Ice Sheet Flow Research Articles

Recumbent folding in ice sheets: a core-referential study

AbstractTo better understand apparent stratigraphic disturbances in ice cores such as Greenland Ice Sheet Project 2 (GISP2), we examine how ice-sheet flow can transform gentle open folds into order-disturbing recumbent folds. The initial disturbances in the stratigraphy have their roots in transient dynamic processes and local rheological inhomogeneities, but the kinematics of even a simple ice-flow model can deform these disturbances enough to alter paleoclimatic interpretation of an ice core. The local vorticity number suggests which structures can be passively overturned, but analyzing the finite strain along particle paths gives a more complete picture, especially when taken relative to a hypothetical core location. Core-relative isochrones, or “pre-cores”, predict which stratigraphic disturbances will appear as obviously overturned layers in a core. The deformation-gradient tensor along particle paths allows us to calculate the rotation of segments of various reference slopes. These calculations suggest that observed 20° dips in the GISP2 core are rotating on a time-scale of a few hundred years and could result from distortions with much smaller slopes produced upstream. The time during which they can be recognized to be overturning is short because the rotation rate is high. Once overturned they are flattened further and may be hard to recognize, especially in the small cross-section of a core.

Vostok (Antarctica) ice-core time-scale from datings of different origins

AbstractThree different approaches to ice-core age dating are employed to develop a depth–age relationship at Vostok, Antarctica: (1) correlating the ice-core isotope record to the geophysical metronome (Milankovich surface temperature cycles) inferred from the borehole temperature profile, (2) importing a known chronology from another (Devils Hole, Nevada, USA) paleoclimatic signal, and (3) direct ice-sheet flow modeling. Inverse Monte Carlo sampling is used to constrain the accumulation-rate reconstruction and ice-flow simulations in order to find the best-fit glaciological time-scale matched with the two other chronologies. The general uncertainty of the different age estimates varies from 2 to 6 kyr on average and reaches 6–15 kyr at maximum. Whatever the causes of this discrepancy might be, they are thought to be of different origins, and the age errors are assumed statistically independent. Thus, the average time-scale for the Vostok ice core down to 3350m depth is deduced consistent with all three dating procedures within the standard deviation limits of ±3.6 kyr, and its accuracy is estimated as 2.2 kyr on average. The constrained ice-sheet flow model allows, at least theoretically, extrapolation of the ice age–depth curve further to the boundary with the accreted lake ice where (at 3530m depth) the glacier-ice age may reach ∼2000 kyr.

Optimal estimation of changes in the mass of ice sheets

We describe a new approach for estimating changes in ice sheet mass. Two methods are in common use: the ice budget and geodetic methods. The first makes use of separate estimates of the mass fluxes into and out of a domain, differencing them to obtain the local mass balance. The second estimates mass balance directly, using measurements of the change in surface elevation, often from aircraft or satellites. Here we combine ice budget and geodetic approaches to obtain an optimal estimate of mass balance. We seek maximum likelihood solutions for three terms: (1) the rate of change of surface elevation, (2) the rate of snow accumulation, and (3) the local divergence of the ice flux. These estimates are constrained to obey the continuity equation. We allow the location and temporal averaging interval of the estimates to be chosen arbitrarily. This approach can use all relevant measurements. The fidelity of any measurement is lowered by measurement error, and by fluctuations in each of the three terms driven by random year‐to‐year snowfall variations. We take full account of both error sources, weighting the data so as to minimize the confounding effect of these influences. Realistic covariance between randomly forced fluctuations are provided by a linearized model of ice sheet flow. We test the approach by applying the algorithm to synthetically generated measurements. The method performs better than either ice budget or geodetic methods applied in isolation, and has the important advantage that good estimates may still be derived when measurements appropriate to either technique are lacking or inaccurate.

