Glacier

C

Rock Glaciers

Rock glaciers are an entirely different type of glacier in which rock, not ice, is the main material. They resemble regular glaciers in shape but no ice is visible. Ice fills the space between the rocks, however, and allows the glacier to move downslope, although only very slowly.

III

Glacial Formation and Movement

Glaciers wax and wane not only with long-term climate change but also on a seasonal basis. Snow falls in winter, accumulates, and slowly turns to ice when it is compressed by additional snow loads. In summer the upper snow layers thaw and meltwater runs off; this loss is called ablation. The glacier’s mass balance or budget then turns from positive to negative as it loses mass. The balance is neutral or in equilibrium from year to year if the glacier does not experience any mass loss or gain. A complex series of processes determines the glacier’s health.

A

Transformation of Snow to Ice

Snow falls as small crystals on a glacier and accumulates if temperatures are below freezing. These snow crystals are found in an infinite variety of hexagonal shapes that are formed when the hydrogen and oxygen atoms of water arrange themselves in a six-sided symmetry upon freezing. Under the weight of successive snowfalls the snow is compressed. Snow crystals settle and change shape and size as the weight packs them closer and closer together. Air passages between crystals disappear and only small air bubbles remain. The snow has now turned into ice with a density of 820 kg per cu m (1,380 lb per cu yd). As snow piles up, the ice crystals that at the surface were smaller than a millimeter (0.04 in) merge, growing to 3 to 5 cm in size (1.2 to 2 in) or even larger.

B

The Anatomy of a Glacier

Most glaciers have two parts, an accumulation area and an ablation or wastage area. In the accumulation area snowfall exceeds melting in each year. In the ablation area melting exceeds snowfall. The boundary between the two areas is called the annual snowline or sometimes the firn limit. In winter most glaciers are entirely snow-covered. In spring the snow cover begins to melt in the lower reaches, exposing the ice surface. As temperatures increase, the melting moves up the glacier. The snowline is the highest position the melting reaches during the year. Firn is old granular snow. The firn limit may not exactly coincide with the annual snowline since in some years rapid melting leaves behind firn patches below the snowline.

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Some glaciers exhibit features called ice streams and icefalls. Ice streams are valley glaciers that form tributaries to a common compound glacier that fills a valley. The tributary glaciers do not intermix but maintain their individual streams of ice, despite compression and extension as they move along side by side. The streams can easily be recognized as individual ice streams by the deposits of boulders, gravel, sand, and mud that separate them. Icefalls occur where a glacier flows over very steep terrain that accelerates the flow. The ice is stretched and fractures into large blocks and a maze of ice pinnacles called séracs and cracks called crevasses. Icefalls are spectacular features that can extend over the entire width of the glacier and over a height of up to a kilometer (3,300 ft). Ogives, regular undulations in height on the surface of the ice, form below icefalls. Scientists believe different rates of flow in summer and winter create ogives, and that ogives therefore present some indication of annual ice movement.

C

Glacier Movement

Although ice is normally brittle, it can flow under pressure. The speed at which glaciers move depends on a number of factors, including their temperature, the amount of meltwater at the bottom of the ice, the steepness of the slope, and the nature of the rock surface over which they move. The big ice sheets move by internal deformation as ice building up in the middle forces the edges to expand. Valley glaciers move by sliding over their rock beds. Friction with the ground produces heat that in turn melts ice and helps to lubricate the sliding. Measurements of glacier movement indicate that they move fastest in the middle of the glacier where the ice is thickest. Valley glaciers typically sustain velocities of 30 to 60 cm (1 to 2 ft) per day or 100 to 200 m (300 to 700 ft) per year, but some can reach speeds of 3 to 6 m (10 to 20 ft) per day. On the large outlet glaciers in Greenland velocities of over 30 m (100 ft) per day have been measured. Short-term advances in so-called galloping or surging glaciers can reach 80 m (250 ft) per day.

As glaciers move downslope they twist and stretch. This may cause the ice to crack, forming crevasses that can be more than 30 to 40 m (100 to 130 ft) deep and up to tens of meters wide. Freshly formed crevasses have clean, straight sides. Over time, crevasses deform and may be covered over by snow bridges when drifting and blowing snow accumulates on their lips. Vehicles or people traveling on the snow’s surface can fall through the snow into a crevasse.

Scientists measure the thickness and movement of glaciers using a variety of methods, including conventional surveying techniques to record the movement of marker stakes drilled into the ice. The latest techniques use lasers on aircraft to determine height changes, and satellite interferometry for movement. Ice thickness was previously measured through seismic methods in which the time it takes for a sound wave from an explosion at the surface to travel to bedrock and back was recorded. More recently radio echo sounding—bouncing radio waves from aircraft from both the surface and bedrock of the glaciers—has replaced seismic methods. Once they gather information on how the thickness and rate of movement of a glacier vary over time, scientists can calculate the glacier’s mass balance. The enormity of the ice sheets of Antarctica and Greenland, however, make it difficult to accurately determine mass balances in those locations.

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