Showing papers in "Journal of Glaciology in 1968"

General theory of subglacial cavitation and sliding of temperate glaciers

Abstract: Earlier theories of Weertman and the present author are reviewed and compared; both are insufficient to account for the facts observed at the tongue of the Allalingletscher. A calculation of the stresses and heat flow at the bed of a glacier with a sinusoidal profile is given which takes account of any degree of subglacial cavitation. The sliding due to plasticity and that due to pressure melting are related to this degree of cavitation and it is shown that these two terms are additive. There results an expression for the friction f ω in terms of the total sliding velocity u and the height of the bumps a. For a given and large enough value of u, f ω (a) exhibits two maxima which are equal and independent of u. The paper then considers a more realistic model of the bed consisting of a superposition of sine waves all having the same roughness r, and a decreasing in a geometrical progression. The biggest a may be inferred from the overall profile of the bedrock; the resulting frictional force can be regarded either as part of the total frictional force f in an overall view for which f = ρgh sin α holds, or else as a correction to such a value on the small scale (the best point of view for crevasse studies). To a first approximation Coulomb’s law of friction holds provided one takes account of the interstitial water pressure at the ice-rock interface. This interstitial pressure p is next related to the thickness of the glacier h. If the subglacial hydraulic system is at atmospheric pressure, p is proportional to h. Next, if the sliding velocity is not too large, the surface slope approaches 1.6r ≈ 0.12 and kinematic waves (which move four times as fast as the ice) disappear rapidly. If the hydraulic system is not at atmospheric pressure the surface slope is smaller and flow instabilities can occur.

On the calving of ice from floating glaciers and ice shelves

Abstract: The deformation and the state of stress in the frontal part of a floating glacier is analysed by a method analogous with the beam theory, applied in engineering practice for determining stresses and deflections of a beans of an elastic material. Very rough approximations are made, the most severe being that of assuming the viscosity of the ice constant. Curves showing the progress in time of the deflections and the stresses in the frontal part of the glacier are given for the case of an infinitely wide glacier. The curves show, that the stresses are greatest at a cross-section situated at a distance of about the thickness of the glacier from the front, and that the stresses are of a magnitude which very likely will lead to fracture, resulting in the formation of an iceberg. It is shown that the magnitude of the icebergs as well as the frequency of the calving is a function of the thickness, the density, and the temperature of the glacier. Observations from nature supporting the theory are described. Finally other calving mechanisms for floating glaciers are briefly discussed.

The formation and shape of drumlins and their distribution and orientation in drumlin fields

Abstract: If glacial till contains more than a certain minimum boulder content, it is dilatant and requires a much larger stress to initiate shear deformation than to sustain it. If the stress level at the glacier–terrain interface drops below a certain critical level, or the till reaches its critical boulder-content density, then the till beneath the glacier packs into stable obstructions. These are shaped into streamlined forms by the glacier and are found distributed at random in drumlin fields. Due to drumlin coalescence there is a normal distribution of drumlin axes about the direction of ice movement.

Recent Glacial History of an Alpine Area in the Colorado Front Range, U.S.A.: II. Dating the Glacial Deposits

Abstract: Recent glacial deposits in the Indian Peaks area of the Colorado Front Range have been dated lichenometricaily, using a growth curve developed locally for Rhizocarpon geographicum. Radiocarbon dates, where available, tend to support the lichen chronology. Three distinct intervals of glaciation, each consisting of several minor pulsations, have occurred in the area during the past 4500 years. The earliest advance (Temple Lake Stade) is dated at 2500–700 b.c. A later advance (Arikaree Stade) began in about a.d. 100 and ended in a.d. 1000. The most recent advance (Gannett Peak Stade) is dated at a.d. 1650–1850. It remains to be seen whether the Arikaree Stade was purely a local development or whether glaciers were advancing elsewhere in the cordilleran region during this interval. Alluviation on the plains east of the Colorado Front Range seems to have occurred during the waning stages of mountain glaciation.

