Crevasses, once formed, are subject to rotation and bending according to the velocity field through which they travel. Because of this, crevasse shapes can be used to infer something about the velocity field of a glacier. This is done using a model in which each crevasse opens perpendicularly to the principal extensional strain-rate, when that strain-rate exceeds some specified critical value, and is then deformed according to the same velocity gradients that formed the crevasse. This model describes how crevasses are formed, translated, rotated, bent, and lengthened.

Velocity fields are sought for which calculations produce crevasses approximating those found in three example areas on Ice Stream B, Antarctica. The first example is the hook-shaped crevasses that occur just outside the chaotic shear zone at the ice-stream margin. They are used to infer a rate of lateral shearing, and side drag. The second example, a pattern of splaying crevasses, is satisfactorily simulated by a model with side-drag stress varying linearly across the ice stream. This confirms that this region is restrained almost entirely by side drag. The third example is transverse crevasses and their change in orientation, but many different velocity fields can produce the observed pattern. Of these three examples, the shapes of hook-shaped marginal crevasses and splaying crevasses can provide useful information whereas transverse crevasses are less helpful.

The Transantarctic Mountains of East Antarctica provide a new milieu for retrieval of ice-core records. We report here on the initial findings from the first of these records, the Dominion Range ice-core record. Sites such as the Dominion Range are valuable for the recovery of records detailing climate change, volcanic activity, and changes in the chemistry of the atmosphere. The unique geographic location of this site and a relatively low accumulation rate combine to provide a relatively long record of change for this potentially sensitive climatic region. As such, information concerning the site and general core characteristics are presented, including ice surface, ice thickness, bore-hole temperature, mean annual net accumulation, crystal size, crystal fabric, oxygen-isotope composition, and examples of ice chemistry and isotopic composition of trapped gases.

The surface and bottom topography of the central Greenland ice sheet was determined from airborne ice-radar soundings over a 180 km by 180 km grid centered on the 1974 “Summit” site (lat. 72°18′N., long. 37°55′W.), using the Technical University of Denmark 60 MHz ice radar. Over 6100 km of high-quality radar data were obtained, covering over 99'% of the grid, along lines spaced 12.5 km apart in both north-south and east-west directions. Aircraft location was done with an inertial navigation system (INS) and a pressure altimeter, with control provided by periodically flying over a known point at the center of the grid. The ice radar was used to determine ice thickness; the surface topography was determined independently using height-above-terrain measurements from the aircraft’s radar altimeter. The calculated surface topography is accurate to about ±6 m, with this error arising mostly from radar-altimeter errors. The ice thickness and bottom topography are accurate to about ±50 m, with this error dominated by the horizontal navigation uncertainties due to INS drift; this error increases to about ±125 m in areas of rough bottom relief (about 12% of the grid).

The highest point on Greenland is at lat. 72°34′ N., long. 37°38′W., at an altitude of 3233 ± 6 m a.s.l. The ice surface at this point divides into three sectors, one facing north, one east-south-east, and one west-south-west, with each having a roughly uniform slope. The ice divide between the last two sectors is a well-defined ridge running almost due south. The ice is about 3025 m thick at the summit. Excluding the mountainous north-east corner of the grid, where the ice locally reaches a thickness of about 3470 m and the bed dips to about 370 m below sea-level, the maximum ice thickness, approximately 3375 m, occurs about 97 km south-south-west of the summit. The average bed altitude over the entire grid is 180 m and the average ice thickness is 2975 ± 235 m. The ice in most of the south-west quadrant of the grid is over 3200 m thick, and overlies a relatively smooth, flat basin with altitudes mostly below sea-level. There is no predominant direction to the basal topography over most of the grid; it appears to be undulating, rolling terrain with no obvious ridge/valley structure. The summit of the ice sheet is above the eastern end of a relatively large, smooth, flat plateau, about 10–15 km wide and extending about 50 km to the west. If the basal topography were the sole criterion, then a site somewhere on this plateau or in the south-west basin would be suitable for the drilling of a new deep ice core.

Two seismic arrays recorded in an 11 month field experiment in 1985 the seismicity of Ekström Ice Shelf in the area of an ice rumple and an inlet, situated respectively about 10 km north-west and 7 km north of the German Antarctic station Georg von Neumayer (lat. 70°37′S., long. 08°22′W). Most of the focal depths of the icequakes considered until now are in the range 5–9 m; the ice-rumple area shows extremely high seismic activity. Tensile fracture is the most frequent fault mechanism, although there are a few shear-fracture events. The ice rumple’s seismicity provides information on the dynamics of the ice shelf in this area. A comparison of this time-dependent seismicity with tides suggests that most of this seismicity is induced by tides. The most active period of this seismicity starts at the beginning of low tide and ends at low tide. The location of the epicentres of icequakes recorded at that time and the digital recording on tapes of the seismicity without interruption for 396 h shows a jerky vertical movement of the ice shelf in response to tides; this can be interpreted as a kind of “grater effect”, especially at the southern ice-rock boundary of the ice rumple.

