307
Indeed, there is an obvious positive feedback here. The deeper an ice flow, the larger
the driving basal stress, and the warmer the basal ice (due to increased frictional heat
and decreased conductive heat loss), and hence the softer the ice. Both of these effects
contribute to enhanced ice flow, which explains the formation of such channels, since
the erosive power of ice flow increases with the basal velocity and the basal shear stress.
Indeed, flow of ice over a plane bed is subject to a lateral instability (much as overland
flow of surface water is unstable to the formation of rills and gullies).
A similar kind of mechanism may operate when ice flows over deforming sediments,
as in the Siple Coast of West Antarctica. Here, it is found that the flow is concentrated
into five ice streams, A, B, C, D, and E, which are characterised by their heavily crevassed
appearance. The flow in these ice streams is very rapid and is due to basal sliding over
the underlying sediment (except for ice stream C, which appears to have 'switched off'
several hundred years ago). Measurements on ice stream B indicate that the basal water
pressure is high (within 0.4 bar of the overburden pressure), and that it is underlain by
some eight metres of saturated till. A similar instability to that concerning ice flow over
hard bedrock can explain the streaming nature of the flow. Where ice flow is larger,
there is increased water production. If the drainage system is such that increased water
production leads to increased water pressure (as one might expect, e.g. for a Darcy flow),
then the higher water pressure decreases the viscosity of the till, and hence enhances the
ice flow further. This is an instability mechanism, and the limiting factor is that when ice
flow increases, there is increased heat loss from the base, which acts to limit the increase
of melt rate. Although this mechanism is viable, it has not yet been shown that it works.
1.5 Jokulhlaups
It will be clear by now that basal water is tremendously important in determining the
nature of ice flow. Equally, the basal water syst.em can fluctuate independently of the
overlying ice dynamics, most notably in the outburst floods called jokulhlaups. In Iceland,
in particular, these are associated with volcanoes under ice caps, where high rates of
geothermal heat flux in the confines of a caldera cause a growing subglacial lake to occur,
which eventually overflows, propagates downglacier, and releases enormous floods over
the southern coastal outwash plains. These floods carry enormous amounts of volcanic
ash and sediments, which create vast beaches of black ash. Despite their violence, the
ice flow is hardly disturbed. Jokulhlaups are essentially internal oscillations of the basal
drainage system. They are initiated when the rising subglacial lake level causes leakage
over a topographic rim, and t.he result.ant water flow leads to an amplifying water flow by
the following mechanism. Water flow through a channel in ice enlarges it by meltback of
the walls due to frictional heating. The increased channel size allows increased flow, and
Indeed, there is an obvious positive feedback here. The deeper an ice flow, the larger
the driving basal stress, and the warmer the basal ice (due to increased frictional heat
and decreased conductive heat loss), and hence the softer the ice. Both of these effects
contribute to enhanced ice flow, which explains the formation of such channels, since
the erosive power of ice flow increases with the basal velocity and the basal shear stress.
Indeed, flow of ice over a plane bed is subject to a lateral instability (much as overland
flow of surface water is unstable to the formation of rills and gullies).
A similar kind of mechanism may operate when ice flows over deforming sediments,
as in the Siple Coast of West Antarctica. Here, it is found that the flow is concentrated
into five ice streams, A, B, C, D, and E, which are characterised by their heavily crevassed
appearance. The flow in these ice streams is very rapid and is due to basal sliding over
the underlying sediment (except for ice stream C, which appears to have 'switched off'
several hundred years ago). Measurements on ice stream B indicate that the basal water
pressure is high (within 0.4 bar of the overburden pressure), and that it is underlain by
some eight metres of saturated till. A similar instability to that concerning ice flow over
hard bedrock can explain the streaming nature of the flow. Where ice flow is larger,
there is increased water production. If the drainage system is such that increased water
production leads to increased water pressure (as one might expect, e.g. for a Darcy flow),
then the higher water pressure decreases the viscosity of the till, and hence enhances the
ice flow further. This is an instability mechanism, and the limiting factor is that when ice
flow increases, there is increased heat loss from the base, which acts to limit the increase
of melt rate. Although this mechanism is viable, it has not yet been shown that it works.
1.5 Jokulhlaups
It will be clear by now that basal water is tremendously important in determining the
nature of ice flow. Equally, the basal water syst.em can fluctuate independently of the
overlying ice dynamics, most notably in the outburst floods called jokulhlaups. In Iceland,
in particular, these are associated with volcanoes under ice caps, where high rates of
geothermal heat flux in the confines of a caldera cause a growing subglacial lake to occur,
which eventually overflows, propagates downglacier, and releases enormous floods over
the southern coastal outwash plains. These floods carry enormous amounts of volcanic
ash and sediments, which create vast beaches of black ash. Despite their violence, the
ice flow is hardly disturbed. Jokulhlaups are essentially internal oscillations of the basal
drainage system. They are initiated when the rising subglacial lake level causes leakage
over a topographic rim, and t.he result.ant water flow leads to an amplifying water flow by
the following mechanism. Water flow through a channel in ice enlarges it by meltback of
the walls due to frictional heating. The increased channel size allows increased flow, and