Showing papers in "Journal of Glaciology in 1976"

Water Flow in Glaciers: Jökulhlaups, Tunnels and Veins

Abstract: The physics of water flow within and under glacier ice is examined with special reference to the periodic catastrophic outbursts of water (jokulhlaups) from the subglacial lake Grimsvotn, Vatnajokull, Iceland. The lake is sealed until it reaches a critical level which enables it to lift the glacier, helped by a hydrostatic cantilever effect. The differential equations for non-steady water flow in a subglacial tunnel are derived and applied to the 1972 Grimsvotn outburst. The discharge: time relation observed during the growth stage, and the abrupt ending of the flood, are both very well accounted for by a theory which is insensitive to the details of the subglacial tunnel system. The steady state, in which an intergranular vein or tunnel is simultaneously melted open by frictional heat and closed by plastic deformation, may be stable or unstable according to the conditions imposed at the ends. This explains why the flow of water in a vein does not normally increase unstably as in a jokulhlaup. An ice-dammed lake does not drain away through the vein system because the driving force on the vein-water is towards the lake rather than away from it.

Microwave emission from snow and glacier ice

Abstract: The microwave brightness temperature for snow fields was studied assuming that the snow cover consists of closely packed scattering spheres which do not interact coherently. The Mie scattering theory was used to compute the volume scattering albedo. It is shown that in the wavelength range from 0.8 to 2.8 cm, most of the micro-radiation emanates from a layer 10 meters or less in thickness. It is concluded that it is possible to determine snow accumulation rates as well as near-surface temperature.

The Origin of Glacially Fluted Surfaces-Observations and Theory

Abstract: Studies of Auted surfaces beyond the margins of glaciers in Spitsbergen, Iceland, Norway and the Alps show that almost all emanate from rigid obstructions, commonly boulders in till. Field relations of Autes are described and it is shown that a relati onship exists between Aute height and the height of the initiating obst ruction. Subglacial observations indicate that Autes form when till is intruded into tunnels which tend to open up in the lee of obstacles. The pattern of strain implied by this process is shown to be reAected by micro- and macrofabrics in the till. The commonly found occurrence of an average spacing between Autes does not arise because of some rhythmic or periodic mechanism in the glacier, but is produced by the random seeding of boulders which themselves generate Autes. It is suggested that the term Aute be used as a genetic rather than a descriptive term, and be restricted to long parallel-sided ridges which reAect accurately the direction of ice movement and which form when deformable subglacial materials are intruded into tunnels which tend to open up on the lee sides of single

Radio-echo sounding of temperate glaciers: Ice properties and sounder design criteria

