Neal R. Iverson

Flow mechanism of glaciers on soft beds.

Subhourly measurements of bed deformation, bed shear strength, subglacial water pressure, and surface speed at Storglaci�ren, a glacier in northern Sweden, showed that the shear-strain rates of the bed decrease during periods of high water pressure and surface speed. High water pressures appear to be accompanied by a reduction in the coupling of ice with the bed that is sufficient to reduce or eliminate shearing. The instability of large ice masses may result from similar decoupling rather than from pervasive bed deformation, as has been commonly thought.

Grain-size distribution in deforming subglacial tills; role of grain fracture

New ways of looking at grain-size distributions may yield insights into sedimentary processes or environments. For example, during shearing of a granular material, alignments of grains, or bridges, develop with orientations such that compressive forces parallel to these alignments support most of the applied shear stress. If deformation is due to failure of such bridges by fracture, rather than by, say, dilation, the grain-size distribution will tend toward one that provides the maximum support for the grains. This size distribution is fractal and has a fractal dimension of 2.6. We analyzed the grain-size distribution of three deforming tills collected from beneath modern glaciers. The size distributions are fractal, and the mean fractal dimension is ∼2.9, suggesting an excess of fines. For comparison, grain-size distributions of samples from some other common sedimentary environments were also analyzed. Samples of dune sand and of glacial outwash were not fractal, but a debris-flow sample was, and had a fractal dimension of 2.8.

Potential effects of subglacial water-pressure fluctuations on quarrying

Water-pressure fluctuations beneath glaciers may accelerate rock fracture by redistributing stresses on subglacial bedrock and changing the pressure of water in bedrock cracks. To study the potential influence of water-pressure fluctuations on the fracture of subglacial bedrock, ice flow over a small b edrock step with a water-filled cavity in its lee is numerically modeled, and stresses on the bedrock surface are calculated as a function of transient water pressures in the cavity. Stresses on the bed are then used to calculate principal stress differences within the step. Rapid reductions in cavity water pressure increase principal s tress differences in the bed, increasing the likelihood of crack growth in the step and the formation of predominantly vertical fractures. Relatively impermeable bedrock may be most susceptible to fracturing during water-pressure reductions b ecause high water pressure in cracks within the rock can be maintained, as water pressure decreases in cavities. These results, when considered in conjunction with the strong likelihood that increases in water pressure accelerate the removal of rock fragments loosened from the bed, suggest that in zones of icebed separation where waterpressure fluctuations typically are large, rates of quarrying may be higher than along other parts of glacier beds.

Rheology of till beneath Storglaciären, Sweden

In order to study, in situ, the rheology of a deforming subglacial till, various instruments were emplaced in till beneath Storglaciaren, Sweden. Boreholes were used to gain access to the till beneath about 100 m of ice. Tiltmeters provided an estimate of the shear strain rate in the till. Two other instruments yielded measures of till strength. In addition, water pressures were recorded in boreholes and in the till, a computer-controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag, based on a force-balance calculation. Tilt rates varied directly with effective pressure, so decreases in water pressure apparently increased the coupling between the glacier and the bed. Surface speed was either out of phase with tilt or varied independently of tilt. Thus, increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed; they were not a result of till deformation! Till strength varied directly with effective pressure, which is consistent with it being a Mohr-Coulomb, or frictional material. The devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed. This implies that the residual strength of the till is insensitive to strain rate. Thus, the appropriate constitutive equation for till rheology may be of the form: e˙∞ e kτ where k is a constant. This is consistent with experimental data reported in the geotechnical literature.

A ring-shear device for the study of till deformation: Tests on tills with contrasting clay contents

Abstract Deformation of subglacial sediment may have had a profound influence on the geomorphic effects and dynamics of Pleistocene ice sheets, but data that bear on the rheology of such sediment are few and contradictory. A means of studying systematically the mechanical properties of glacial tills is provided by a ring-shear device that shears a large annular specimen to high strains at variable rates. Special features of the device allow continuous observation of the distribution of shear strain, measurement of local stresses normal to the direction of shear, and isolation of wall effects. Thus far, a linear-viscous putty and two tills with different clay contents (4 and 32% by weight) have been tested. The experiments with till indicate, as expected, that the presence of dispersed clay minerals reduces markedly both the residual strength and hydraulic diffusivity of till. Clay also reduces spatial variations in normal stress associated with grain bridges. The walls bounding the specimen support only 9–17% of the total resistance to shearing. Strain was localized in both tills, but not in the linear-viscous putty. Strain localization in the tills indicates that their theology departs significantly from linear- and Bingham-viscous models.

Clast-fabric development in a shearing granular material: Implications for subglacial till and fault gouge

Elongate clasts in subglacial till and in fault gouge align during shearing, but the relation between clast-fabric strength and cumulative shear strain for such materials is effectively unknown. This relation was explored in experiments with a large ring-shear device in which a till and a viscous putty that contained isolated clasts were sheared to high strains. As expected, rotation of clasts in the putty is closely approximated by the theory of G.B. Jeffery, who derived the orbits of rigid ellipsoids in a slowly shearing fluid. Clast rotation in the till, however, is strikingly different. Rather than orbiting through the shear plane as predicted by Jeffery, most clasts rotate into the shear plane and remain there, resulting in strong fabrics regardless of the aspect ratios and initial orientations of clasts. This divergent behavior is likely due to slip of the till matrix along the surfaces of clasts, which is a natural expectation in a granular material but violates the no-slip condition of Jeffery9s model. These results do not support the widespread belief that subglacial till deformation results in weak clast fabrics. Thus, many tills with weak fabrics thought to have been sheared subglacially to high strains, like many basal tills of the Laurentide Ice Sheet, may have been sheared only slightly with little effect on either ice-sheet dynamics or sediment transport. In addition, these results indicate that in simple shear the rotation of clasts in till and in fault gouge is best analyzed with the model of A. March, who treated inclusions as passive markers.

