HR: 13:55h
AN: C22A-02    [PDF]
TI: Why Do Ice Streams Have Fast Velocities in Transtion Region of Low Surface Slope?
AU: * Weertman, J
EM: j-weertman2@nwu.edu
AF: Northwestern University, Materials Science & Engineering Dept. and Geologicla Sciences Dept., Evanston, IL 60208 United States
AB: It seems generally accepted that when water at the base of a glacier flows mainly through channels (cut into the basal ice) the basal water itself has little influence on the sliding speed of the glacier. On the other hand, sliding speed can be increased if channel water flow is disrupted and water is forced to flow in some kind of broad distributed pattern. (If basal till underlies the ice mass these statements still may hold if flowing basal water is largely confined between ice and till.) In an early paper (Weertman, {\it Rev. Geophys. Space Phys.} (1972), {\bf 10}, 287-333) we analyzed the basal pressure gradient in the direction normal to a channel axis and showed that the presence of the basal shear stress can change the sign of the pressure gradient. The distance from the channel at which the sign reversal first occurs is smaller the larger is the shear stress and the greater is the water pressure within the channel. Water is driven away from a channel in the region of sign reversal. This effect makes it more difficult for water flow to occur primarily through channels and more likely that basal water will cause increased sliding velocity. In this presentation it is pointed out that the same sign reversal effect occurs if a longitudinal (with respect to the channel axis direction) deviatoric stress exits within the basal ice. A pressure gradient sign reversal around water channels arising from a longitudinal deviatoric stress that lead to speed up of sliding could be important in two physical situations. (1) In the transition zone where an ice stream of an ice sheet becomes part of an ice shelf. In this region the basal shear stress is greatly reduced (because the upper ice surface becomes almost horizontal). However, the longitudinal deviatoric stress is increased to almost the level of that of an ice shelf. (2) In a surging glacier. A surging glacier elongates by a significant fraction of its length in a relatively short period of time. As a consequence, a large longitudinal deviatoric stress must exist during a surge. In each of these situations the presence of a large longitudinal deviatoric stress will help disrupt water flow through channels and lead to increased speed of sliding. This mechanism may be the solution to the existing vexing problem: Why do ice streams have fast velocities in the transition region of low surface slope?
DE: 1827 Glaciology (1863)
SC: Cryosphere [C]
MN: 2003 Fall Meeting