Glacier variation and weather

Abstract: Statistical results of measurements of glacier variations in Switzerland 1891–1965 are compared with deviations from average temperature and precipitation in summer for stations above 2000 m. The result is interpreted in terms of meso-scale glacier–weather relations. A correlation between glacier mass budget and deviation from the average height of the 500 mbar surface indicates the importance of cyclonic and anticyclonic conditions during the ablation period, and therefore of the frequency of certain Grosswetterlagen which has been studied for the period 1881–1965. The pattern of the difference of Grosswetterlagen favourable and unfavourable for glaciers resembles the pattern of glacier variations and also of mass-budget data. Atmospheric circulation in zonal and meridional forms, as expressed by the frequency of Grosswetterlagen has to be considered by seasons. There is no simple relation between the intensity of zonal circulation and glacier variation. Glacier recession of the last few decades seems to be caused by the fact that meso-scale weather conditions mainly in the ablation season have become much less cyclonic than in the decades 1886–95 and 1906–15.

Coastal Geomorphology, McMurdo Sound, Antarctica

Abstract: Well-developed elevated beaches. deltaic deposits, marine-boulder pavements and wave-washed bedrock surfaces are found from Cape Bernacchi north to Granite Harbour. The highest measured marine feature, an elevated beach at Dunlop Island, is 20 m (67 ft) above sea-level. The highest beaches at Marble Point and Cape Roberts, about 48 km (30 miles) apart, are about 20 m (66 ft) above sea-level. The marine limit between these two points is, therefore, essentially horizontal. The highest beach at Cape Bernacchi. approximately 4.8 km (3 miles) south of Marble Point, is about 12 m (40 ft) above sea-level, Well-developed elevated beaches disappear about 3.2 km (2 miles) south of Cape Bernacchi and are not found between this point and Koettlitz Glacier. These beaches post-date the youngest glaciation recognized in the lower Wright Valley. A 14C analysis of an elephant seal buried in a 13 m (44 ft) beach at Marble Point indicates that this beach is 4450 ± 150 years old. As sea-level at this time was approximately 3 m (10 ft) lower than at present. the Marble Point area has risen isostatically about 16 m (54 ft) during the last 4450 ± 150 years. Pitted beaches, beaches deposited on ice, a buried elephant seal and gravel ridges deposited by ice indicate that all of the beaches were formed in a climate like that now found in the area.

Bubbles and bubble pressures in Antarctic glacier ice

Abstract: Application of the gas law to fourth-place density measurements of ice samples from two deep drill holes at “Byrd” station and “Little America V”, Antarctica, shows that virtually all density increase beyond the pore close-off density (0.830 g cm−3) can be attributed to compression of the entrapped bubbles of air. Data from “Byrd” station also indicate that the lag between overburden pressure and bubble pressure, initially 4–5 kg cm−2 at pore close-off, diminishes to less than 1.0 kg cm−2 at about 200 m depth. By substituting the overburden pressure for the bubble pressure in the pressure-density relationship based on the gas law, one can determine ice densities below 200 m more accurately than they can be measured per se on cores, because of the relaxation that occurs in samples recovered from high confining pressures. This relaxation, resulting in a progressive increase in the bulk volume of the ice with time, is generally attributed to decompression of the entrapped air bubbles following removal of the ice from high confining pressures. However. calculations of the stress in ice due to bubble pressure, together with measurements of bubble sizes in cores from various depths at “Byrd” station, both tend to indicate that there has been negligible decompression of the inclosed bubbles. It is suggested that most of this relaxation may be due to the formation of micro-cracks in the ice. Anomalous bubble pressure–density relations at “Little America V” tend to confirm abundant petrographic evidence of the existence of considerable deformation in the upper part of the Ross Ice Shelf. Studies of crystal–bubble relations at “Byrd” station revealed that the concentration of bubbles in ice remains remarkably constant at approximately 220 bubbles/cm3. Bubbles and crystals were found to be present in approximately equal numbers at pore close-off at 64 m depth, at which level the average bubble diameter was 0·95 mm, decreasing to 0.49 mm at 116 m and to 0·33 mm at 279 m. Despite a ten-fold increase in the size of crystals between 64 and 279 m, the bubbles showed no tendency to migrate to grain boundaries during recrystallization of the ice. The observation that most of the bubbles had assumed substantially spherical shapes by 120 m depth points to essentially hydrostatic conditions in the upper layers of the ice sheet at “Byrd” station.

