306
between channelised flow and linked cavities, implicitly for ice flowing over (hard) bedrock.
A rather different situation appears to operate in Trapridge Glacier, another well-studied
surging glacier in the Yukon. Here the glacier is cold (unlike the temperate (at the melting
point) Variegated) and rests on a thick ('" 6 metres) layer of till - a rather sludgy
mixture of water and rock particles. The till has a bimodal distribution, consisting of
coarse pebbles and boulders interspersed with finer clay material. Till is produced by
the erosion of brittle underlying bedrock, and is evacuated by the slow motion of the ice
downstream.
The sequence of events which appear to be occurring as the Trapridge thickens is that
the basal ice reaches melting point (and the till thaws). The till is then deformable, and
the surge will be associated with a slump of the ice riding over the soft till; it therefore
depends on the rheology of the till. While this is presently uncertain, it is clear that
water pressure in unconsolidated sediments has a major effect on till rheology. As water
pressure increases, the till dilates and its effective viscosity is reduced. Thus, surging here
could also be explained by a sudden alteration of drainage mechanism. Quite what this
could be, however, is unclear. At the moment, water is evacuated from the base through
the till to a subglacial aquifer, and emerges at an outlet stream in front of the glacier. One
possibility is that, as the ice thickens and more basal melt water is produced, the water
pressure at the base necessary to evacuate the meltwater gradually increases towards
overburden pressure, with an associated increase in basal till deformation. One might
expect a runaway phenomena, as the accelerating ice flow produces yet more melt water,
a process eventually relieved by the surge and the re-freezing of the base due to thinning
of the ice. However, the long-awaited next surge of Trapridge has not yet occurred and
such mechanisms are highly speculative.
1.4 Ice streams
Although ice sheets also flow under the horizontal pressure gradients induced by the
glaciostatic pressures beneath their sloping surfaces, they rest on essentially unsloping
bases, and therefore have no advective component in their dynamics. Thus ice sheets do
not, at least on the large scale, exhibit wave motion: the governing equations are essentially diffusive in character. On a more local scale, however, ice sheets have interesting
phenomena of their own.
Principal among these may be ice streams. Ice sheets do not tend to drain uniformly to
the margin from their central accumulation zones, but rather the outflows from catchment
areas are concentrated into fast-moving ice streams. Examples are the Lambert Glacier in
Antarctica and Jakobshavn in Greenland, a fast-moving (8 metres per day) outlet glacier.
These ice streams gain their speed by carving out deep channels through which they flow.
between channelised flow and linked cavities, implicitly for ice flowing over (hard) bedrock.
A rather different situation appears to operate in Trapridge Glacier, another well-studied
surging glacier in the Yukon. Here the glacier is cold (unlike the temperate (at the melting
point) Variegated) and rests on a thick ('" 6 metres) layer of till - a rather sludgy
mixture of water and rock particles. The till has a bimodal distribution, consisting of
coarse pebbles and boulders interspersed with finer clay material. Till is produced by
the erosion of brittle underlying bedrock, and is evacuated by the slow motion of the ice
downstream.
The sequence of events which appear to be occurring as the Trapridge thickens is that
the basal ice reaches melting point (and the till thaws). The till is then deformable, and
the surge will be associated with a slump of the ice riding over the soft till; it therefore
depends on the rheology of the till. While this is presently uncertain, it is clear that
water pressure in unconsolidated sediments has a major effect on till rheology. As water
pressure increases, the till dilates and its effective viscosity is reduced. Thus, surging here
could also be explained by a sudden alteration of drainage mechanism. Quite what this
could be, however, is unclear. At the moment, water is evacuated from the base through
the till to a subglacial aquifer, and emerges at an outlet stream in front of the glacier. One
possibility is that, as the ice thickens and more basal melt water is produced, the water
pressure at the base necessary to evacuate the meltwater gradually increases towards
overburden pressure, with an associated increase in basal till deformation. One might
expect a runaway phenomena, as the accelerating ice flow produces yet more melt water,
a process eventually relieved by the surge and the re-freezing of the base due to thinning
of the ice. However, the long-awaited next surge of Trapridge has not yet occurred and
such mechanisms are highly speculative.
1.4 Ice streams
Although ice sheets also flow under the horizontal pressure gradients induced by the
glaciostatic pressures beneath their sloping surfaces, they rest on essentially unsloping
bases, and therefore have no advective component in their dynamics. Thus ice sheets do
not, at least on the large scale, exhibit wave motion: the governing equations are essentially diffusive in character. On a more local scale, however, ice sheets have interesting
phenomena of their own.
Principal among these may be ice streams. Ice sheets do not tend to drain uniformly to
the margin from their central accumulation zones, but rather the outflows from catchment
areas are concentrated into fast-moving ice streams. Examples are the Lambert Glacier in
Antarctica and Jakobshavn in Greenland, a fast-moving (8 metres per day) outlet glacier.
These ice streams gain their speed by carving out deep channels through which they flow.