Transmission of basal variability to a glacier surface

Transmission of basal variability to a glacier surface is investigated using analytical models for a linearly viscous medium. The three‐dimensional transient response of the surface to both bedrock undulations and spatial variations in basal slipperiness for perturbations of arbitrary wavelengths is determined using perturbation methods. Both information transfer toward the surface and lateral transmission of horizontal stresses are strongly affected by the slip ratio, that is, the ratio of basal sliding to deformational velocity. For any mean bedrock slope, and above a minimum value of slip ratio, the amplitude transfer of bedrock undulations toward the surface has a local maximum at undulation span corresponding to about 3–8 times the mean ice thickness. The transmission of basal variability to a glacier surface increases quite significantly with increasing slip ratio. This explains why the surfaces of fast flowing ice streams are more undulating than the slower moving bordering areas. At slip ratios higher than about 100, the flow of glaciers and ice sheets becomes nonlocal in the sense that surface velocities and buildup and propagation of surface undulations cannot be calculated accurately on the basis of local thickness and slope. Using linearized long‐wave theories at these slip ratios, instead of the more accurate arbitrary wavelength theory, gives estimates of decay times that are an order‐of‐magnitude too small and phase velocities several times too large. The problem of the propagation and decay of small‐amplitude surface undulations on glaciers in three dimensions is solved. Small‐amplitude surface waves on glaciers are strongly diffusive and dispersive. Redistribution of mass on ice sheets and glaciers is a diffusion process, and it is misleading, albeit not mathematically incorrect, to describe the reaction of glaciers to surface perturbations in terms of a wave propagation.

Spatial and temporal scales of anisotropic effects in ice-sheet flow

AbstractAlong most particle paths in polar ice sheets, ice experiences a slowly changing local deviatoric-stress pattern, and develops a fabric characteristic of its current stress state, in which there is generally a correspondence between non-zero strain-rate components and non-zero deviatoric-stress components. Where the stress pattern changes rapidly in special transition zones, fabric may evolve more slowly than the local stress field, and unusual or unexpected deformation patterns can result. The degree to which fabric tracks the local stress is determined by the relative characteristic times for changes in stress, given by the transition-zone width and ice velocity, and for changes in crystal orientation, given, in the absence of recrystallization, by the inverse of the local strain rate due to the principal stress. Recrystallization can significantly reduce the time-scale of fabric adjustment. We examine transition zones where ice (a) enters ice-stream margins, (b) is overrun by a migrating divide, and (c) flows through a strong saddle. Stress and fabric tend to be significantly misaligned in ice-stream margins and in flow through a saddle. When stresses that are markedly different from in situ stresses are applied to ice specimens during creep tests, deformation may be difficult to interpret.

On a Doubly Nonlinear Parabolic Obstacle Problem Modelling Ice Sheet Dynamics

This paper deals with the weak formulation of a free (moving) boundary problem arising in theoretical glaciology. Considering shallow ice sheet flow, we present the mathematical analysis and the numerical solution of the second order nonlinear degenerate parabolic equation modelling, in the isothermal case, the ice sheet non-Newtonian dynamics. An obstacle problem is then deduced and analyzed. The existence of a free boundary generated by the support of the solution is proved and its location and evolution are qualitatively described by using a comparison principle and an energy method. Then the solutions are numerically computed with a method of characteristics and a duality algorithm to deal with the resulting variational inequalities. The weak framework we introduce and its analysis (both qualitative and numerical) are not restricted to the simple physics of the ice sheet model we consider nor to the model dimension; they can be successfully applied to more realistic and sophisticated models related to other geophysical settings.

Plane ice-sheet flow with evolving and recrystallizing fabric

AbstractA plane, gravity-driven, steady flow of a polar ice sheet over a horizontal bedrock, with no-slip basal conditions, is considered. The ice is modelled as a linearly viscous, incompressible and anisotropic fluid, with evolving orthotropic fabric depending on local strain rates and deformations. For prescribed free-surface elevation and non-uniform temperature field, the ice velocities required to maintain the assumed geometry are calculated by using the finite-element method. The focus is on the mechanism of dynamic (migration) recrystallization occurring near the base ofan ice sheet and leading to significant weakening, and ultimately to the complete loss, of the anisotropic fabric developed in the upper part of the ice cap. The weakening of the fabric with increasing strain rate and temperature is modelled by means of a scaling function depending continuously on a single effective strain-rate invariant. The results of numerical calculations demonstrate the effect of the recrystallization process on the overall flow rate of the ice sheet, and normalized velocity and strain-rate depth profiles are compared for flows with and without recrystallization involved to illustrate the effect of this mechanism on the large-scale behaviour of polar ice sheets.