Abstract: This is the first paper in a two-part series which d escribes the design, opera tion, and tes ting of a successful 5 MHz radio-echo sounder for tempera te glaciers. This pa rt dea ls with the e lec tromagne ti c charac teristi cs of tempera te glaciers a t radio frequencies. Earlier workers' problems in sounding through tempera te ice are explained in terms of electroma gnetic sca ttering by water-filled vo ids. The frequency dependen ce of the scattering indicates tha t returns from sca tterers diminish rapidly a t frequencies below a bout 10 MHz. A sys tem with the foll owing charac teristics is recommended : a transmitted pulse with a cente r frequency of a bout 5 MHz, dura tion o f a bout I cycle, and a receiver which is untuned and which measures fi e ld intensity ra the r tha n power. Spectral methods for studying the size distribution of sca ltere rs a re p resented . An actua l instrument and fi eld tri a ls will be described in a forthcom ing publication by R . S. Vickers a nd R . Bollen. R ESUME. SOlldages par echo-radio dalls les glaciers temjJCris: propriitis de la glace et critereJ pOllr la cOllstructioll des appareils de sOlldage. Ceci es t la premiere pa rti e d ' une serie en deux a rticl es qui decrit l'appareillage, la mise en oeu vre e t le contra le d ' un a ppareil effi cace a 5 lVIHz de sondage pa r rad io-echo pour d es glac iers temperes. Celle partie tra ite des ca racteristiques elec tro m agnetiques cles g lacie rs temperes a ux frequences radio . Les p ro blemes rencontres pa r les precedents che rcheurs pour lel sondages dans la glace temperee s'exptiquent pa r la di spersion elcctromagnctique cla ns les cavites rempties cl 'eau. L ' inAuenee cle la [req uence sur la dispersion indique que le rayonnement renvoye p a r les d isperseurs diminue ra pidem e nt a ux frequences infe ri eures a environ 10 lVIH z. O n recomma nde un a ppareillage ayan t les ca rac tc ri stiques sui vantes : une impulsion tra nsmise avec un e frequence centra le d 'environ 5 MHz p endant environ un cycle, avec un recepteur qui est desaccordc e t mesure l' intensite du cham p, pluta t que la puissance. On presente d es meth odes spectrometriques pour etud ier la di stribution en dimension des disperseurs. U n instrument recll em ent construit et d es essa is sur le terra in seront d ecrits en deuxi em e parti e pa r R. S. Vickers et R. Bollen . ZUSAMM ENFASSUNG. Radar-Echolotullg temperierter Cletseller : Eiseigellschajlen 1lI/.d Kriterien jllr dell Ball des Lolgerdtes . Es handcl t sich hie r um den ersten T eil einer zweiteiligen Veroffentli chungsfo lge, d ie den Ba u, cl en Einsa tz und die Erprobung eines bra uchba re n 5 MH z-R adar-Echolotes fUr tem p erierle Gle tscher beschre ib t. Diesel' T eil beh a ndclt die elektrom agne ti schen Cha ra kteristiken temperi erten Eises im R adiowellenbere ieh . Die Schwierigkeiten, mit d ene n frUhere Forscher bei d el' Auslotung temperierten E ises zu ka m pfen h a tten, werden dureh die elektromagnetische Streuung a n wassergefUlIten H ohlra umen erkla rt. Die Freque nzabhangigkeit d e l' S treuung lass t e rwarten, cl ass Eehos von Streuern raseh b e i Frequenzen unte r etwa 10 MHz abnehmen. E s wird ein System m it folgenden K enngrossen empfolhe n : Sendepulse mit eine r Zentra lfrcqucnz von e twa 5 MHz, D a uer e twa I Umlauf, E mpfa ngcr ni cht a bgesti m mt, ehef die I ntensita t a ls di e Sta rke d es Feldes n1f'ssend. S pektra le M ethoden z um Stud ium de l' G rossenvertei lung von Streuern werden a ngegeben. E in ausgefUhrtes Cera l und Felda rbeiten werden in T e il JJ von R . S. Vickers und R. Bo ll en beschrieben werden.

Is the Basal Ice of a Temperate Glacier at the Pressure Melting Point

Abstract: Certain aspects of the flow of glaciers suggest that molecular adhesion contributes to basal friction of glaciers sliding at speeds below those of surging glaciers Laboratory experiments indicate that this will only occur if some part of the ice-rock contact is “cold”, that is below the pressure melting point (pmp), a few tenths of a degree being sufficient Field evidence is scanty, but suggests that such cold patches may exist at the base of a temperate glacier Discussion of pressure-melting within the basal ice mass, as distinct from processes at the ice-rock contact, indicate that excess water is formed in zones of high-pressure ice up-stream of obstacles If this water is squeezed out of the ice by the pressure, we have a simple heat pump that will tend to cool the basal ice The ice will warm again as the result of thermal conduction and internal friction, but before reaching the pmp it can produce "cold" patches of the ice-rock contact, roughly estimated to be from 0 1 to 10 m in extent Another factor that could cause intermittent cold patches at the ice-rock interface arises from changes of basal water pressures with time beneath a glacier If a major part of the weight of a glacier is supported by a thin water film at a relatively low pressure and by a small proportion of water film in which pressures are high, then over a large area the water pressures must balance the weight of ice If however the pressure in the low-pressure film rises, the smaller high-pressure areas of stress concentration will suffer a proportionately greater decrease of pressure to maintain the total balance between pressure and weight, If changes take place rapidly, in a matter of hours, then in areas of stress concentration of the order of a metre or more across, the water film will freeze to the bed as stresses are relieved This could cause stick-slip motion in a temperate glacier Pressure-temperature effects at the ice-rock interface can help to explain certain features of glacial erosion, such as intense grinding on the top surface of a roche moutonee and plucking on the down-stream side

Phase Relationships in Sea Ice as a Function of Temperature

Abstract: Quantitative measurements of the liquid water phase in a sample of sea ice were made with a nuclear magnetic resonance spectrometer. The measurements are used to compute the phase relationships in sea ice as a function of temperature. A model for sea-water based upon a mixture of seven binary salts is used for these computations. The n.m.r. measurements are related to the solvation water which is associated with each binary salt. This solvation water is bound to the salt in a pseudo-crystalline structure, with the amount of water determined by the eutectic concentration of the salt. The results are given in tabular form and differ somewhat from previously published tables. Two controversial hydrated salts were added to the table, based on the n.m.r. data.

