Glacier

I

Introduction

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.

II

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.

III

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.

IV

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.