Radio-frequency attenuation beneath Siple Dome,West Antarctica, from wide-angle and profiling radar observations

AbstractKnowledge of the spatial distribution of bed lubrication regimes, i.e. frozen vs wet conditions, is crucial for understanding ice-sheet flow. Radar sounding can probe differing reflectivities between wet and frozen beds, but is limited by uncertainty in attenuation within the ice of bed echoes. Here we present two methods to estimate attenuation: (1) wide-angle radar sounding, in which source and receiver locations are varied so as to vary propagation path length, and thus echo amplitude; and (2) profiling, inwhich similar variations are obtained by sounding through varying ice thicknesses (assuming constant bed reflectivity). Siple Dome, West Antarctica, provides unusually favorable circumstances for application of these methods: the bed beneath Siple Dome is flat and uniform in its radar reflectivity, while ice thickness varies by several hundred meters. Wide-angle data 4 km from the summit yield an estimate for characteristic attenuation length of 124 m (35 dB km–1 loss), whereas profiling yields an estimate of 168 m.The difference between estimates is modest compared to the range of attenuation lengths reported in the literature. It may nonetheless prove informative by bounding effects of two ice properties to which the methods respond differently: (1) wide-angle sounding sampled relatively warm (lossy) ice beneath the summit, whereas the profiling method sampled relatively cold ice beneath the flanks as well; and (2) strain-induced crystal orientation fabrics and resulting dielectric anisotropy in the ice would vary from summit to flank, and may influence wide-angle sounding more strongly than profiling.

Stochastic forcing of Pleistocene ice sheets: Implications for the origin of millennial‐scale climate oscillations

Considerable evidence now exists for high‐frequency climate changes in the last ice age cycle (Dansgaard‐Oeschger and Bond cycles). Though large‐scale ice sheet flow has too long a time constant to play a significant role in these events, stochastically forced changes in the mass balance of the Pleistocene ice sheets may have resulted in significant variation in freshwater flux to the North Atlantic. In this paper we mimic the effects of “natural climate variability” with small (≤0.5W/m2) stochastic changes in insolation and examine the response in terms of ice volume change in an ensemble of coupled energy balance/ice sheet model runs. The total change in ice mass is comparable to a third of the volume of the current Greenland ice sheet, with meltwater discharge potentially sufficient to trigger Dansgaard‐Oeschger type events. In general, the Eurasian ice sheet is more sensitive to perturbations than the North American ice sheet. The distribution and occurrence of spectral peaks in the 1–9 kyr band is also comparable to the record of millennial oscillations over the last 500,000 years in a subpolar North Atlantic core. Our results suggest that considerable caution is necessary in interpreting the physical significance of sub‐Milankovitch spectral peaks in paleo time series.

Reply to comment by P. Duval and M. Montagnat on “Superplastic deformation of ice: Experimental observations”

[1] In a recent paper, we reported the results of a detailed experimental investigation of the rheological properties of ice [Goldsby and Kohlstedt, 2001]. One of the innovative aspects of our investigation was the use of fine-grained samples that enabled us to explore deformation not only by grain-size insensitive mechanisms involving grain matrix dislocation processes but also by grain-size sensitive flow mechanisms involving grain boundary sliding (GBS). An exciting discovery in our research was an extensive creep regime in which GBS contributes substantially to the flow of ice. Most importantly, this deformation regime in which basal slip is accommodated by GBS, referred to in our paper as the ‘‘superplastic flow’’ regime, governs the flow of ice over a wide range of temperature, grain size and stress conditions found in glaciers, ice sheets and icy planetary interiors. [2] In their comment on our paper, Duval and Montagnat [2002] repeat the objection that they had raised previously [Montagnat and Duval, 2000] concerning our conclusion that superplastic deformation controls the flow of glaciers and ice sheets. They assert ‘‘that GBS as a significant creep mechanism is not compatible with observations on the development of fabrics and microstructures in ice sheets. Special emphasis is placed [instead] on the role of grain boundary migration as a recovery process in glacier ice.’’ They then continue by arguing that basal slip accommodated by grain boundary migration (GBM) rather than by GBS is the dominant deformation mechanism in ice. [3] In this reply, we extend the results presented in detail by Goldsby and Kohlstedt [1997] and Goldsby and Kohlstedt [2001] to further demonstrate that GBS is a very important deformation process in natural ice bodies. Our reasoning is based on three points: (1) The microstructures reported for samples from large natural ice bodies are remarkably similar to those observed in samples deformed in the laboratory in the superplastic flow regime. (2) Our constitutive equation for ice is in excellent agreement with the flow behavior determined from field measurements on glaciers and ice sheets. (3) Contrary to the assertion of Montagnat and Duval [2000] and Duval and Montagnat [2001], fabric development is fully compatible with deformation in the superplastic regime described in our paper. In addition, we note that, while GBM associated with dynamic recrystallization is an important recovery mechanism in ice, it is not a deformation mechanism and hence cannot accommodate basal slip.