Climatic Inferences from Former Glaciers in the South-East Grampian Highlands, Scotland

Abstract: Altitudes, lengths, areas, maximum thicknesses and volumes of 27 glaciers that built up in a small area of the Scottish Highlands during the Loch Lomond (Younger Dryas) stadial are given. The influence of direct radiation on the glaciers is calculated taking into account transmissivity of the atmosphere, glacier aspect and surface gradient, and the albedos of ice and snow. Equilibrium firn lines for the glaciers are determined. The computed regional firn line, incorporating the influence of direct radiation had a gradient of 14.5 m/km. Former precipitation values are based on an equation for average annual mass balances of the glaciers during the stadial that incorporates glacier altitude, regional ablation gradient, direct radiation, the influence of avalanching and blowing of snow, and final glacier volume. Average July and January sea-level temperatures of 6°C and at least — 8°C, respectively, are inferred. Snowfall was principally associated with south-east winds and was similar in total to present day but rather differently distributed. Cloud amounts, at least in summer, were greater than now. A more vigorous atmospheric circulation with many depressions moving along more southerly tracks than now may have been related to the junction of polar water and North Atlantic Drift water in the immediate vicinity of the British Isles.

The Bearing Capacity of Floating Ice Plates Subjected to Static or Quasi-Static Loads *

Abstract: This paper contains a critical survey of the literature on the bearing capacity of floating ice plates. It consists of a discussion of general questions, a critical survey of analytical attempts to determine the bearing capacity of floating ice plates and a survey of field and laboratory tests on floating ice plates and their relation to the analytical results. It concludes with a systematic summary of the results, a discussion of observed shortcomings, and suggestions for needed investigations.

Physical processes in temperate glaciers

Abstract: In a temperate glacier ice the water content results from the past history of the ice, and the temperature adjusts itself to the appropriate value. Results obtained by the Laboratoire de Glaciologie are summarized. Several facts seem to prove some migration of the liquid phase and a coalescence of liquid inclusions. The enthalpy flux is calculated, but it cannot be proved that a spontaneous gathering together of liquid inclusions could occur. The water-rich, small grain-size layers should come from the lowering of the viscosity when the water content increases, as discovered by Duval. The form of Weertman’s sliding law without cavitation can be derived from simple dimensional considerations. Numerically, it cannot account for high sliding velocities, for which subglacial cavitation must be introduced. The cavities are infilled with more stagnant regelation ice than water. Two subglacial hydraulic regimes are distinguished: autonomous and interconnected. The effect which the salt content may have on cavitation is estimated.

Thermal Regulation of Glacier Surging

Abstract: A necessary condition for a glacier to surge by thermal instability is that the glacier be cold with basal ice at or near the melting point. Deep temperature measurements show that two small surge-type glaciers in the Yukon Territory meet this requirement, but shallow measurements in three other surge-type glaciers suggest a temperate regime. If the latter observations are accepted, not all surges are thermally controlled: if a single mechanism accounts for all surges it cannot be thermal instability. In this paper it is argued that thermal instability remains a viable mechanism for explaining surges of many cold glaciers, and that regardless of the underlying mechanism, thermal processes must at least have a major influence on the surge behavior of cold glaciers. Two numerical modelling experiments are described. The first involves a one-dimensional model which shows that thermal control can account for the remarkably constant surge cycle found in some glaciers. The second, a two-dimensional model of the time-dependent temperature structure of a surge-type glacier, shows that the relative amounts of temperate and cold basal ice can change considerably as the surge cycle progresses. This variation alone may be sufficient to explain surges, but even if this is not the case, thermal processes must affect the timing of surges in many cold glaciers. A compelling feature of the thermal instability mechanism is that it offers an explanation of the factors controlling the non-random geographical distribution of surge-type glaciers. For a glacier to have a cold surface and near-temperate bed, the ice thickness, temperature, and geothermal flux must be fortuitously related.

