Stability of Temperate Ice Caps and Ice Sheets Resting on Beds of Deformable Sediment

Abstract

Although theories of glacier movement ge n erally assume that glaciers flow over rigid rock beds, there are many places where glacie rs r est on beds of d eforma ble sedim e nt, a nd the great Pleistocene ice sheets which extend ed from time to time over much of Northern Europe a nd N orth America were largely underlain by such b eds. Observations sh ow that a large proportion of the forward movement ofa glacier lying on such a bed may be contributed by d eformation of the bed ra ther than the gl acier. A theory is d eveloped in which the glacier surface profile is rela ted to the hydraulic and strength prope rti es of potentially deform-able bed materials. If these have a high hydraulic transmiss ibility, melt wa te r is readily discharged sub-glacially, the bed is stable, and the profile is a normal p a ra boli c one, governed by the rheologica l properties of ice. If bed transmissibility is low, wa ter pressures build up, the bed begins to d eform, and a lower equilibrium profile will develop, so tha t in a n extreme case the gl acier a pproxim a tes to a thin flat sheet, similar to an ice shelf. It is suggested tha t su ch behaviour may h ave occurred a t the m a rgins of large Pleistocene ice sheets over North America and Europ e, a nd evidence in support of this is dra wn from the reconstructed shapes of these ice margins, a nomalo usly sm all a mounts of isostatic rebound, a n o ma lously high retreat-rates, and the presence of glaciotectonic structures. R easons a re suggested to expl a in why this behaviour should have been important for Pleistocene glaciers which p en etra ted into currently te mperate latitudes but does not appear to be important in la rge m od ern glaciers. REsuME. Stabiliti de calottes de glace tempi ree reposant sur des lits de sidiments diformables. Bien que les theories sur le mouvem e nt d es glaciers supposent en general que les glaciers s'ecoule nt sur d es lits rocheux rigides, il arrive souvent que les glaciers reposent e n fait sur des sed iments deforma bles et la grande calotte glaciaire pleistocene qui s' es t pa rfois etendue sur la plus grande pa rti e de l'Europe du N o rd et de l'Amerique du Nord etait en gran de partie supportee p a r d e tels lits. Des o bservati ons montre nt qu'une fort e proportion des avancees de glacie rs r eposant sur un tellit peut et re le res ultat de la deform atio n du lit plus que d e glacier. On a developpe un e theorie selon laquell e le profil de la surface du gl acier es t mis en relation avec les pro-prietes hydrauliques et plastiques d es m a te ria ux d'un lit d e fo rma ble. S' ils o nt un e forte permeabilite, I'eau d e fonte est rapid ement tra nsmise so us le glacier, le lit est sta ble et le profil est normalement parabolique, conformement aux lois rheologiques d e la glace. Si la pe rm eabili te a I'eau est fa ible, la pression hydrosta tique croit, le lit commence a se deforme r et un profil d'equilibre plus bas en a ltitude se developpe d e sorte que, e n d es cas extre m es, le glacier ressembl e a un e feuille min ce pl...

On Subglacial Stone Pavements in Till

“…Therefore, the till is deformed easily under the moving glacier if the water is mainly held in the pores of the till (e.g. Boulton & Jones 1979) and does not form a significant lubricating film for basal sliding over the till. The deforming till would have a velocity/displacement gradient that decreases beneath the glacier sole (e.g.…”

Mechanisms leading to the 2016 giant twin glacier collapses, Aru range, Tibet

“…The sustained very low basal drag under Aru glaciers (<20kPa) may be similar to ice stream mechanisms whereby water-saturated till enables fast flow at low driving stresses (≈20kPa) (Cuffey and Paterson, 2010). It has been shown that 15 glacier till behaves with a plastic rheology with a shear strength strongly dependent on the effective normal stress (Clarke, 2005;Iverson et al, 1998;Iverson, 2010;MacAyeal, 1992). Such behaviour was found to be well described by a Coulombtype friction law (Boulton and Jones, 1979;Clarke, 2005;Tsai et al, 2015) as follows:…”

A shear‐diffusion model of till genesis based on the dispersal pattern of indicator rocks in the Grand‐Volume Till of central Gaspésie, Québec, Canada

“…Glacial shearing has been inferred to explain certain aspects of glacial sedimentation (Elson 1961;Mac-Clintock & Dreimanis 1964;Lavrushin 1970;Rappol 1987), including the development of debris dispersal patterns (Rappol & Stoltenberg 1985;Hirvas & Makinen 1989). Until recently, however, subglacial sedimentation by active glaciers has been attributed principally to subglacial lodgement, but subglacial sediment deformation has received renewed attention through the recognition of deformable beds (Boulton & Jones 1979;Alley 1991;Clark & Walder 1994).…”

“…Deforming subglacial debris appears as the most probable site for the shear-diffusion process. The glacier bed, made of preglacial regolith, may undergo pervasive shear when the pore water pressure reduces friction among the particles (Boulton & Jones 1979) under an active ice. However, shearing also takes place in the lower englacial environment, as revealed by the observation of debris-rich basal ice deforming more readily than pure ice at the base of a cold glacier (Echelmeyer & Zhongxiang 1987).…”

“…Current hypotheses on till-forming processes have been deduced from a variety of sources, such as direct observation of subglacial environments (Boulton 1979;Echelmeyer & Zhongxiang 1987;Blake et al 1992;Lawson 1982;Tison et al 1993), glaciological theory and experimentation (Weertman 1968;Boulton & Jones 1979;Hallet 198 l), geophysical investigations (Alley et al 1986), and from glacial geologic studies involving erosional landforms (Boulton 1979), depositional landforms (Goldstein 1989;Clark & Walder 1994) and sedimentary properties of tills (Elson 1961;Eyles et al 1982;Shaw 1982;Bouchard et al 1984;Rappol 1987;Alley 1991;Hicock & Dreimanis 1992). The latter category includes attributes such as grain size (Haldorsen 198 1 ;Kemmis 198 l), fabric (Mac-Clintock & Dreimanis 1964;Mickelson 1973;Drake 1977), lithological composition (Goldstein 1989;Rappol & Stoltenberg 198.5) and dispersal patterns (Clague 1975;Drake 1983;Clark 1987;Puranen 1990).…”