Geography Reference
In-Depth Information
tion zone is greatest in the summer, because temperatures are higher and there is more
meltwater for lubrication (Ilken and Bindschadler 1986). Ives and King (1955), working
on Morsárjökull in southeast Iceland, recorded some of the earliest observations that
indicated a significant increase in rate of glacier flow following the onset of heavy rain.
Velocities are highly variable, ranging from a few centimeters to several meters per
day. In steep reaches of the glacier, particularly where icefalls occur, velocities may be
much higher. The highest velocities occur during the so-called surges of glaciers, when
speeds exceeding 100 m (330 ft) per day may occur for short periods (Sharp 1988). Sur-
ging mechanisms are divided into two models: the hydrological switch model, where
surging is controlled by basal hydrology, and the thermal switch model, where surging
is controlled by basal temperature (Murray et al. 2003). This still little-understood phe-
nomenon has received increasing attention within the last several decades; it is not fully
understood, for instance, why some glaciers surge while neighboring glaciers do not
(Meier 1969).
Glaciers are thought to move by one or two basic mechanisms (internal deformation
and basal sliding), depending on whether the portion of the glacier being considered is
frozen to its bed (cold based) or at the pressure melting point (warm based). Several
theories have been developed over the years to describe the internal deformation of ice.
It is currently believed that ice behaves as a viscoplastic or pseudoplastic polycrystal-
line solid where internal deformation occurs because of flow or creep, as in the creep of
metals. This idea has been mathematically formalized in Glen's Flow Law (Glen 1955;
Paterson 1994; Hooke 2005). Ice, of course, is much weaker than most crystalline solids.
It deforms easily through the action of gravity, producing shear stress on its mass and
causing intragranular yielding, in which the ice crystals yield to shear stress by gliding
over one another along basal planes within the lattices of the ice crystals. The individual
ice crystals should become internally elongated, but since no such crystal deformation
is found in glaciers, a progressive recrystallization apparently accompanies the deform-
ation (Sharp 1988: 46; Hooke 2005). The primary factors controlling the rate of internal
deformation are the depth of the ice, the surface slope of the glacier, and ice temperat-
ure. The steepness of the bedrock slope beneath the ice is less important, since plastic
flow may continue, even where there are bedrock depressions and obstacles (Paterson
1994; Barry and Gan 2011).
The other major mechanism involved in glacier movement is that of basal sliding,
which involves the slippage of ice en masse over the rock surface at its base. The ab-
rasions and striations left on bedrock across which glaciers have moved are evidence
for this kind of movement. The processes involved are even less well understood than
those of viscoplastic flow, since the base of a glacier is inaccessible to direct observa-
tion except in rare cases. The important controls on basal sliding are the temperature
of the ice at the base and the presence of water to serve as a lubricant (Sharp 1988).
Basal sliding does not generally occur in polar glaciers, since the ice is frozen to the
underlying rock surface. In other regions, the temperature of the glacial ice is higher,
and water may be present along the base. This is part of the explanation for why polar,
or cold, glaciers move rather slowly, except where they calve into a lake or the sea.
Water may also be released when ice reaches the pressure melting point. This can
happen when an obstacle is encountered during glacial movement: The ice is com-
pressed on the upstream side of the obstruction, and the increased pressure causes
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