Distributed shear of subglacial till due to Coulomb slip

In most models of the flow of glaciers on till beds, it has been assumed that till behaves as a viscoplastic fluid, despite contradictory evidence from laboratory studies. In accord with this assumption, displacement profiles measured in subglacial till have been fitted with viscoplastic models by estimating the stress distribution. Here we present a model that illustrates how observed displacement profiles can result from till deformation resisted solely by Coulomb friction. Motion in the till bed is assumed to be driven by brief departures from static equilibrium caused by fluctuations in effective normal stress. These fluctuations result from chains of particles that support intergranular forces that are higher than average and that form and fail at various depths in the bed during shearing. Newtons second law is used to calculate displacements along slip planes and the depth to which deformation extends in the bed. Consequent displacement profiles are convex upward, similar to those measured by Boulton and colleagues at Breidamerkur-jokull, Iceland. The model results, when considered together with the long-term and widespread empirical support for Coulomb models in soils engineering, indicate that efforts to fit viscoplastic flow models to till displacement profiles may be misguided.

Shear resistance and continuity of subglacial till: hydrology rules

The field observations of G.S. Boulton stimulated widespread interest in deformable beds. Shear resistance of till in its critical state is insensitive to strain rate and increases linearly with effective pressure. During unsteady deformation, pseudo-viscous shear resistance can be caused by dilation of consolidated tills and resultant pore-pressure decline. This effect is probably uncommon, however, because susceptible tills of low hydraulic diffusivity are also those least likely to consolidate significantly during effective-pressure transients. Stick-slip motion at Whillans Ice Stream, Antarctica, indicates that its basal till must weaken during rapid slip and strengthen during longer periods of slower slip. Recurrence intervals for rapid-slip episodes there (6-18 hours) indicate that till-strength variations, if driven by changes in pore pressure either related or unrelated to basal freezing, are focused in the uppermost several centimeters of the bed. Ploughing of grains at the bed surface and associated excess pore pressures in adjacent till can account for rate-weakening during rapid slip, with pore-pressure decay causing strengthening between slip episodes. By promoting shallow, sluggish subglacial water flow and low effective pressure, soft beds may help sustain themselves by slowing their own transport. Soft-bed shear resistance, kinematics and continuity are problems rooted in subglacial hydrology.

Slow episodic shear of granular materials regulated by dilatant strengthening

Slow, stable shear of granular materials in landslides, beneath glaciers, and along fault surfaces is common, despite little or no intrinsic strengthening of such materials with increasing deformation rate. Compacted, water-saturated sediments, subjected to constant stresses in a ring-shear device, sheared slowly without unstable acceleration in repeated episodes that included pore dilation during shear, attendant pore-water-pressure decline, and consequent strengthening, followed by gradual pore-pressure recovery and weakening. Time-averaged shear velocities (2–800 mm/d) depended inversely on the magnitude of pore dilation with shear and were significantly lower for fine-grained sediment than for coarse-grained sediment, owing to different rates of pore-pressure diffusion in the two materials. When sediment had dilated to its critical-state (steady) porosity and therefore could not dilate further, shear accelerated catastrophically. These data indicate that pore-pressure decreases and consequent strengthening caused by shear-induced dilation may not only suppress rapid shear of landslide debris, subglacial till, and fault gouge, but may also result in slow episodic shear at rates that depend on both material porosity and hydraulic diffusivity.

Intrusion of ice into porous media by regelation: A mechanism of sediment entrainment by glaciers

Unlithified sediment at glacier beds should be entrained in ice as a result of melting above individual grains and refreezing below grains (regelation). To test this hypothesis, a device that simulates subglacial conditions was used to push ice downward through idealized and natural porous media in contact with a flat bed. Ice regelated toward the bed at speeds proportional to the gradient in ice pressure across the thickness of particles. Ice temperatures responded predictably to changes in driving stress and demonstrated that the ice-particle mixtures were at or very near the pressure-melting temperature. A theory of pore ice motion by regelation predicted the measured speed usually to within a factor of two, although at low driving stresses the speed was less than the predicted value by as much as a factor of five. The low speeds were probably caused by imperfect temperature control that resulted either in heat loss from the ice-particle mixtures or in slightly subfreezing pore ice temperatures that were not detected by thermistors. Ice should intrude the pores of underlying sediment to a steady depth at which the downward regelation speed equals the rate of basal melting. Departures from the steady state should cause release of debris from ice or further entrainment. An approximate calculation of the steady intrusion depth yields values comparable to typical thicknesses of debris in basal ice, a few centimeters to more than a meter. Intrusion of sediment by regelation is consistent with the isotopic composition of debris-bearing ice, if melting exceeds refreezing.