Emerging Applications of Interferometric Synthetic Aperture Radar (InSAR) in Geomorphology and Hydrology

Interferometric synthetic aperture radar (InSAR) is a powerful geodetic tool used to construct digital elevation models of the earth’s topography and to image centimeter–scale displacements associated with crustal deformation and the flow of ice sheets. The past decade has seen significant improvements in our understanding of earthquakes, volcanoes, and glaciers as a direct result of this technology. Geomorphology and hydrology can also benefit from InSAR. A small but growing body of work shows that it is possible to interferometrically measure centimeter–scale motions associated with unstable slopes, land subsidence, fluctuating soil moisture and water levels, and deforming river–ice cover. Maps of interferometric correlation provide information about the structure of snow and reveal areas disturbed by erosion, sedimentation, flooding, snowfall, and aufeis growth. At present, such techniques are underdeveloped and largely overlooked by the geographic and radar communities. This article reviews these emerging geomorphic and hydrologic applications of InSAR and presents a first demonstration of motion detection for river ice.

New ways of studying ice sheet flow directions and glacial erosion by computer modelling—examples from Fennoscandia

A computer ice sheet model has been used to study regional ice flow directions and glacial erosion of the Weichselian ice sheet, adopting a new method of presenting modelled ice flow directions. Ice sheet model results from different time periods during the Weichselian were extracted for five regions and presented in rose diagrams. When comparing these computer-generated results with information on flow directions obtained from current conceptual geological models based on field data, large similarities were observed. In comparing the ice flow directions as such, the similarities were strikingly good. In many cases there was also a good agreement in the timing of the events, while in some cases a certain flow direction was assigned to a different time period by the ice sheet model than in the current interpretation of geological information. Nevertheless, the overall agreement between the data sets shows that results from ice sheet models can aid in placing geological information into a coarse timeframe and chronostratigraphic context, and also fill in time gaps in the glacial geological record where chronological control is sparse. Ice sheet model results thus constitute a new data set against which glacial geological information can be compared and tested. A new quantity, basal sliding distance, has also been calculated from the ice sheet model results, describing the over time accumulated length of ice that has passed over the landscape by basal sliding. The results show high basal sliding distance values in SW Sweden/SE Norway, in Skagerrak, and along the Gulf of Bothnia, implying relatively large amounts of glacial erosion in these regions. On elevated parts of the Scandinavian mountain range and on adjacent plains in the east the basal sliding distance values are low, implying weaker glacial erosion. This compares well with different types of geological and morphological data, suggesting that basal sliding distance is a useful entity for studying regional patterns of glacial erosion intensity.

Continuum Mechanics and Applications in Geophysics and the Environment

I Applied Continuum Mechanics.- Numerical Investigation of Shock Waves in a Radiating Gas Described by a Variable Eddington Factor.- Anisotropic Fluids: From Liquid Crystals to Granular Materials.- Integration and Segregation in a Population - A Thermodynamicists's View.- Asymptotic and Other Properties of Some Nonlinear Diffusion Models.- The Binary Mixtures of Euler Fluids: A Unified Theory of Second Sound Phenomena.- Continuously Distributed Control of Plates by Electric Networks with PZT Actuators.- II Soil Mechanics and Porous Media.- Hydraulic Theory for a Frictional Debris Flow on a Collisional Shear Layer.- The Beavers and Joseph Condition for Velocity Slip at the Surface of a Porous Medium.- Porous Convection, the Chebyshev Tau Method, and Spurious Eigenvalues.- Mechanics of Multiphase Porous Media - Application to Unsaturated Soils.- III Glacier and Ice Dynamics.- Modelling Iceberg Drift and Ice-Rafted Sedimentation.- Modelling the Flow of Glaciers and Ice Sheets.- Notes on Basic Glaciological Computational Methods and Algorithms.- Constitutive Modelling and Flow Simulation of Anisotropic Polar Ice.- Influence of Bed Topography on Steady Plane Ice Sheet Flow.- IV Climatology and Lake Physics.- Glacial Isostasy: Models for the Response of the Earth to Varying Ice Loads.- Arctic Sea Ice and Its Role in Climate Variability and Change.- The Role of Simple Models in Understanding Climate Change.- Comparing Different Numerical Treatments of Advection Terms for Wind-Induced Circulations in Lake Constance.