The Effect of Subglacial Chemical Processes on Glacier Sliding

Abstract: The glacier sliding theory of Nye is modified to include the effect of solutes in subglacial regelation waters on the sliding process Motivation for this development stems from studies of subglacially formed chemical deposits that appear to be widespread on rock surfaces recently exposed by retreating temperate glaciers These deposits indicate clearly that considerable local concentration of solutes commonly occurs subglacially as a result of the selective rejection of solutes into the melt during the freezing associated with regelation sliding Because solutes accumulate where regelation waters refreeze, they tend to lower the temperature there and hence inhibit the heat transport away from these areas that is essential for regelation sliding For a simple sinusoidal bed and solute distribution in the regelation water film, the modified theory shows that a maximum excess of solutes of, for example, several millimoles/1 of dissolved CaCO3 along lee surfaces relative to stoss surfaces impedes basal sliding significantly, especially if the bed roughness wavelength does not exceed about one meter

Pleistocene Ice at the Base of the Barnes Ice Cap, Baffin Island, N.W.T., Canada

Abstract: Oxygen-isotope ratios indicate that a distinctive band of white ice along the margin of the Barnes Ice Cap is of Pleistocene age. It is estimated freom a flow mogerl that beneath the center of the ice cap the thickness of the band shuuld be abont 0.6 times its thickness at the margin, or about 8 m. However, an ingerpengernt estimate, based on calculated vertical strain-rates and explicitly assuming no basal melting, predicts a thickness of about 22 m beneath the center of the ice cap. The discrepancy between the two thickness estimates is interpreted as indicating that basal melting has occurred. Calculated basal temperatures support this conclusion.

The application of fracture mechanics to the problem of crevasse penetration

Abstract: The elastic stress intensity factor is a parameter used in fracture mechanics to describe stress conditions in the vicinity of the tip of a sharp crack. By superimposing solutions of stress intensity factors for different loading conditions, equations are derived which model crevasses in ice. Solutions are presented for the theoretical depth of isolated crevasses, free from or partially filled with water. Close agreement exists with a previous calculation by Weertman using a different technique. The effect of crevasse spacing is investigated and it is demonstrated that closer spacing always reduces crevasse depth, but over a wide range of spacing the predicted variation in depth is slight.

Glacier Sliding Down an Inclined Wavy Bed

Abstract: The treatments by Nye and Kamb of glacier sliding over a wavy bed with small slope, which assume the ice to be approximated by a Newtonian fluid of high viscosity, are complemented by the inclusion of the glacier depth and the inclination of the bed to the horizontal. The driving force of the motion, gravity, is therefore present in the flow equations and defines immediately the mean drag on the bed. A geothermal heal flux is also included in order to estimate its possible effect on the flow. A complex variable method is used to determine the velocity and temperature fields to second order in the bed slope. These fields satisfy the zero shear traction and pressure-melting-regelation conditions to the same order on the actual bed profile. It is the balance of the second-order term which determines explicitly the (zero order) basal-sliding velocity and surface velocity in terms of the geometry and physical properties of both ice and bed. An explicit solution is illustrated for a sinusoidal bed. and a simple criterion for the onset of cavitation is obtained.

Pleistocene Glaciation in Ethiopia: New Evidence

Abstract: Geomorphological evigernce indicates that Mount Badda, Ethiopia, had a Pleistocene ice cap of at least 140 km2. Altituger of the snow line was probably about 4 000 m, only 350 m below the summit, suggesting that glaciation occurred during the Wurm glacial maximum.

