“Striations” produced by catastrophic subglacial drainage of an ice-dammed lake were investigated in front of Mjelkedalsbreen, Jotunheimen, southern Norway. At each site, length, width, and orientation of at least 50 “striations” were recorded. These data are compared with similar measurements from a glacially abraded “control” site. On the basis of length or width measurements alone, “striations” produced by subglacial drainage are not consistently distinguishable from those produced by glacial abrasion. However, the former display more variable orientations and cross-cutting relationships which could be misinterpreted as indicating changing ice-flow directions. They can be recognized by the occurrence of some “striations” with relatively low length : width ratios, which indicate the transport of boulders by saltation.

VLF surface-impedance measurements were made along four traverses on the Antarctic ice sheet in the vicinity of Casey base. Computer modelling of the surface-impedance data allowed ice-depth predictions to be made; predictions which are almost independent of ice temperature for ice depths less than 800 m. Results agree with ice-radar and other ice-depth data. Surface-impedance anomalies were observed close to moraines and crevasses in the ice sheet. The technique is fast and the instrumentation sufficiently portable for single-person operation.

The length of time TM over which a glacier responds to a prior change in climate is investigated with reference to the linearized theory of kinematic waves and to results from numerical models. We show the following: TM may in general be estimated by a volume time-scale describing the time required for a step change in mass balance to supply the volume difference between the initial and final steady states. The factor f in the classical estimate of τM = ƒl/u, where I is glacier length and u is terminus velocity, has a simple geometrical interpretation. Ft is the ratio of thickness change averaged over the full length I to the change at the terminus. Although both u and f relate to dynamic processes local to the terminus zone, the ratio f/u and, therefore, Tm are insensitive to details of the terminus dynamics, in contrast to conclusions derived from some simplified kinematic wave models. A more robust estimate of Tm independent of terminus dynamics is given by TM= h/(–b) where h is a thickness scale for the glacier and –b is the mass-balance rate (negative) at the terminus. We suggest that Tm for mountain glaciers can be substantially less than the 1O2–103 years commonly considered to be theoretically expected.

Preliminary results are presented on a solution of the two-dimensional time-dependent continuity equation for mass conservation governing ice-sheet dynamics. The equation is solved using a column-averaged velocity to define the horizontal flux in a finite-element formulation. This yields a map-plane model where flow directions, velocities, and surface elevations are defined by bedrock topography, the accumulation/ablation pattern, and in the time-dependent case by the initial surface configuration. This alleviates the flow-band model limitation that the direction of flow be defined and fixed over the course of the modeling experiment. The ability of the finite-element method to accept elements of different dimensions allows detail to be finely modeled in regions of steep gradients, such as ice streams, while relatively uniform areas, such as areas of sheet flow, can be economically accommodated with much larger elements. Other advantages of the finite-element method include the ability to modify the sliding and/or flow-law relationships without materially affecting the method of solution.

Modeling experiments described include a steady-state reconstruction showing flow around a three-dimensional obstacle, as well as a time-dependent simulation demonstrating the response of an ice sheet to a localized decoupling of the bed. The latter experiment simulates the initiation and development of an ice stream in a region originally dominated by sheet flow. Finally, a simulation of the effects of a changing mass-balance pattern, such as might be anticipated from the expected carbon dioxide warming, is described. Potential applications for such a model are also discussed.

SYMBOLS USED

a(x,y) Accumulation/ablation rate.

A Flow-law parameter.

B Sliding-law parameter.

CijC Global capacitance matrix.

f Fraction of the bed melted.

Fij,F Global load vector.

g Acceleration of gravity.

hj,h Ice-surface elevation.

H Ice thickness.

k(x,y) Constitutive equation constant of proportionality.

kij Global stiffness matrix.

m Sliding-law exponent.

n Flow-law exponent.

ρ Density of ice.

σ(x,y) Ice flux.

t Time.

U Column-average ice velocity.

UF Column-average deformation (flow) velocity.

US Sliding velocity.

v Variational trial function.

x,y Map-plane coordinates.

Tiltmeters that can detect changes in slope of a glacier surface as small as 0.1 μ rad have been used on Storglaciären. The records obtained to date have been from the upper part of the ablation area, where the bed of the glacier is overdeepened. A total of 82 d of records has been obtained for various time periods between early June and early September.

There is generally a gradual change in inclination of the glacier surface over periods of several days, but these changes do not appear to be systematic. In particular, they are not consistent with vertical movements of stakes located 2–3 ice thicknesses away from the tiltmeters. This suggests that the tiltmeters are sensing disturbances over areas with diameters comparable to the local ice thickness.