Surface melt-induced acceleration of Greenland ice-sheet flow.

Ice flow at a location in the equilibrium zone of the west-central Greenland Ice Sheet accelerates above the midwinter average rate during periods of summer melting. The near coincidence of the ice acceleration with the duration of surface melting, followed by deceleration after the melting ceases, indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice-bedrock interface. Interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting, with larger increases accompanying higher amounts of summer melting. The indicated coupling between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming.

Calculating basal temperatures in ice sheets: an Excel spreadsheet method

AbstractThe flow of ice sheets and their geomorphological impact is greatly influenced by their basal thermal regime. Calculations of basal temperatures in ice sheets are therefore fundamental in evaluating glacier dynamics and in determining the spatial distribution of zones of erosion and deposition beneath ice masses. Calculations of basal temperatures are not frequently attempted, however, primarily because of the techniques required to solve the heat conduction equation between the ice surface and the base. This paper describes a new Excel spreadsheet method of solving this equation that can readily be applied to both former and contemporary ice sheets. The application of the spreadsheet is illustrated with two examples. The first provides a calculation of basal thermal regime beneath the north eastern part of the Scottish ice sheet during the last glacial maximum; the second shows how basal ice temperatures can be calculated beneath the modern Antarctic ice sheet. Copyright © 2002 John Wiley & Sons, Ltd.

The physiography of modern Antarctic subglacial lakes

The size and distribution of Antarctic subglacial lakes have been investigated using airborne radio-echo sounding (RES) and satellite radar altimetry. Over 70 lakes have been identified beneath the ice sheet from distinctive, mirror-like reflectors observed on RES records. Almost 60% of lakes are found within 200 km of an ice divide and only about 15% are located >500 km distant. The total volume of water stored in lakes beneath the Antarctic Ice Sheet is between 4000 and 12,000 km 3. The bedrock topography of the ice-sheet interior is characterized by large subglacial basins separated by mountain ranges. More than 60% of lake records have marginal bed gradients of <0.1, implying that many Antarctic subglacial lakes are found in areas of relatively low bed relief, in and on the margins of subglacial basins. First, there are those lakes located where subglacial topography is relatively subdued, often towards the centre of subglacial basins. Secondly, some lakes occur in significant topographic depressions, closer to subglacial basin margins. Lakes are also found perched on the sides of subglacial mountains. Sixteen lakes are located close to the transition from slow to enhanced ice-sheet flow. Warm-based fast-flowing ice streams provide a possible route by which subglacial lakes may establish a hydrological connection with the ice-sheet margin. At a continental scale, the locations of Antarctic subglacial lakes match the modeled distribution of pressure melting at the ice-sheet bed.

Topographically‐independent ice flow in northwestern British Columbia: implications for Cordilleran Ice Sheet reconstruction

AbstractStudies in southern British Columbia have shown that Cordilleran Ice Sheet flow was controlled by topograph, even in full glacial time. New ice‐flow evidence from the Nass River region, northern British Columbia, however, indicates that ice was thicker there and that the continental ice‐sheet phase of glaciation was reached. Inspection of high elevation sites has revealed a suite of ice‐flow indicators (mainly striae) undetected by earlier work. These suggest that at the Last Glacial Maximum (Fraser Glaciation), ice flowed southwestward across the Nass River region from an ice divide that probably was located in the Skeena Mountain area. Comparisons with adjacent work allow this divide to be mapped over a wide area. The results suggest that maximum ice thicknesses in the northern part of the Cordilleran Ice Sheet were larger than reported previously. The location of storm tracks in full glacial time may have played an important role in the production of an ice sheet that was thicker in northern British Columbia than it was in the southern half of the province. During deglaciation, ice thinned and gradually became confined to fiords and valleys, resulting in numerous and variable ice‐flow directions at that time. Topographic control was thus exerted on ice flow only after the glacial maximum was reached, despite the significant amount of relief in this region. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Comment on “Superplastic deformation of ice: Experimental observations” by D. L. Goldsby and D. L. Kohlstedt

