the lower end of the glacier the moraines increase in size. When two glaciers from neighbouring valleys meet, the moraines at their adjoining sides coalesce to form a medial moraine in the middle of the resulting glacier. As the ice melts at the lower end of a glacier, rock and debris that have been ploughed up by its progress over the valley floor, in addition to rock material that may have fallen into crevasses, are deposited in a series of semicircular hillocks called the terminal moraine.
As a glacier moves down its valley, it eventually reaches a point at which the ablation, or melting and evaporation, from the surface exceeds the amount of snow falling on it. At this point, often called the névé, or firn, line, the surface of the glacier is névé rather than snow.The speed at which glaciers flow varies within wide limits. Most glaciers move downwards at the rate of less than 1 m (3 ft) per day, but observation of the Black Rapids Glacier in Alaska, during 1936-1937, showed that it was moving more than 30 m (100 ft) per day. This is the swiftest advance ever recorded for any glacier in the world and was probably due to the extremely heavy snowfalls that had occurred in the area some years earlier.With variations in climate, glaciers shrink and expand to a marked extent. An excess of precipitation creates a situation analogous to a river flood and causes the glacier to increase in size. Similarly, when precipitation decreases, the glacier shrinks.Glaciers of the alpine type are found in high mountain ranges throughout the world, even in the tropics. In the United States, for example, alpine, or valley, glaciers exist on the slopes of Mount Rainier, Mount Baker, and Mount Adams, in Washington, Mount Hood in Oregon, and Mount Shasta in California. The Hubbard Glacier in Alaska is one of the longest alpine glaciers in the world.Piedmont Glaciers When a number of alpine glaciers flow together in the valley at the foot of a range of mountains, they frequently form extensive glacier sheets known as piedmont glaciers. Glaciers of this type are especially common in Alaska, the largest of which is the Malaspina Glacier, which has an area of approximately 3,900 sq km (1,500 sq mi). The lower portion of this glacier is almost flat and is covered with so much soil and rock debris that it supports a thick forest.
Icecap Glaciers The glacier system that covers a large portion of the Norwegian island group of Svalbard, in the Arctic Ocean, is unusual in form, being a type intermediate between the alpine glacier and the Greenland glacier described below. The entire centre of each island is covered with an ice sheet that overlies a high plateau. At the edges of the plateau the sheet breaks up into a series of alpine glaciers that move down steep valleys, sometimes reaching the sea.Continental Glaciers Covering almost the entire extent of Greenland is a huge glacial blanket over 1.8 million sq km (700,000 sq mi) in area and more than 2,700 m (9,000 ft) in maximum thickness. This gigantic glacier flows slowly outward from two centres, one on the southern part of the island and one in the north. Because of its thickness the Greenland ice sheet rises far above both the valleys and hills of the land beneath it, and the underlying rock is exposed only near the seacoast, where the glacier breaks up into tongues of ice somewhat resembling valley glaciers. From the ends of these tongues, where they reach the sea, large and small fragments of ice break off during the summer, forming icebergs. A glacier of a similar type covers the whole of the Antarctic continent and has an area of about 13 million sq km (5 million sq mi). Continental glaciers covered all of Britain, part of Europe, and much of North America during the Pleistocene Ice Age in the Quaternary period, which ended about 10,000 years ago.Glacial Erosion As a glacier moves down a valley, or cross-country in the case of a large ice sheet, it sculpts the land in a characteristic manner. Rocks in its path are ploughed out of the way, and rocks beneath it are broken up by frost action and then carried away. The rocks embedded in the bottom of the glacier act as abrasive particles, scratching and scouring the rocks beneath.
At the head of a valley in which a glacier is formed, the headwalls are eroded into a characteristic semicircular form called a cirque. Progressive erosion of headwalls occurring simultaneously on several sides of a mountain produces what is called a horn; the most famous example being the Matterhorn in the Swiss Alps. Valleys down which glaciers have travelled are eroded to a U shape rather than the V shape caused by stream erosion.Frequently the valley is excavated so deeply that the mouths of tributary valleys are left high above the new valley floor ashanging valleys. Fiords are glaciated valleys that have been partly flooded by the sea.