Hexagonal and Cubic Ice at Low Temperatures

Abstract: The formation of hexagonal and cubic forms of ice was studied by the use of a cold stage in an electron microscope within the temperature range −90° to −180° C. Ice crystal specimens were made on cold substrates, i.e. a collodion film, gold foil, or copper grid on the specimen holder of the cold stage. The hexagonal form of ice formed on the cold substrates at temperatures from−90° to−100° C. At −100° to −130° C, both hexagonal and cubic forms of ice were detected. From −130° to −160° C only cubic ice was found. At temperatures below −160° C, minute crystals of cubic ice were detected. No transformation of the structural form of ice from hexagonal to cubic or from cubic to hexagonal occurred when the temperature of the specimens was varied in the range −90° to −160° C. The lattice constants of hexagonal and cubic ice, and the coefficient of thermal expansion of ice were calculated from the experimental results.

Diffusion Law for the Dispersion of Hard Particles in an Ice Matrix that Undergoes Simple Shear Deformation

Abstract: A diffusion equation is obtained that describes the mechanical dispersion of a dilute mixture of solid particles within an ice matrix that is undergoing deformation. It is shown that within the limits of time intervals and strain-rates appropriate to the movement of glaciers and ice sheets, the dispersal distance usually is no larger than a distance about one order of magnitude greater than the size of the particles themselves.

Determination of the rate of snow accumulation at the pole of relative inaccessibility, eastern Antarctica: a comparison of glaciological and isotopic methods

Abstract: The rate of snow accumulation has been measured at the USSR station situated at the Pole of Relative Inaccessibility (eastern Antarctic plateau) Five independent methods were used The results, expressed in g cm−2 year−1, are as follows: (1) Accumulation-stake measurements show an accumulation of 3·1±0·5 in 1965 A maximum value of 3·6 for the 6–year interval 1959–65 is derived from snow-thickness measurements (2) Firn stratigraphy was found difficult to interpret in terms of annual layers An upper limit of 12 g cm−2 year−1 is suggested (3) 18O/16O ratios were measured along the upper 235 cm Assuming that each maximum in the isotope profile corresponds to a summer season, the accumulation would be 8·6±0·9 (4) Gross β radioactivity as a function of depth was measured on three firn sections The 1955 radioactive fall-out horizon is clearly marked, enabling the derivation of an average accumulation of 3·0±0·3 (5) The average accumulation over the last 80 years, derived from the 210Pb distribution with depth, is 3·1±0·3 A multi-annual average accumulation rate of (3·0±0·3) is thus firmly established, as well as the validity of both the 210Pb method and the 1955 radioactive fall-out horizon method Stratigraphie studies lead to uncertain results Oxygen isotope variations give accumulation values which are too high by a factor of two at least Accumulation values of the same order of magnitude (2·8±0·2) were derived from radioactive fall-out measurements at the US “Plateau” station

The complexities of the three-dimensional shape of individual crystals in glacier ice

Abstract: A block of ice from Blue Glacier. Washington, was successively sectioned at close intervals as a means of determining exact crystal shape in three dimensions. Two crystals occupying over 20 per cent of the entire sample are used as examples, and their volumes calculated. These crystals were found to be much larger and more complex in shape than expected from thin-section examination alone. The surface-to-volume ratio was calculated using a simple relationship between the length of lines in a grid crossing the crystal and grid-line intercepts with the crystal boundary. From this ratio the surface area of each crystal was calculated. A measurement of irregularity or jaggedness is introduced in order to compare crystals of different size with one another. This is necessary because surface-to-volume ratio of a body of the same shape decreases as the size increases.