Superimposed on these trends are diurnal signals suggesting rises and falls of the surface just up-glacier from the riegel that bounds the overdeepening on its down-glacier end. These may be due to waves of high water pressure originating in a crevassed area near the equilibrium line. If this interpretation is correct, the waves apparently move down-glacier at speeds of 20–60 m h−1 and become sufficiently focused, either by the bed topography or by conduit constrictions, to result in local uplift of the surface. Also observed are abrupt tilts towards the glacier center line shortly after the beginning of heavy rainstorms. These appear to be due to longitudinal stretching as the part of the glacier below the riegel accelerates faster than that above. Water entering the glacier by way of a series of crevasses over the riegel is believed to be responsible for this differential acceleration. In June 1987, a dramatic event was registered, probably reflecting the initial summer acceleration of the glacier.

Calculations of the regional variation of net mass balance around Crary Ice Rise, Antarctica, show significant rates of thickening up-stream of the ice rise and significant thinning down-stream (assuming zero basal melting or freezing). Thickening also is occurring on the south-west side of the ice rise nearest the Transantarctic Mountains. These imbalances imply migration of the rise and help explain its current non-equilibrium shape. The pattern of a suture line down-stream of the rise indicates a quasi-periodic mechanism of rift formation.

A practical method is developed for calculating stresses and velocities at depth using field measurements of the geometry and surface velocity of glaciers. To do this, it is convenient to partition full stresses into lithostatic and resistive components. The horizontal gradient in vertically integrated lithostatic stress is the driving stress and it describes the horizontal action of gravity. The horizontal resistive stress gradients describe the reactions. Resistive stresses are simply related to deviatoric stresses and hence to strain-rates through a constitutive relation.

A numerical scheme can be used to calculate stresses and velocities from surface velocities and slope, and from ice thickness. There is no mathematical requirement that the variations in these quantities be small.

Uniaxial compression experiments at –10°C at 10−3s−1 on fresh-water, granular ice have established through the use of high-speed photography that internal cracks nucleate preferentially away from the ice/platen (i/p) interface under conditions of i/p contraint, but near the interface under conditions of i/p expansion. Under conditions of little i/p interaction, cracks nucleate more or less randomly throughout the specimen. Correspondingly, the brittle-fracture strength decreases as the i/p interaction changes from compressive to tensile. These effects are explained in terms of the spatial variation of the maximum shear stress and the crack density.

Strong winds can disrupt the thermal regime in seasonal snow because of the variation in surface pressure associated with surface features like dunes and ripples. Topographical features of shorter wavelengths produce stronger surface flows, but the flow decays rapidly with depth. Longer-wavelength features produce weaker surface flows but the flow decays more slowly with depth. The flow may only be strong enough to disrupt the temperature field for features of wavelengths on the scale of meters or tens of meters at wind speeds of 10 m/s or more.

Other possible causes of windpumping have been examined but they do not appear to be as significant. Rapid pressure perturbations due to turbulence produce very little displacement of the air because of the high frequency and low amplitude. Barometric pressure changes cause compression and expansion of the air in the pore space, but the rate is too low to have much effect.

Resistive stresses and velocities at depth are calculated along the Byrd Station Strain Network, Antarctica, using field data. There are found to be large longitudinal variations in basal drag and this result is little affected by errors in the input data or by uncertainties in the constitutive relation for ice. Basal drag varies by a factor of about 2 along the strain network, and is usually equal to the driving stress to within 10–20%. Sites of high drag are not always correlated with basal topographic highs, indicating that some process such as basal water drainage is involved in controlling the friction at the bed. Basal sliding velocities are very sensitive to errors in measured surface velocities and the rate factor in Glen’s flow law. As a result, calculated sliding velocities are much less reliable than deep stresses, and need to be interpreted with caution.

Stresses at the surface and at depth are calculated for a stretch of Byrd Glacier, Antarctica. The calculations are based on photogrammetrically determined velocities and elevations, and on radio-echo-determined ice thicknesses. The results are maps of drags from each valley wall, of normal forces laterally and longitudinally, and of basal drag. Special challenges in the calculation are the numerical gridding of velocity, ensuring that unreasonable short-wavelength features do not develop in the calculation, and inference of ice thickness where there are no data.

The results show important variations in basal drag. For the floating part, basal drag is near zero, as expected. Within the grounded part, longitudinal components of basal drag are very variable, reaching 300 kPa with a dominant wavelength of 13 km. Generally, these drag maxima correlate with maxima in driving stress. Usually the across-glacier component of basal drag is small. An important exception occurs in the center of the grounded part of the glacier where the flow shows major deviations from the axis of the valley.

Other results are that side drag is roughly constant at 250 kPa along both margins of the glacier, tension from the ice shelf is about 100 kPa, and tension in the grounded part cycles between 250 and 150 kPa. Calculated deep velocities are too large and this is attributed to deficiencies in the conventional isotropic flow law used.

