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Glaciers
In
regions where average temperatures hover below zero degrees Celsius, glaciers
grow with each snowstorm. Compressed by overlying snow, buried layers
slowly grow together to form a thickened mass of ice.
The pressure created from overlying snow squashes snow grains together.
Individual grains eventually metamorphose, growing to the size of rock
salt. If these enlarged crystals survive one melt season, they are considered
firn.
Firn grains are generally four to 16 times the size of the original snow
crystal and increase in size as the weight of the overlying snow increases.
As the grains grow, they slowly snuff out pockets of existing air between
the grains.
 Over
time, individual firn grains are pressed together to form larger crystals,
ultimately forming slabs of glacier ice.
When the mass of compressed ice reaches a critical thickness of about
18 meters, it begins to deform and move. Its sheer girth, in combination
with the forces of gravity, causes a glacier to slowly move, or flow.
Glacier ice flows down mountain valleys, fans across plains, and spreads
into the sea. As a glacier moves over the ground surface, friction causes
the underside of the glacier to move more slowly while overlying glacier
ice moves unimpeded
Most of the world's glaciers are found at the poles, but glaciers exist
on all of the world's continents, even Africa.
Glaciers require very specific geographical and climatic conditions. Most
are found in regions of high snowfall in winter and cool temperatures
in summer. These conditions assure that the snow accumulating in the winter
remains throughout the summer. Such conditions typically prevail in the
polar and high alpine regions.
The amount of precipitation (whether in the form of snowfall, freezing
rain, avalanches, or wind-drifted snow) is important to glacier survival.
In areas such as Siberia and parts of Antarctica, where low temperatures
meet glacier growth requirements, the lack of adequate precipitation prevents
glacier development.
In areas of glacier growth, upon reaching a critical mass, the slabs of
ice begin to flow and dramatically impact the surrounding environment.
The great weight and slow movement causes glaciers to reshape the underlying
and surrounding landscape.
Acting as an enormous push broom, the ice erodes the land surface, carrying
broken rocks and soil debris far from their place of origin. Glaciers
slowly push earth and rock forward as they advance and leave these same
materials behind in the form of moraines and other glacial deposition
features as they retreat.
In the northern half of North America, glacial remnants from the last
ice age may be reincarnated as vegetated hillsides. Views from an airplane
window over the midwestern states and provinces reveal lines of eskers
and herds of drumlins dotting the landscape.
Throughout advance and retreat, glacial debris (till) is jostled in all
directions. Till is thrust forward with the glacier, brushed aside as
the glacier pushes past less mobile objects, such as a mountainside, or
drawn along on the glacier's journey
As large glaciers retreat, the underlying ground surface is typically
scoured of most materials, leaving only scars on the underlying surface.
Glacier retreat, melt, and ablation, result from increasing temperature,
evaporation, and wind scouring. Ablation is a natural and seasonal part
of glacier life. As long as snow accumulation equals or is greater than
melt and ablation, glacier health is maintained.
Over the past 60 to 100 years, glaciers worldwide have tended toward retreat.
Alpine glaciers, which are typically smaller and less stable to begin
with, seem particularly susceptible to glacial retreat. Whether this is
due to a predictable climate trend or because of increased human impacts
on global climate remains to be determined.
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