Geography Reference
In-Depth Information
46
GLACIERS AND CREVASSES
The Antarctic continent is blanketed by the largest sheet of ice on Earth. The
transition to glacier ice from snow is one of compaction and recrystallization. The
process begins with ice crystals (snowflakes) dropping from vapor-laden clouds,
or, in the interior of the continent, directly out of clear atmosphere. At the atomic
level, the repetitive union of molecules of water builds these delicate crystals.
Although the forms that snow crystals may take would appear to be infinite, the
one characteristic that they all share is hexagonal symmetry in their basic crystal-
line structure. The snow crystals fall to the Earth, are blown across the surface
and blunted, and eventually come to rest as tiny particles of ice that accumulate
over time.
The average rate of accumulation (water-equivalent) for the whole of
Antarctica is around eight inches per year. As snow is buried, the process of
recrystallization begins. Because of slightly higher pressures at points of con-
tact between the ice crystals, water molecules break free there, jump into the
surrounding air space or dance along grain boundaries between the crystals, to
points with less pressure where they recombine (recrystallize), building on the
existing crystal structure. Larger crystals grow at the expense of smaller ones.
The effect tends to segregate the ice from the air. The pervasive air space at
the beginning of the process is reduced to interconnected tunnels and finally to
discrete bubbles with no connectivity. By definition, glacier ice begins when the
air space is no longer interconnected. At first the ice is white, but in time the air
bubbles compress or seep out, and the purified ice assumes a deepening shade
of blue.
“Like mighty rivers,” glaciers flow down valleys under the force of gravity,
but unlike rivers in which the water is a liquid (noncrystalline), glacier ice flows
as a
ductile
or plastic solid (crystalline, albeit in motion). Flow rates for the major
outlet glaciers of the Transantarctic Mountains have been measured in the range
of two to six feet per day. If you sat on the side of a glacier and watched all day,
the flow would not be quite perceptible. But if you set out some markers and
surveyed them at the beginning and end of a field season, the distance that the
glacier traveled would become clear. At the scale of the glacier, the movement
is steady and uniform, but at the scale of individual crystals of ice, the frantic
migration of molecules from spots of higher to lower pressure is causing the solid
crystals of ice to recrystallize and change their shapes.
One of the characteristics of a ductile solid like ice is that, if the rate at which
it flows is too rapid, then it will lose its coherency and break. The ice is then said
to be behaving as a
brittle
solid. Because the ductility of ice increases with depth
due to its overlying weight, ice is most brittle at the surface. As brittle fractures
open they propagate downward, producing gashes in the glacier that are known