Geology Reference
In-Depth Information
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for glaciers. Of course, temperature varies with elevation and
latitude, so we would expect to fi nd glaciers in high moun-
tains and at high latitudes, if these areas receive enough snow.
Many small glaciers are present in the Sierra Nevada of
California, but only at elevations exceeding 3900 m. In fact,
the high mountains in California, Oregon, and Washington
all have glaciers because they receive so much snow. Mount
Baker in Washington had almost 29 m of snow during the
winter of 1998-1999, and average accumulations of 10 m or
more are common in many parts of these mountains.
Glaciers are also found in the mountains along the Pacifi c
Coast of Canada, which also receive considerable snowfall,
and of course, they are farther north. Some of the higher
peaks in the Rocky Mountains in both the United States and
Canada also support glaciers. At even higher latitudes, as in
Alaska, northern Canada, and Scandinavia, glaciers exist at
sea level.
What Would You Do
Suppose you are a high school earth science teacher trying
to explain to students that ice is a mineral and rock, and how
a solid like ice can fl ow like a fl uid. Furthermore, you explain
that the upper 40 m or so of a glacier is brittle and fractures,
whereas ice below that depth simply fl ows when subjected to
stress. Now that your students are thoroughly confused, how
will you explain and demonstrate that ice can behave like a
solid yet show properties of fl uid fl ow?
basal slip is far more important in valley glaciers as they fl ow
from higher to lower elevations, whereas continental glaciers
need no slope for flow. Although glaciers move by plastic
fl ow, the upper 40 m or so of ice behaves like a brittle solid
and fractures if subjected to stress. Large crevasses commonly
develop in glaciers where they fl ow over an increase in slope
of the underlying surface or where they fl ow around a corner
(
THE GLACIAL BUDGET—
Just as a savings account grows and shrinks as funds are
deposited and withdrawn, a glacier expands and contracts
in response to accumulation and wastage. We describe a
glacier's behavior in terms of a
glacial budget
, which is
essentially a balance sheet of accumulation and wastage.
For instance, the upper part of a valley glacier is a
zone of
accumulation,
where additions exceed losses and the sur-
face is perennially snow covered. In contrast, the lower part
of the same glacier is a
zone of wastage,
where losses from
melting, sublimation, and calving of icebergs exceed the rate
of accumulation (
Figure 14.6). In either case, the ice is stretched (subjected
to tension) and crevasses open, which extend down to the
zone of plastic fl ow. In some cases, a glacier descends over
such a steep precipice that crevasses break up the ice into a
jumble of blocks and spires, and an icefall develops.
◗
As you might suspect, the amount of snowfall and tempera-
ture are important factors in determining where glaciers form.
Parts of northern Canada are cold enough to support glaciers
but receive too little snowfall, whereas some mountain areas
in California receive huge amounts of snow but are too warm
Figure 14.7).
At the end of winter, a glacier's surface is completely cov-
ered with the accumulated seasonal snowfall. During the spring
and summer, the snow begins to melt, fi rst at lower elevations
and then progressively higher up the glacier. The elevation to
which snow recedes during a wastage season is the
firn limit
(Figure 14.7). You can easily identify the zones of accumulation
and wastage by noting the location of the fi rn limit.
The fi rn limit on a glacier may change yearly, but if it
does not change or shows only minor fluctuations, the
glacier has a balanced budget. That is, additions in the zone
of accumulation are exactly balanced by losses in the zone
of wastage, and the distal end, or terminus, of the glacier
remains stationary (Figure 14.7a). If the fi rn limit moves up
the glacier, indicating a negative budget, the glacier's termi-
nus retreats (Figure 14.7b). If the fi rn limit moves down the
glacier, however, the glacier has a positive budget, additions
exceed losses, and its terminus advances (Figure 14.7c).
Even though a glacier may have a negative budget
and a retreating terminus, the glacial ice continues to
move toward the terminus by plastic fl ow and basal slip.
If a negative budget persists long enough, though, the
glacier continues to recede and it thins until it is no longer
thick enough to maintain fl ow. It then ceases moving and
becomes a
stagnant glacier
; if wastage continues, the glacier
eventually disappears.
◗
Total
surface
movement
Flow
direction
Zone
of
plastic
flow
Basal
slip
Plastic
flow
Ice
Rock
◗
Figure 14.5
Part of a Glacier Showing Movement by a
Combination of Plastic Flow and Basal Slip Plastic fl ow involves
internal deformation within the ice, whereas basal slip is sliding
over the underlying surface. If a glacier is solidly frozen to its bed, it
moves only by plastic fl ow. Notice that the top of the glacier moves
farther in a given time than the bottom does.
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