Sub-Surface Melt Pools in the McMurdo Ice Shelf, Antarctica

Abstract: Sub-surface melt pools were discovered during construction of an airfield located on glacier ice in the western part of the McMurdo Ice Shelf, Antarctica. The melt pools occur beneath areas of blue glacier ice and they are impossible to detect by visual examination. They vary in size and shape but are usually 1.0 to 1.5 m deep and span circular areas 10 to 15 m in diameter. Sub-surface melting starts in mid-December at depths of 40 cm or more and it progresses until late January when refreezing begins. The ice cover over melt pools may thin to as little as 7 cm and this creates a serious hazard to aircraft operations. The melt pools are caused by the greenhouse effect of intense solar radiation, low albedo of the blue glacier ice and heat absorption by rock particles and dust. The high-albedo layer of chipped ice and powdered ice that was produced during runway construction was completely successful in preventing sub-surface melting. The thickness of this protective layer appeared to be of little importance, providing it exceeded 3 cm.

Snow Accumulation Studies on the Thule Peninsula, Greenland

Abstract: Data from stake measurements, marker boards and pits along a 136 km trail crossing the Thule peninsula sector of the Greenland ice sheet have been used to determine both the regional and local distribution of snow accumulation, On a regional scale trend surfaces of mean annual accumulation can be adequately predicted from a model using distance from moisture source and elevation as independent parameters. A series of step- or wave-like features break the smooth profile of the ice. sheet and cause profound changes in accumulation rates on a local scale. The accumulation pattern over these features can be predicted from surface slope and departure from regional elevation. Profiles of’ surface and subsurface topography indicate a direct relationship between subsurface hills and step-like features, but cannot be quantitatively accounted for by existing ice-flow theory. Detailed accumulation studies in conjunction with a program of spirit leveling in the vicinity of Camp Century has revealed the development a shallow valley-like feature. Within this feature accumulation rates have increased indicating that it is the result of flow phenomena.

Slip in single crystals of ice

Abstract: Rectangula r spec imens of ice (c. 5 cm X 2 cm X 0.5 cm ) were cut from la rge sing le crysta ls (c. 10 cm X 5 cm ' ) grown from pure water by a modified Bridgman technique. vVhen these specimens were d eformed under controlled conditions, sli p lines wh ich were predominantly para lle l to the basa l pla ne became visible. In some cases short , perpendicu la r segments were a lso seen which can be interpreted as ev idence for cross-slip in ice. M easurenlents of slip-band spacings \\Nere nlade o n sil vered \" fornlva r \" replicas of some d eformed crysta ls. These measu rements showed tha t \"coarse\" slip occurred when the reso lved shea r st ress on the basa l plane, a , was greater than about 0 . 2 ba rs, and that the average thi ckness of the slip la mellae. d (cm ) was approximately g iven by Wakaha ma's relationship. (a o.2 ) d = 0-4-5 X 10 3. At lower stresses \"fine\" slip occurred, and the relationshi p between the average thickness of the lamellae and the resolved shea r stress was more adequately described by Tay lor 's formula, ad = 7.2 X 10 ' . I t is. however. possible that both coarse a nd fi ne slip occurred at higher stresses, but that the fine slip was then below the limi t of resolution. R ESUME. Glissement dans des monocrislallx de gLace. Des echant illons de g lace recta ngula ires (environ 5 X 2 X O.5 e lll ) ont ete cou pes dans de la rges tllonoeristaux (environ 10 enl X 5 C111 2) iss us d ' une ea u pure par la technique de Bridgman modifiee. Lorsque ces echan ti ll ons eta ient c1eformes da ns c1 es condi t ions contro lees. des lignes de glissement principalement pa ra ll eles au plan de base devenaient visibles. Dans quelques cas , des segments cou rts perpendicu la ires devena ien t a uss i visibles ce qui peut et re interprete comm e I'evidence d'un glissement devie dans la glace . Des mesures des espacements entre les ba ndes de glissement ont ete [aites su r des repliques a rgentees \" formvar\" de quelques crista ux deformcs. ees mesures ont montre que le \" gros\" g lissement a lieu lorsq ue la tension de cisa ill ement su r le p lan de base a cta it plus grand e que 0.2 bar, et que I'epa isseu r moyenne de la lamelle de glissemenl d (cm) eta it approximativement don nee par la rela tion de Wakahama (a O,2) d = 0-4-5 X 10 3. Pour des cont ra intes p lus fa ibl es , un gli ssement \"' fin \" ava il li eu. et la relat ion entre I'epa isseur moyenne de la lamelle et de la contra inte de cisa ill ement eta it mieux c1ecrite pa r la fOl'mu le de Taylor ad = 7 .2 X 10 ' . II est cependa nt possible que les c1eux gli ssements \" fin \" et \" gros\" ont lieu pour des contra intes p lus fortes . mais que le glissement \"fin \" cta it clans ce cas en-dessous c1 e la Iimite de resolution. ZUSAMMENFASSUNC. Gleilen in Einkrislallen von Eis. Aus grossen E is-E inkrista llen ( 10 cm X 5 cm ') . die aus reinem \\\"'asser nach ein er mod ifizierten Bridgman-Technik gezogen worden wa ren. wurden quaderformige E ispro ben ( 5 cm X 2 cm X 0 .5 cm ) geschnitten. Bei kontrollierter Verformung dieser Proben wurden Gleitlinien sichtba r , di e vorwiegend pa ra ll el ZlIr Grund Aache verliefen. In einigen Fa llen wLIl·den kurze Tei lli nien senkrecht dazu beobachtct. d ie a ls Beweis fllr ein Quergleiten d es Eises geclcutet werclen konnen. Die Abstande der G leitlinien wurclen in versilberten \" Form var\" -Nachbildllngen von e inigcn dcformier ten Krista llen ausgcl1lessen. Diese M essungen zeigten class \"gro bes\" G leiten a uftrat. wenn die wirksame Scherspannung a au[ die G rundAache grosser a ls etwa 0,2 bar war. und dass die mittlere Dicke d (cm) der G leitschichten annahernd del' Bez iehung von \\ \"'a ka ha ma (a o.2) d = 0-45 X 10 ) en tsprach . Bei nied rigeren Spann ungen trat \" fein es\" G leiten ein ; di e Beziehung zwischen mi tt lerer La mell endicke und Scherspa nnung wird dann besser durch T ay lor's Formel ad = 7.2 X 10 5 beschri eben . Es ist j edoch moglich. dass sowohl gro bes a ls auch fcines Gle ilen bei hoheren Spa nnllngen au ftrat. dass a ber dann das f\"e in e G leiten unterhalb der Aufl osungsgrenze lag.

