Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Juan Franco
are large, natural accumulations of ice, advancing, retreating, or standing
stationary on the land. A glacier forms where the annual snowfall, on the
average, exceeds the amount of snow and ice lost by melting during the warm
season. At such a place a large snow field is created, and the region is known
as one of permanent snow. Snow fields lie at elevations varying from about sea
level in Polar Regions to more than 18,000 feet in equatorial areas. (New
Standard Encyclopedia, Volume 5)
Each year a
layer of snow is added to that already accumulated. Gradually, the fresh snow
is compacted and changed into a granular ice called a névé or firn. Later it is
compressed by the weight of overlying layers into hard, crystalline ice.
Eventually, in response to gravity and the weight of snow above it, the ice
begins to move slowly downhill, and an active glacier comes into being. (New
Standard Encyclopedia, Volume 5)
Glaciologists usually recognize two distinct types of glaciers. The two main
types are valley and continental glaciers. A valley glacier is a glacier that
forms in a mountain valley. (See figure 1) A continental glacier is a glacier
that covers a major portion of a continent or landmass, such as Antarctica. (See
glacier moves in much the same manner as a river. Although it appears to be
rigid, a glacier can follow an irregular valley, moving around bends and even
pressing through narrows. Such movements are possible because the tremendous
pressure on lower layers causes the ice to become almost plastic when necessary.
At the same time, stresses are created in the glacier’s upper layers that
result in formation of deep crevasses. (New Standard Encyclopedia, Volume 5)
The speed of a glacier is mainly governed by the steepness of its slope and the amount of ice issuing from its source. Some glaciers have little or no movement; others have advanced 30, 50, and even 100 feet per day. All parts of a glacier, however, do not move at the same speed. Largely because of friction with the earth, the bottom and sides move less rapidly than the middle and surface sections. (New Standard Encyclopedia, Volume 5)
The melting and evaporating of a glacier’s surface, called ablation, follows exposure to sun, rain, or warm dry wind. The water, called meltwater, runs over the surface streams, pools, waterfalls, and deep potholes of swirling water, or moulins. Melting also occurs within the glacier, especially at the bottom where friction with the earth is greatest. As a result, subglacial tunnels and streams develop. They collect water surface draining down through crevasses, make their way to the foot of glacier, and flow out through openings called glacier snouts. (New Standard Encyclopedia, Volume 5)
Glaciers are powerful forces and
are capable of great erosive work. On lowlands, they tend to level the surface;
in mountains, where the ice is not thick enough to override the higher peaks and
ranges, they make the terrain more rugged. Glacial ice is heavily laden with
rock fragments, both large and small. This material comes from the rock floor
upon which the ice moves or from the valley walls. Under the great weight of the
glacier, the land is slowly eroded by the grinding and scraping action of the
ice itself. Deep grooves and scratches, called striations, can still be seen in
most areas that were once covered by glaciers. (New Standard Encyclopedia,
Volume 5) Glaciers also tend to cause glacial grooves and glacial polish.
Glacial grooves are formed as a glacier moves across bedrock, the large rocks
that the glacier is dragging along gouge long grooves. (See figure 3) Glacial
polish occurs when a glacier moving across a bedrock surface polishes the
bedrock by abrasion with the fine rock material that is embedded in the ice.
(See figure 4)
While wearing down old landforms, glaciers are actively forming new ones
elsewhere, either by the direct action of the ice or by the water issuing from
it. There is a continuous process of eroding, transporting, and depositing the
glacial material, or debris. (New Standard Encyclopedia, Volume 5)
Some landforms that are created as a result of a glaciers’ great erosive work include arêtes, cirques, drumlins, hanging valleys, and horns.
An arête is a skinny mountain ridge shaped on opposite sides by a series of glacial cirques.
A cirque is a
scoop-like depression on the side of a mountain formed by a valley glacier
eroding the mountain.
A drumlin is a long,
skinny, cigar-shaped hill that has been sculpted by a glacier flowing over it.
A hanging valley is a
small U-shaped glacial valley exposed on the upper side of a larger U-shaped
(Diagram courtesy of R. Weller)
A horn is a mountain
with very steep slopes which was carved with cirques on three or more by valley
glaciers. Of the most famous horns, the Matterhorn in Switzerland is the most
Glaciers are found on all continents except Australia, and on many islands. In
Europe mountain glaciers cap the Alps, Caucasus, Pyrenees, and ranges of
Scandinavia. The Rockies and various coastal ranges in western North America
carry hundreds of glaciers. Glaciers are found on the summits of the Andes, in
South America; the Himalayas, Karakoram, and other great Asian ranges; the
mountains of New Zealand; and the volcanic peaks of equatorial Africa. Largest
and most spectacular of all are the continental glaciers on Antarctica and
Greenland. (New Standard Encyclopedia, Volume 5)
Glaciers can be destructive and erosive forces, but despite that fact without glaciers we wouldn’t have or be able to enjoy the many wonderful scenes and extraordinary range of landforms found on our planet.
New Standard Encyclopedia, Volume 5. Standard Educational Corporation, Chicago.