] Knowledge of the rheological properties of ice at devia-toric stresses lower than 0.1 MPa is of great interest whenmodeling the flow of glaciers and ice sheets. In spite of thedifficulty of obtaining convincing results in the laboratory, thereis now agreement on a flow law with a stress exponent lowerthan 2 [Mellor and Testa, 1969; Duval, 1973; Alley, 1992;Duval and Castelnau, 1995]. This result is also supported byborehole deformation measurements [Dahl-Jensen and Gun-destrup, 1987] and bubbly ice densification modeling [Lipenkovet al., 1997].[

Flow variability in the Scandinavian ice sheet: modelling the coupling between ice sheet flow and hydrology

There is increasing geologic evidence for periodic flow variability within large ice sheets, manifested as spatially and temporally variable areas of fast ice flow, and resulting in the very complex patterns of lineations observed in formerly glaciated areas. However, many ice sheet models do not replicate this behaviour. A possible reason for this is that such models do not include a detailed treatment of basal hydrology. Changes in the character of sub-glacial drainage systems are believed to cause surges in valley glaciers. Recent ice sheet models, which have included basal hydrology or at least a link between basal velocity and the presence of water at the bed, often show flow variability. However, these models have typically assumed a deformable bed, or have made no assumptions about the nature of the bed. Whilst these assumptions seem applicable to areas close to the former margins of Quaternary ice sheets, they are less applicable to interior areas. These areas typically show thin or scanty till cover over eroded bedrock, and the presence of eskers, which are indicative of drainage in sub-glacial tunnels. We have developed a two-dimensional time-dependent ice sheet model that includes hard-bed basal hydrology. This allows calculation of sub-glacial water pressures and the use of a water pressure dependent sliding law to calculate ice sheet velocities. When used to simulate the Weichselian Scandinavian ice sheet, with late Quaternary climate and sea level as forcing functions, this model develops localised areas of fast-flowing ice, which vary in extent and in distance of penetration into the interior of the ice sheet both spatially and temporally. The behaviour of these lobes depends crucially on the influence of the evolving ice sheet topography on the routing of subglacial water flow, due to the resulting variations in the subglacial hydraulic potential which drive the water flow. Bedrock topography also has some influence, but fast flow areas are not confined to obvious topographic troughs. A relatively thin ice sheet with low surface slopes is produced in areas experiencing fast ice flow. Generally, two to four separate areas of fast flow can be recognised, and these are similar in size and shape to the ‘lobes’ identified in some geologically based reconstructions of the Scandinavian ice sheet. Within the fast-flowing areas, sub-glacial drainage is typically in a cavity-based system. However, tunnel-based drainage is predicted to have extended up to 150 km from the ice sheet margin, particularly during deglaciation. Because of the changes in ice sheet topography associated with fast flow, and the resulting changes in the pattern of sub-glacial water flow, the model predicts that these fast-flowing lobes interacted in complex ways, and exhibited quasi-periodic switching between fast and slow flow.

Subglacial volcanic features beneath the West Antarctic Ice Sheet interpreted from aeromagnetic and radar ice sounding

Abstract The West Antarctic Ice Sheet (WAIS) flows through the volcanically active, late Cenozoic West Antarctic rift system. Active subglacial volcanism and a vast (&gt; 10 6 km 3 ) extent of subglacial volcanic structures have been interpreted from aerogeophysical surveys over central West Antarctica in the past decade, combined with results from 1960s and 1970s aeromagnetic profiles over the WAIS. Modelling of magnetic anomalies constrained by radar ice sounding shows volcanic sources at the base of the ice throughout large areas, whose subglacially erupted hyaloclastite edifices have been eroded by moving ice, as in Iceland. The 1800 m-high divide of the WAIS is underlain by the 400 km-long volcanic Sinuous Ridge, which rises above sea level; most hyaloclastite edifices there have also been glacially removed, indicating migration of the ice divide through time. Northeast of the divide of the WAIS there is a 400-nT positive magnetic anomaly over the shallowest, most rugged bedrock topography (elevation +380m above sea level), probably comprising subaerially erupted flows erupted when the Sinuous Ridge area was deglaciated. Uplift of the Sinuous Ridge may have forced the advance of the WAIS. Other aspects of the subglacial volcanism in Antarctica can be observed in Iceland and have a direct bearing on our understanding of the subglacial conditions of the WAIS and its dynamics.