Glaciers

glacier

glacier, moving mass of ice that survives year to year, formed by the compacting of snow into névé and then into granular ice and set in motion outward and downward by the force of gravity and the stress of its accumulated mass. Glaciers are usually found in high altitudes and latitudes.

Classification

Glaciers are of four chief types. Valley, or mountain, glaciers are tongues of moving ice sent out by mountain snowfields following valleys originally formed by streams. In the Alps there are more than 1,200 valley glaciers. Piedmont glaciers, which occur only in high latitudes, are formed by the spreading of valley glaciers where they emerge from their valleys or by the confluence of several valley glaciers. Small ice sheets known as ice caps are flattened, somewhat dome-shaped glaciers spreading out horizontally in all directions and cover mountains and valleys. Continental glaciers are huge ice sheets whose margins may break off to form icebergs (see iceberg). The only existing continental glaciers are the ice sheets of Greenland and Antarctica, but during glacial periods they were far more widespread. Glaciers may be classified as warm or cold depending on whether their temperatures are above or below -10°C (14°F).

Geological Impact

Glaciers alter topography, and their work includes erosion, transportation, and deposition. Mountain glaciers carve out amphitheaterlike vertical-walled valley heads, or cirques, at their sources. They transform V-shaped valleys into U-shaped valleys by grinding away the projecting bases of slopes and cliffs and leveling the floors of the valleys; in this process tributary valleys are frequently left "hanging," with their outlets high above the new valley floor. When the tributary valleys contain streams, waterfalls and cascades are formed, such as Bridal Veil Falls of Yosemite National Park. Elevations over which glaciers pass usually are left with gently sloping sides in the direction from which the glacier approached (stoss sides) and rougher lee sides. Humps and bosses of rock so shaped are known as roches moutonnées.

The debris from glacial erosion is carried upon, within, and underneath the ice. The debris frozen into the underside of the glacier acts as a further erosive agent, polishing the underlying rock and leaving scratches, or striae, running in the direction of the movement of the glacier. Glacial deposits are often known as till or drift. The melting of the ice in summer forms glacial streams flowing under the ice, while the retreat of a large glacier sometimes leaves a temporary glacial lake, such as the ice age Lake Agassiz. Fjords generally owe their origin to glaciers.

Glacial Movement

A glacier moves as a solid rather than as a liquid, as is indicated by the formation of crevasses (see crevasse). The center of a glacier moves more rapidly than the sides and the surface more rapidly than the bottom, because the sides and bottom are held back by friction. The rate of flow depends largely on the volume of ice in movement, the slope of the ground over which it is moving, the slope of the upper surface of the ice, the amount of water the ice contains, the amount of debris it carries, the temperature, and the friction it encounters. Glaciers are always in movement, but the extent of the apparent movement depends on the rate of advance and the rate of melting. If the ice melts at its edge faster than it moves forward, the edge of the glacier retreats; if it moves more rapidly than it melts, the edge advances; it is stationary only if the rate of movement and the rate of melting are the same.

The causes of glacial movement are exceedingly complex and doubtless are not all operative on the same glacier at the same time. Important elements in glacial movement are melting under pressure followed by refreezing, which may push the mass in the direction of least resistance; sliding or shearing of layers of ice one on top of the other; and rearrangement of the granules when pressure causes melting. Sudden, rapid movements of glaciers, called glacier surges, have been observed in Alaskan and other glaciers, with evidence for such abnormal movements as the crumpled lines of surface debris found on them. It is thought that the relatively sudden movement and melting of glaciers may be indicative of climate warming.

The Columbia Encyclopedia, 6th ed. Copyright© 2014, The Columbia University Press.

Selected full-text books and articles on this topic

The World of Ice
James L. Dyson.
Alfred A. Knopf, 1962
Ice and Snow: Properties, Processes, and Applications
W. D. Kingery.
M. I. T. Press, 1963
Glacier Variations and Climatic Fluctuations
H. Wilhelmsson Ahlmann.
American Geographical Society, 1953
Earth Features and Their Meaning
William Herbert Hobbs.
Macmillan Company, 1931
Librarian’s tip: Chap. XX "The Glaciers of Mountain and Continent"
Compendium of Meteorology
Thomas F. Malone.
American Meteorological Society, 1951
Librarian’s tip: "Climatic Implications of Glacier Research" begins on p. 1019
FREE! The Land of the Midnight Sun: Summer and Winter Journeys through Sweden, Norway, Lapland and Northern Finland
Paul B. Du Chaillu.
Harper & Brothers, vol.1, 1882
Librarian’s tip: Chap. XIX "The Glaciers of Scandinavia"
Icy Indicators of Global Warming
Denniston, Derek.
World Watch, Vol. 6, No. 1, January-February 1993
The Little Ice Age
Jean M. Grove.
Routledge, 1990
Himalaya to the Sea: Geology, Geomorphology, and the Quaternary
John F. Jr. Shroder.
Routledge, 1993
Physical Geography of Asiatic Russia
S. P. Suslov; Noah D. Gershevsky; Joseph E. Williams.
W. H. Freeman, 1961
Librarian’s tip: "Glaciation" begins on p. 526
Applied Climatology: Principles and Practice
Russell D. Thompson; Allen Perry.
Routledge, 1997
Librarian’s tip: Chap. 7 "Glaciers"
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