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Glacier climbing
Anatomy of
a glacier
The upper part of a glacier that receives most of the snowfall
is called the accumulation zone. In general, the accumulation
zone accounts for 60-70% of the glacier's surface area. The
depth of ice in the accumulation zone exerts a downward force
sufficient to cause deep erosion of the rock in this area. After
the glacier is gone, this often leaves a bowl or
amphitheatre-shaped depression called a cirque.
On the opposite end of the glacier, at its foot or terminal, is
the deposition or ablation zone, where more ice is lost through
melting than gained from snowfall and sediment is deposited. The
place where the glacier thins to nothing is called the ice
front.
The altitude where the two zones meet is called the equilibrium
line. At this altitude, the amount of new snow gained by
accumulation is equal to the amount of ice lost through
ablation. The downward erosive forces of the accumulation zone
and the tendency of the ablation zone to deposit sediment also
cancel each other out. Erosive lateral forces are not canceled;
therefore, glaciers turn v-shaped river-carved valleys into
u-shaped glacial valleys.
The "health" of a glacier is defined by the area of the
accumulation zone compared to the ablation zone. When directly
measured this is glacier mass balance. Healthy glaciers have
large accumulation zones. Several non-linear relationships
define the relation between accumulation and ablation.
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