The stability of a low Reynolds number flow on an inclined plane is investigated with respect to modelling the initiation of transverse wave-like ridges which commonly occur on the surfaces of rock-glacier forms. In accordance with field observations indicating the presence of stratification in rock glaciers, two models of rock-glacier structure are considered, each stratified and possessing a lower layer which is treated as a Newtonian fluid. An upper, less compliant layer is treated, alternatively, as a Newtonian fluid of viscosity greater than that of the lower layer, or as an elastic solid under longitudinal compression induced by a decrease in the slope of the underlying incline. A linear stability analysis is used to examine the behaviour of each of the proposed models, and both are found to generate instabilities at wavelengths comparable to those associated with transverse surficial ridges on rock glaciers. The growth rates of a flow disturbance predicted by the viscous-stratified model appear to be too slow to account fully for the development of wave forms of finite amplitude, suggesting that other mechanisms are involved in the amplification of an initial disturbance. The results of the stability analysis of the elastic lamina model indicate that finite surficial ridges may develop on rock glaciers as a product of a buckling instability in the surface region if there is a decrease in the slope of the underlying incline. Both of the analyses illustrate that transverse ridges can occur on the surface of a rock glacier in the absence of any variations in debris supply to the system. The results further imply that the use of these features in the paleoreconstruction of Holocene climatic conditions must entail an assessment of the relative roles of external climatically driven forcing versus internal Theologically derived instability.

This paper tests and falsifies the theory that the development of thick sequences of vertically stacked clean and debris-laden ice layers at the margin of the Greenland ice sheet can be attributed solely to simple freezing-on of material at the bed. Isotopic analysis in δD and δ18O of ice from the ice-sheet margin near Søndre Stramfjord indicates that the debris-rich and debris-poor elements of the basal sequence have different origins. While the debris bands display isotopic fractionation consistent with a freezing origin, the intercalated clean ice layers do not. The clean ice layers have isotopic values indistinguishable from debris-bearing ice immediately above the debris-band sequence and from unaltered glacier ice, and are entrained by a different process from the debris bands. Debris may be entrained by freezing at the bed, but the development of a vertically stacked sequence of debris bands must be attributed to some other mechanism.

Two distinct medial moraines are located on the tongue of Ghiacciaio dei Forni (Rhaetian Alps, Italy); they are formed by the confluence of three ice streams. The two ice-cored medial moraines originate from rock outcrops situated below the snow line. The moraines, which on the surface comprise angular clasts, extend for approximately 2 km and reach a maximum height of 10 m. Nourishment of the moraines seems to be due primarily to the supraglacial debris derived from the rock walls. Measurements made in the late summer of 1985 provided evidence of the role played by differential ablation in the development of these moraines. Sedimentological analysis further confirmed the mainly supraglacial origin of the debris comprising the moraines.

Crystalline texture and c-axis orientation of the 2083 m ice core at Vostok Station, covering more than 150kyear, reveal the existence of strong anisotropics. Changes in crystal size with depth are compatible with the growth of grains driven by the free energy of grain boundaries. A smaller growth rate appears to be associated with cold periods. A gradual increase in the horizontal elongation of grains was observed between 350 and 680 m. But, the mean value of the coefficient of the linear dimensional orientation of grains does not change below 700 m.

The c-axis orientation of ice grains tends to orientate perpendicular to the direction of the elongation of grains, forming a vertical girdle pattern. This characteristic fabric has been interpreted as resulting from the gradual rotation of grains by basal glide under uniaxial longitudinal tension. The rotation of grains was calculated with respect to the total strain, simulating the formation of the girdle fabric pattern. The fabric-enhancement factor was calculated at various depths. It appears that Vostok ice hardens gradually with depth when considering the transverse convergent flow. No significant variation of the enhancement factor was observed with changes in climate and impurity content.

The development of strong preferred c-axis alignments in sea ice has been a subject of debate since its initial discovery in 1959. Although numerous theories have been proposed for its development, none has been able to account for all of its field characteristics.

A detailed field investigation was carried out at Pond Inlet, N.W.T., to describe further the occurrence and structure of this ice type. Data collected over the winter of 1983–84 suggest two possible mechanisms of development. The first involves the selective growth of crystals during the formation of the initial ice skim, while the second requires the solid-state rotation of slip planes during constrained plastic deformation.

