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| A slice through a valley glacier.
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Glacier growth
As the glacier flows downhill, it descends
to warmer zones where the snow melts from year to year. The
boundary where loss from melting and evaporation equals accumulation
from snowfall is called the annual snowline or firn limit-"firn"
being the term for partially compacted snow carried over from
previous seasons.
The firn limit fluctuates from year to year in response to
changes in precipitation and temperature. The firn limit can
be as much as 1,000 ft lower in elevation on the shaded north
sides of mountain peaks tan on their sunny south sides. For
this reason, many present-day glaciers are found on the north-facing
mountain slopes.
The section of the glacier through which the maximum amount
of ice flows coincides with the firn limit, because as the
glacier flows toward the firn limit, it is continually augmented
by new net snowfall; and downvalley from the firn limit, more
ice is lost by melting and evaporation-together called ablation-each
year than is added by snowfall. As the glacier flows downvalley
from the firn limit, more and more of the ice ablates, and
the glacier grows thinner or narrower, or both. Ultimately
a point is reached where the ice front can advance no farther
because the ice melts there as rapidly as it is provided by
inflow from up glacier. If the yearly rates of accumulation
and ablation were constant, this point would be fixed. However,
they vary, and for that reason alone the terminus of the glacier
is not likely to be fixed in position. As the climate turns
warmer or drier, a glacier will gradually waste away, rather
than melting catastrophically. |
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