Stress-generated ice crystals in a nearly isothermal two-phase system

Abstract: The use of the two-phase system ice–water to illustrate the effects on equilibrium of both hydrostatic and non-hydrostatic stresses has been a recurrent theme in the history of thermodynamic theory. The effects of hydrostatic pressure on the melting point of ice are firmly established by theory and experiment. Those of non-hydrostatic stress are still a subject of debate today; several theorists have predicted ice re-crystallization under such stress, but the magnitude of any slight melting-point depression is not known with certainty. The recrystallization of ice caused by local variations in hydrostatic stress was predicted and experimentally confirmed over a century ago. Cavities deep within temperate glaciers provide a suitable environment for the occurrence of this latter phenomenon. A water-filled cavity intersected by a tunnel in nearly stagnant ice of the Blue Glacier, Washington State, U.S.A., was lined with large and unusual single ice crystals which apparently owe their origin to the effects of hydrostatic stress. Even the minute differences in pressure melting point around this cavity are adequate to remove the heat of fusion as ice forms within it. There is evidence that interstitial movement of melt water in the surrounding ice also contributes to the heat and mass transfer. The form of these crystals indicates that they grew into slightly supercooled water. It is suggested that this growth pattern is sustained by the existence of oriented stresses at the cavity walls.

Instruments and Methods: A Buoyancy-Stabilized Hot-Point Drill for Glacier Studies

Abstract: Hot point drills are practical tools for penetrating glaciers for ice thickness and temperature measurements and other glaciological studies. Buoyancy stabil ization ensures a vertical attitude of the drill and a plumb hole using a heavy hot point and a light upper section which floats in the surrounding melt water. The buoyant force is less than the weight of the drill in air but its rectifying moment about the fulcrum (the tip) is greater than the tilting moment of the drill weight. Two methods to prevent refreezing of the melt water are proposed to permit drilling in cold ice and to assure continued access to the hole. Ill