The flow of ice over a bump under steady-state conditions has been studied both experimentally and numerically. The experiments were performed with the ice viscometer installed at the Laboratoire de Glaciologie et Géophysique de l'Environnement du C.N.R.S. (LGGE) in Grenoble, France. The flow was observed by means of a marker network photographed at fixed time intervals. The separation of the ice down-stream of the bump and the non-linearity of the generally accepted Glen’s flow law led to the modelling of this flow using a finite-element method. The results describe the velocity and stress fields in ice moving over a bump and obeying Glen’s flow law (until now only relatively rough approximations of these fields have been given in the literature). The comparison of the observed and calculated network deformations shows that, although ice flowing over a bump does not undergo steady creep, the use of Glen’s flow law with an exponent n = 3 remains justified when simulating the flow of a glacier over its bed under steady-state conditions.

The nature of icebergs calved from Svalbard’s 1030 km of tide-water ice cliffs is related to glacier dynamics and morphology. Both iceberg dimensions and rates of relative iceberg production are affected. Valley tide-water glaciers entering fjords typically calve irregular icebergs of <50 m in length. Ice caps and large outlet glaciers, which predominate in eastern Svalbard, yield small, irregular icebergs and large (>500 m length) tabular icebergs which can travel considerable distances before melting. Surge-type ice masses calve very large numbers of small icebergs during short periods of rapid advance, but few icebergs during longer periods of stagnation and retreat between surges. The nature of iceberg interactions with sea ice also influences the timing and pattern of iceberg production. Winter shore-fast ice traps icebergs close to tide-water ice cliffs. They are released as a pulse on spring-time fast-ice break-up. Pack ice damps waves, and therefore increases iceberg stability and reduces the frequency of overturn. Small icebergs often undergo complete melting and sediment release within fjords. Larger icebergs can be trapped close to glacier termini by shallow bedrock or morainic sills, but some larger, tabular icebergs do escape into the Barents Sea. Implications for iceberg rafting of sediments include the production of large numbers of relatively debris-rich icebergs during surges and the absence of floating ice masses. The latter restricts the loss of debris-rich basal ice by undermelt prior to calving.

It is generally assumed that the mass-balance gradient on glaciers is more or less conserved under climatic change. In studies of the dynamic response of glaciers to climatic change, one of the following assumptions is normally made: (i) the mass-balance perturbation is independent of altitude or (ii) the mass-balance profile does not change — it simply shifts up and down. Observational evidence for such an approach is not convincing; on some glaciers the inter-annual changes in mass balance seem to be independent of altitude, on others not at all. Moreover, it is questionable whether inter-annual variation can be “projected“ on different climatic states.

To see what a physical approach might contribute, we developed an altitude-dependent mass-balance model. It is based on the energy balance of the ice/snow surface, where precipitation is included in a parameterized form and numerical integrations are done through an entire balance year (with a 30 min time step). Atmospheric temperature, snowfall, and atmospheric transmissivity for solar radiation are all dependent on altitude, so a mass-balance profile can be calculated. Slope and exposure of the ice/snow surface are taken into account (and the effects of these parameters studied). In general, the calculations were done for 100m elevation intervals.

Climatological data from the Sonnblick Observatory (Austria; 3106 m a.s.l.) and from Vent (2000 m a.s.l.; Oetztal Alps, Austria) served as input for a number of runs. Simulation of the mass-balance profiles for Hinterseisferner (north-easterly exposure) and Kesselwandferner (south-easterly exposure) yields reasonable results. The larger balance gradient on Kesselwandferner is produced by the model, so exposure appears to be an important factor here.

Sensitivity of mass-balance profiles to shading effects, different slope, and exposure are systematically studied. Another section deals with the sensitivity to climatic change. Perturbations of air temperature, cloudiness, albedo, and precipitation are imposed to see their effects on the mass-balance profiles. The results clearly show that, in general, mass-balance perturbations depend strongly on altitude. They generally increase down-glacier, and are not always symmetric about the reference state.

For typical climatic conditions in the Alps, we found that a 1 K temperature change leads to a change in equilibrium-line altitude of 130 m. Three factors contribute to this large value; turbulent heat flux, longwave radiation from the atmosphere, and fraction of precipitation falling as snow. Here, the albedo feed-back increases the sensitivity in a significant way.

Satellite-altimeter data over ice sheets provide the best tool for mapping their topography and its possible climatic variations. However, these data are affected by measurement errors, orbit errors, and slope errors. We develop here a three-step inversion technique which accommodates the a priori information on the expected topography and correctly handles and propagates the data errors: it estimates first a large-scale reference surface, then maps the residuals related to undulations, and finally iteratively corrects the slope error. The method is tested on overlapping small fragments of the Antarctic ice sheet, using a sub-set of Seasat data. Finally, a topographic map of Terre Adélie is produced. Over areas of small slopes, the a posteriori error should be of the order of 0.4 m. Using ERS-I data, it is therefore expected that climatic variations in the ice-sheet topography since the introduction of Seasat will be observable.