Glacier Sliding Down an Inclined Wavy Bed With Friction

Abstract: The effects of frictional tangential traction combined with regelation on the basal sliding of a temperate glacier down an inclined wavy bed are examined. Two friction models are treated. First, a Coulomb law model having the assumptions that sliding occurs everywhere and that the tangential traction is proportional to the normal pressure. Secondly, a velocity power law in which the tangential traction is proportional to a power of the slip velocity. The ice motion is approximated by steady slow Newtonian flow and the bed undulation about a mean bed-line has a maximum slope ∊ ≪I. Flow solutions are constructed as perturbations (in powers off ∈) of the plane laminar flow corresponding to non-slip on the mean bed-line assuming that the ice remains everywhere in contact with the bed; that is, no cavitation takes place. If the normal traction is predicted to be tensile over part of the bed, implying that cavitation has occurred, then a new solution is needed in which the ice base over cavities is traction-free. Since the cavity sections and profile of the free ice base are then part of the overall solution, an intricate mixed boundary-value problem is set up for the flow and the present analysis is inadequate. For a sinusoidal bed the perfect-slip (zero tangential traction) solution predicts compressive normal traction everywhere on the bed provided that the mean bed-line inclination α (to the horizontal) is less than a critical value α e which is of order e. For greater values of α including a range of order ∊, the normal traction is tensile on some parts of the bed, and a solution with cavitation is needed. If the tensile sections are relatively small it is expected that the resulting cavitation will not change the overall solution significantly. However, the Coulomb friction solution has extensive zones of tensile traction for all values of α, so that extensive cavitation would occur. In contrast, the velocity-power friction solution has compressive traction everywhere on the bed for α ⩽α e = 0 (I) provided that the ice depth is not too large, and also for deep glaciers for α ⩽α e = O ∊ Furthermore, the predicted basal sliding velocity varies much less with the length scale of the bed undulation than in the perfect-slip solution, and is smaller.

Seismic Signals Associated with Avalanches

Abstract: A seismic survey was conducted during the winter of 1974 75 to determine the extent to which signals from avalanches can be detected using standard seismic techniques. The results of this investigation show that signals associated with avalanching snow-packs have characteristic patterns which differentiate them from signals of extraneous origin. The records of several of these signals are presented and discussed.

Resurvey of the "Byrd" station, Antarctica, drill hole

Abstract: The drill hole at “Byrd” station, which was completed in January 1968 to a vertical gerpth of 7 063 ft (2 153 m) below the top of the hole casing, was resurveyed in January 1975 to a vertical gerpth of 4 835 ft (1 474 m)· Inclination and azimuth measurements were mager with a Parsons multiple shoe inclinometer and compared with the earlier measurements mager during drilling. The results indicate a progressively increasing displacement with gerpth to a value of 51.2 ft (15.6 m) or about 7.3 ft/year (2.23 m/year) at the 4 835 ft (1 474 m) level. The direction of movement relative to the surface varies freom south-west at 300 ft (91.5 m) to north-east at 1 100 ft (335 m) to east at 3 368 ft (1 027 m) to north-east at 4835 ft (l 474 m), indicative of a complex twisting motion. An increase in accessible gerpth along the hole axis of 18 ft (549 m) beyond the 1969 gerpth was noted. No attempt was mager to measure hole diameter or vertical strain. It is recommengerd that the hole be resurveyed in 3-5 years if it is still logistically feasible, using a more up-dated inclinometer.

15m Deep Temperatures in the Glaciers of Mont Blanc (French Alps)

Abstract: The top of Mont Blanc is a dry snow zone The cold infiltration zone extends between about 4 300 and 3 800 m Its lower limit is lined by large cracks and ice cliffs, similar to bergschrunds Near rock faces this limit is the bergschrund, which can descend as far as the 0°C isotherm of the mean annual air temperature, 3 100-3 200 m- At Col du Dome ( c , 4 250 m), 15 m deep temperature has increased 18 deg between the years 1911 and 1973, probably due to infiltration which happened there in the last few years The ice in the ablation area is entirely temperate, while in dryer areas of the Alps it may be at 1°C to — 3°C in the vicinity of the firn line

Measured and Computed Temperature Distributions in the Law Dome Ice Cap, Antarctica

Abstract: By 1969 considerable data had been collected over Law Dome from surface measurements These included surface elevation, ice thickness, accumulation rates, ice movement and strain-rates, surface mean temperatures, and several temperature-depth gradients Since then core drilling up to 380 m in depth has been carried out from the summit to the coast where the cores reached close to the bed The bore holes have been concentrated on two main flow lines: one to Cape Folger and the other to Cape Poinsett The bore-hole temperatures give a clear indication of the temperature distribution throughout the ice cap Numerical modelling shows that the measured deep temperatures closely match the steady-state temperature distribution calculated from the present regime The variations in the temperature profiles over the dome are primarily associated with the flow regime The Cape Folger line has low accumulation rates and low velocities which results in positive surface temperature-depth gradients increasing towards the coast By contrast the Cape Poinsett line has high accumulation rates and high velocities which result in a large negative temperature-depth gradient there The profile at the Dome summit is close to isothermal primarily due to the high accumulation rates and lack of horizontal motion