DESCRIPTION:
Glaciers and Glaciation


Glaciers exist where, over a period of years, snow remains after summer's end. They exist in environments of high and low precipitation and in many temperature regimes; they are found on all the continents except Australia and they span the globe from high altitudes in equatorial regions to the polar ice caps. There is a delicate balance between climatic factors that allows snow to remain beyond its season. ... Snow deposits may accumulate by direct precipitation, by avalanching, or by wind. For instance, during winter it is not uncommon to find barren wind-blown ridges on the slopes above valleys with deep snow deposits. In addition, wind, relative humidity and precipitation are variable around the mountain. By these capricious agents, snow layers are accumulated.

Scientists and skiers alike can note that within a few days of falling, snowflakes have noticeably begun to change. ... The snowflakes are compressed under the weight of the overlying snowpack. Individual crystal near the melting point have slick liquid edges allowing them to glide along other crystal planes and to readjust the space between them. Where the crystals touch they bond together, squeezing the air between them to the surface or into bubbles. During summer we might see the crystal metamorphosis occur more rapidly because of water percolation between the crystals. By summer's end the result is FIRN -- a compacted snow with the appearance of wet sugar, but with a hardness that makes it resistant to all but the most dedicated snow shovelers! Several years are usually required for the snow to settle and to season into the substance we call glacier ice.

We can best determine the health of a glacier by looking at its MASS BALANCE. Each year glaciers yield either a net profit of new snow, a net loss of snow and ice, or their mass may remain in equilibrium. Scientists divide each glacier into upper and lower sections termed the ACCUMULATION AREA, where snowfall exceeds melting during a year; and the ABLATION AREA, where melting exceeds snowfall. An EQUILIBRIUM LINE, where mass accumulation equals mass loss, separates these areas. You can see it as the boundary between the winter's snow and the older snow or ice surface. Its altitude changes annually with the glacier's mass balance. To find mass balance, scientists measure the area of each region and observe amounts of accumulation and ablation relative to preset stakes. After density measurements are made they may calculate how much water has been added or lost to the glacier. ...

After a series of positive mass balance years, the glacier may respond to the increased thickness by making a GLACIAL ADVANCE downvalley. A series of negative years may cause a GLACIAL RETREAT, meaning that the TERMINUS is melting faster than the ice is moving downvalley.

Because each glacier has a unique valley geometry and setting on the mountain, each responds slightly differently to the same climatic event. Only after a lag period unique to each glacier will a higher-than-normal snow accumulation cause an advance at the glacier terminus. ...

Glaciers have been likened to mighty rivers of ice. Although they move many times more slowly, glaciers have equivalent changes in flow rate and often form falls of fast-moving ice above slow-moving ice pools. Glaciers flow faster down their centers than at ice margins, and more quickly at the surface than at the bed.

... Glaciologists have defined two distinct types of glacial movement -- deformation of the ice, and sliding of the glacier upon its rock bed. You can see where deformation has taken place by observing the wavelike flow patterns within the ice. ... Note the effects of glacial sliding where the ice has towed rocks that scratched the bedrock.

How fast a glacier moves is mostly dependent on the thickness of the ice, and on the angle of its surface slope. Glacier speeds vary when changes are made in this geometry. They respond to excessively high seasonal snow accumulations by generating bulges of thicker ice that may move downvalley many times faster than the glacier's normal velocity. We can measure those KINEMATIC WAVES using instruments to survey the glacier surface. These waves leave a legacy of severely cracked ice and often advance the glacier terminus. Kinematic waves may occur on all large glaciers. ...

... Crevasses form where the speed of the ice is variable, such as in icefalls and at valley bends. The surface may appear blistered with crevasses where the ice flows over bedrock knobs and ridges. ...

... the glaciers themselves are slowly reducing the size of the mountain they depend on. Glacier ice alone is too soft to be a powerful rock-cutting agent. Many glaciers are armed with rock fragments embedded within the ice that are effective cutting tools. The rock-choked ice grazes over the glacier bed, removing rock obstacles and leaving the bedrock rounded and smoothed. In some places find-grained debris polishes the bedrock to a lustrous surface finish called GLACIAL POLISH. Coarser rocks may gouge scratches called STRIATIONS. ...

Look for evidence of glacial erosion in the streams and rivers ... Those rivers originating beneath glaciers are choked with GLACIAL FLOUR, the silty fine-grained sediment produced by the abrasion of rocks at the glacier bed.

Not all glacier beds are alike. Some glaciers rest upon sturdy bedrock foundations, while others engulf so much rock that there is no distinct boundary between the glacier and its bed. As glaciers melt, their remaining load or rocks is distributed in several ways. Rocks may be dropped in place by the melting ice; they may be rolled to the ice margins, or they may be deposited by meltwater streams. Collectively, these deposits are called GLACIAL DRIFT. TILL refers to the debris deposited directly by the glacier. Rock debris rolls off the glacier edges and builds piles of loose unconsolidated rocks called GLACIER MORAINE. LATERAL MORAINES form along the side of a glacier and curl into a TERMINAL MORAINE.


Return to Glaciers Return to:
[Glaciers and Glaciation Menu] ...
[Glossary of Hazards, Features, and Terminology] ...



ButtonBar

URL for CVO HomePage is: <http://vulcan.wr.usgs.gov/home.html>
URL for this page is: <http://vulcan.wr.usgs.gov/Glossary/Glaciers/description_glaciers.html>
If you have questions or comments please contact: <[email protected]>
11/25/97, Lyn Topinka