The seismicity in the inlet is much less and tensile fracture seems to be the only fault mechanism.

In spring and winter, there is a considerable amount of solid precipitation in the western part of the Tian Shan mountains. Avalanches frequently occur here, and often endanger national economic construction and human life. Observation shows that the avalanche occurrences are closely related to the physical characteristics of the snow, especially to the development of depth hoar in the snow cover. The conditions for the development of depth hoar are determined by thickness of the snow cover, air temperature, ground-temperature regime, and the duration of negative temperatures. The results of this study have revealed that the optimum depth of snow cover for the development of depth hoar is about 80 cm in the Tian Shan mountains, and the mean maximum depth of the snow cover in this region is 78 cm. Therefore, depth hoar develops extraordinarily well. The thickness of depth hoar can reach more than 80% of the total snow-cover depth. That is one of the main reasons why avalanches still occur frequently under the conditions of a limited snow-cover depth in the western part of the Tian Shan mountains.

Freezing processes in temperate ice consisting of a mixture of pure ice with water inclusions are studied for the case that the initial amount of moisture content is uniform. By introducing a cold source at the center of the ice specimen, the cold front propagates outwards leaving behind pure cold ice with a temperature distribution dictated by the exact set-up of the cold source. The speed of the front is directly related to the water content of the temperate ice and depends essentially on the Stefan condition.

Three types of initial and boundary conditions are considered and realized in uniaxial, cylindrical, and/or spherical symmetry: (1) a metallic core at a temperature below the freezing point is initially brought into contact with the ice and the system is left free to evolve; (2) the metallic core is kept at constant temperature below freezing; (3) Case (2) is repeated with an insulating air layer between the metallic core and the ice.

Two-dimensional finite-element calculations of velocity and temperature fields have been applied to the energy balance of a cross-section of Barnes Ice Cap, Baffin Island, Canada. The flow plane currently is cooling near the ice divide and warming near the margin. Long-term simulations show a net warming trend followed by a cooling trend with a steady-state average temperature similar to the present. Sensitivity studies on an idealized version of the flow plane show that the overall temperature responds less than surface-temperature forcing, because a negative feedback in temperature advection is substantially larger than a positive feed-back in strain heating. The response times of the flow plane by itself are somewhat faster but of the same magnitude as response times that would be estimated from one-dimensional modeling. When bedrock-temperature calculations are included, response times increase an order of magnitude, but these do not substantially affect the short-term response.

Since its inception in 1975, the Northwest Avalanche Center (NWAC) has developed and produced micro- and mesoscale weather forecasts to support avalanche forecast and control needs for the Olympic and Cascades Mountains of Washington and Oregon, U.S.A. This paper describes NWAC’s array of data, observational results, and analytical techniques that make “avalanche weather” forecasting possible. In addition, NWAC’s operational program and the general terrain and climate of the area are described. A sample forecast is also included.

Measured subglacial water pressures are frequently higher than theoretical values calculated by assuming that subglacial conduits are straight and either circular or semi-circular in shape. While this may be attributed to errors in the estimates of conduit roughness or ice viscosity, we suggest here an alternative explanation: namely, that the conduits are actually broad and low rather than semi-circular. Good agreement between measured and calculated pressures can be obtained by assuming that the cross-sectional shape of conduits resembles the space between the arc of a circle and its chord. The angle subtended by the arc, θ, is treated as an adjustable parameter. In four cases studied, θ ranged from 2° to 36°.

A small ice-dammed lake near Søndre Strømfjord, West Greenland, drained suddenly during the night of 15–16 July 1988. This drainage was observed from within the lake basin near the tunnel mouth. The drainage had four main effects: the collapse of adjacent moraine ridges; the shearing of ice around the tunnel mouth; rapid incision of the lake bed; and the transport of sediment from the lake basin into the glacier. All of the above were confined to a period of 15–30 min following the exposure of a circular tunnel c. 6 m in diameter.

Airborne 60 MHz radio echo-sounding data were collected over 250 km of flight track from an ice cap (Kvitøyjøkulen) on Kvitøya in north-eastern Svalbard. These were combined with Landsat imagery to provide ice-surface and bed topographic information and inferences about the thermal and climatic conditions on the ice cap. Quantitative analysis of radio-echo strength measurements and the existence of offshore plumes of sediment-laden melt water indicate significant basal melting and a warmer thermal regime than would be expected from the geographical location of the ice cap. Analysis of the dielectric absorption of radio waves by the ice and data from a recently recovered ice core from a neighbouring ice cap (Austfonna) suggest considerable anthropogenic contamination of the precipitation falling on Kvitøya, most probably originating from Eurasia.

