Geology Reference
In-Depth Information
Geo-Focus
Glaciers and Global Warming
As you know, global warming is a
phenomenon involving warming of
Earth's atmosphere during the last
100 years or so. Many scientists think
that the concentration of greenhouse
gases (carbon dioxide, methane, and
nitrous oxide) as a result of human
activities, especially the combus-
tion of fossil fuels, is the cause of
global warming. There are, however,
dissenters who acknowledge that
Earth's surface temperatures have
increased but attribute the increase
to normal climatic variations. Need-
less to say, the issue has not been
resolved.
Whatever the cause of climate
change, no one doubts that glaciers
are good indicators of short-term
variations in climate. According
to one estimate, there are about
160,000 glaciers outside Antarctica
and Greenland, with Alaska alone
having several tens of thousands.
What do valley glaciers tell us about
climate? Remember that the behav-
ior of valley glaciers depends on
their glacial budget, which is in turn
controlled by temperature and pre-
cipitation.
It is true that not very many of
the 160,000 or so glaciers on Earth
have been studied, but those that
have reveal an alarming trend:
Many are retreating, and in some
cases they have nearly or completely
disappeared. For example, of the
approximately 150 glaciers in Glacier
National Park in Montana in 1850,
only a few very small ones remain.
Many of the valley glaciers in Alaska
at lower elevations are much smaller
than they were a few decades ago,
and so it goes just about everywhere
glaciers are studied, including those
in the Cascade Range of the Pacific
Northwest.
Recall from Chapter 5 that the
Cascade Range is made up of sev-
eral large composite volcanoes
and hundreds of smaller volcanic
cones and vents (see Chapter 5).
All of the higher peaks in the range
have glaciers, although some are
very small and most are retreating.
When Mount St. Helens erupted
in May 1980, all of its 12 glaciers
were destroyed or at least consid-
erably diminished. But by 1982,
the lava dome in the crater had
cooled sufficiently for snow to ac-
cumulate yearly and it is now as
much as 190 m thick. In any case,
it is thick enough so that pressure
on the snow at depth converts it to
glacial ice, and “giant cracks in the
ice, called crevasses, and other flow
features, indicate that the ice body
is transforming into a glacier”*
(
Figure 1).
Three factors account for the
birth of this new glacier. First,
the Cascade Range receives huge
amounts of snowfall. Second, the
crater provides protection for the
accumulating snow. And third,
rockfalls from the crater walls help
insulate the forming glacier. This
new glacier formed in little more
than 20 years, and it is the only one
in the continental United States that
is advancing.
The story for the other Cascade
Range glaciers is not so comforting.
Glacier Peak in Washington, which last
erupted in 1880, has more than a dozen
glaciers, all of which are retreating, and
Whitechuck Glacier will soon be inac-
tive (
◗
◗
Figure 2). During the Little Ice
Age, Whitechuck Glacier had northern
and southern branches, each with a sep-
arate accumulation zone, that merged
to form a single glacier. It covered about
*U.S. Forest Service,
Volcano Review
, Summer
2002, contribution by Charlie Anderson, Director
of the International Glaciospeleological Survey
Continental glaciers ordinarily fl ow at a rate of centi-
meters to meters per day. One reason continental glaciers
move comparatively slowly is that they exist at higher lati-
tudes and are frozen to the underlying surface most of the
time, which limits the amount of basal slip. But some basal
slip does occur even beneath the Antarctic ice sheet, al-
though most of its movement is by plastic fl ow. Nevertheless,
some parts of continental glaciers manage to achieve
extremely high fl ow rates. Near the margins of the Green-
land ice sheet, the ice is forced between mountains in what
are called
outlet glaciers
. In some of these outlets, flow
velocities exceed 100 m per day.
In parts of the continental glacier covering West Antarc-
tica, scientists have identifi ed ice streams in which fl ow rates
are considerably higher than in adjacent glacial ice. Drilling
has revealed a 5-m-thick layer of water-saturated sediment
beneath these ice streams, which acts to facilitate movement
of the ice above. Some geologists think that geothermal heat
from subglacial volcanism melts the underside of the ice,
thus accounting for the layer of water-saturated sediment.
A
glacial surge
is a short-lived episode of accelerated fl ow
in a glacier during which its surface breaks into a maze of
crevasses and its terminus advances noticeably. Glacial
surges are best documented in valley glaciers, although
they also take place in ice caps and perhaps in continen-
tal glaciers. In 1995, for instance, a huge ice shelf at the
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