The lower 80 km of the fast-moving Jakobshavns Isbræ, West Greenland, is subject to significant melting during the summer season. The melt water drains into large supraglacial rivers which pour into moulins or feed into beautiful supraglacial lakes, some of which are observed to drain periodically. Except for a few streams that drain directly off the margins of the ice sheet within the drainage basin of this glacier, the fate of this melt water is unknown. However, a localized upwelling of highly turbid water is often observed during the melt season in the fjord adjacent Io the terminal cliff of the glacier, indicating that water from some source does move along the glacier bed.

As part of an investigation on the mechanisms of rapid flow on Jakobshavns Isbræ, measurements of surface velocity at several (∼25) locations along the ice stream at and below the equilibrium line were made in order to investigate the effects of this seasonally varying input of melt water on the speed of the glacier.

No significant seasonal variation in speed was found at any location. This indicates that, unlike many other sub-polar and temperate glaciers, surface melt-water production does not affect the motion of this glacier on a seasonal basis, and, thus, does not cause a significant temporal variation in basal sliding. This finding has important ramifications on the mechanisms of flow for this ice stream.

During the summers of 1987 and 1988, 15 dye-tracer tests from a total of eight injection points were conducted in the ablation area of Midtdalsbreen, a northern outlet of Hardangerjokulen, southern Norway. The spatial and temporal patterns of water discharge, shapes of the dye-return curves, through-flow velocities, dye-recovery rates, dispersivities, and velocity/discharge relationships suggest the existence of distinct catchments beneath the eastern and western halves of the glacier which are characterized by different types of drainage sytem. Experiments on the eastern side were associated with high melt-water discharges and produced short-lived and highly peaked dye-return curves, fast through-flow velocities, high dye-recovery rates, low dispersivity values which decreased through the melt season, and a velocity/discharge relationship with an exponent of 1.0. Experiments on the western side were associated with low melt-water discharges and produced flat, extended dye-return curves which often displayed secondary peaks, slow through-flow velocities, low dye-recovery rates, high dispersivity values which increased during the melt season, and a velocity/discharge relationship with an exponent of 0.6. Comparison of observed through-flow velocities with values calculated theoretically using various hypothetical drainage-system structures suggests that water flows in a major sinuous conduit beneath the eastern half of the glacier and in a system of linked cavities beneath the western half. A model for the seasonal evolution of the whole drainage network is postulated which has important implications for temporal variations in subglacial water pressures and glacier-sliding velocity.

During the winter of 1987–88, an average of seven tensile tests was made for each of 66 snow layers in the Rocky Mountains of western Canada. The precision of the mean strength for seven tests, expressed in terms of the coefficient of variation, was 15% with 90% confidence. Snow with a faceted micro-structure was approximately half as strong as partly settled or rounded snow of the same density. Notch sensitivity in the strength data and critical strains of 1% or less indicate that the test fractures were essentially brittle.

Tensile tests have been performed on notched and unnotched cylindrical samples of randomly oriented polycrystalline ice of controlled grain-size (between 2.2 and 7.3 mm) at a loading rate of 100Pa s−1 and at a temperature of −10°C In the notched samples, the notch-root diameter was 80% of the base diameter. A notch-strengthening effect was observed in the large-grained ice, with fracture stresses being up to 50% higher than that for unnotched samples of the same grain-size. This notch-strengthening effect diminished as grain-size decreased, disappearing at a grain-size of ≈3 mm.

The notch-strengthening effect is explained in terms of the triaxial stress constraint at the notch root. This triaxial constraint results in a change in the controlling mechanism of fracture from crack propagation in the unnotched samples to crack initiation in the notched samples.

A new short-pulse digital profiling radar system that operates at lower frequencies than most ice radars used in polar regions to date has been designed and built by the U.S. Geological Survey. The transmitter is an avalanche transistor pulser which drives a resistively loaded dipole transmitting antenna. A similar, but separate antenna is connected to the receiver. The receiver has adjustable sensitivity time control (STC) of as much as 60 dB to compensate for attenuation and geometric spreading factors. A fiber-optic cable is used to transmit both control signals and data. The data-acquisition and display system incorporates very high-speed digitizing and signal averaging, real-time profile display, and data storage on standard computer nine-track magnetic tape.

The system was successfully used on Ice Stream B in West Antarctica at centre frequencies of 1, 2, 4, 8, and 12.5 MHz. Bottom-return signal-to-noise ratios of more than 40 dB were obtained at 2 MHz through 800 m of ice. Convoluted internal surfaces not related to present bottom topography were resolved within the ice streams and anomalous strong reflections or “bright spots” were identified near the base of the ice. At present, there is no satisfactory glaciological explanation for either of these observations.