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Crevasse Patterns and the Strain-Rate Tensor: a High-Resolution Comparison

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Crevasse Patterns and the Strain-Rate Tensor: a High-Resolution Comparison University of Montana ScholarWorks at University of Montana Geosciences Faculty Publications Geosciences 1998 Crevasse patterns and the strain-rate tensor: a high-resolution comparison Joel T. Harper University of Montana - Missoula, [email protected] Neil Humphrey University of Wyoming W. Tad Pfeffer University of Colorado Boulder Follow this and additional works at: https://scholarworks.umt.edu/geosci_pubs Part of the Glaciology Commons Let us know how access to this document benefits ou.y Recommended Citation Harper, Joel T.; Humphrey, Neil; and Pfeffer, W. Tad, "Crevasse patterns and the strain-rate tensor: a high- resolution comparison" (1998). Geosciences Faculty Publications. 37. https://scholarworks.umt.edu/geosci_pubs/37 This Article is brought to you for free and open access by the Geosciences at ScholarWorks at University of Montana. It has been accepted for inclusion in Geosciences Faculty Publications by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. JournalojGlaciology, Vo!. 44, No. 146, 1998 Crevasse patterns and the strain-rate tensor: a high-resolution comparison 2 J. T. HARPER,i N. F. HUMPHREY,I W. T. PFEFFER IDepartment ojGeology and Geophysics, University ojT!lYoming, Laramie, TiJYoming 82071, US.A. 2Jnstitute oj Arctic and Alpine Research, University ojColorado, Bouldel; Colorado 80309, US.A. ABSTRACT. Values of the strain-rate tensor represented at a 20 m length scale a re found to ex plain the pattern and orientation of crevasses in a 0.13 km 2 reach of Worthing­ ton Glacier, Alaska, U.S.A. The flow fi eld of the reach is constructed from surveyed dis­ placements of 110 markers spaced 20- 30 m apart. A velocity gradient m ethod is then used to calcul ate values of the principal strain-rate axes at the nodes of a 20 m x 20 m ortho­ gonal grid. Crevasses in the study reach are of two types, splaying and transverse, and are everywhere normal to the trajectories of greatest (most tensile) principal strain ra te. Splaying crevasses exist where the longitudinal strain rate (Ex ) is :'S 0 and transverse cre­ vasses are present under longitudinally extending flow (i. e. Ex > 0). The orientation of cre­ vasses changes in the down-glacier direction, but the calculated rota tion by the flow field does not account for this change in orientation. Observations suggest that individual cre­ vasses represent local values of the regional flow field and are transient on the time-scale of 1- 2 years; they are not persistent features that a re translated a nd rotated by flow. Crevasse patterns are thus found to be a useful tool for mapping the strain-rate ten or in this reach of a temperate valley glacier. INTRODUCTION different flow regimes. For example, Nye (1952) described the crevasse patterns that would be expected under various Crevasses, features that are common to virtually all glaciers configurations oflateral shear and longitudinal stress. and ice shee ts, are often interpreted as an indication of ten­ While some observa tional case-studies of crevasses have sional stress in surface layers of ice. For example, the cre­ been m ade (e.g. Nye, 1952; Meier a nd others, 1957; M eier, vasses in the upper reaches of an icefall are commonly 1958; Gunn, 1964; K ehle, 1964; Holdsworth, 1969b; Col beck assumed to represent fas t and extending flow. \tVhile such and Evans, 1971; H ambrey and Mii 11 er, 1978; Hambrey and assumptions are broadly valid, glaciologists often need a others, 1980; Vornberger and Whill ans, 1990), field-based more complete description of the flow fi eld than the currenL testing of theory has been limited a nd results have been in­ understanding of crevasses allows. A better knowledge of conclusive. Previous work has been conducted at a variety of the relationship between crevasses and flow-field dynamics length scales, from measuring the strain fi eld over a few cen­ might enable quick and useful assessments of the surface timeters around a single crevasse (Colbeck and Eva ns, 1971) flow conditions to be made from these features. A rough ap­ to modeling the development of kilometer-long Antarctic praisal of the surface flow field would thus be possible for crevasses (Vornberger and Whill ans, 1990). Most fi eld regions of many glaciers and ice sheets. This paper presents studies have made comparisons of crevasse occurrence with a case-study that compares observations of crevasses and measured values of the principal strain rates. A review of detailed measurements of the flow fi eld as a means ofinves­ nearly all published fi eld measurements by Vaughan (1993) tigating the relationship between crevassing and the strain­ found that the thermal activation energies for creep and fail­ rate tenso r. ure appear to be closely related, although no systematic var­ The problem of crevassing has been addressed by theo­ iation with tensile strength was found. Other workers have retical, laboratory, and field-based observational studies. At suggested a value of 0.01 a I as the critical level of ex tending a small scale, work such as that by Rist and Murrell (1994) strain rate for the formation of new crevasses in temperate and Rist and others (1994) has examined the orientation of ice (e.g. Meier, 1958; Holdsworth, 1969a). This, however, brittle fractures a nd microcracks in stressed polycrystalline has not been well establi shed, as H a mbrey and Mi.iller ice. Various workers have addressed the larger-scale (1978) found new crevasses opening over values ranging problem of crevasse formation in glaciers with force-balance from 0.004 to 0.163 a I. considerati ons. Much of this work has fo cused on the fr ac­ In addition to the uncertainties associated with crevasse ture mechanics controlling the spacing and penetration initiation, other questions remain about the relationship depth of crevasses (e.g. Loewe, 1955; Nye, 1955; Robin, 1974; between crevassing a nd flow dynamics. In particular, rela­ Smith, 1976; \"'eertman, 1977; Nemat-Nasser and others, 1979; tively little research has been directed toward the develop­ Sassolas and others, 1996). Still other work has addressed the ment of crevasse fi elds. Does an array of adjacent crevasses theoretical patterns of crevasse families associated with represent various stages in the life cyele of a crevasse as it is 68 Harper and others: Crevasse patterns and strain -rate tensor translated and rotated through the fl ow fi eld? Or is each center line of the glacier to the north val ley wall. Boreholes crevasse a transient event representing only the stress condi­ drilled to the bed and ice-penetrating radar measurements tions of its current location? Nye (19 83: 75) stated, "the cre­ show that ice thickness ranges from 185 to 210 m (Welch and vasses one observes on a glacier have been carried away others, 1996). Other work on this study reach has addressed from the places where they were originally formed and have the subglacial hydrology (Stone a nd others, 1994), surface been rotated by the fl ow". For this reason, he suggested that velocity variations (H arper and others, 1996) and the char­ the trajectories of principal stress and strain rate may not acteristics of the bed and subsurface ice (Harper and Hum­ necessaril y match the crevasse pattern. H oldsworth phrey, 1995). (1969b) found that a seri es of transverse crevasses on Kaska­ wulsh Glacier, YukonTerritory, Canada, are a train with two METHODS new crevasses formed at a starting zone each year. Further­ more, Vornberger and \l\Th illans (1990) modeled the transla­ Nye (1959) pioneered a procedure for calculating the princi­ tion and rotation of splaying crevasses on Ice Stream B, pal axes of strain rate from measurements of the relative dis­ Antarctica, as individual crevasses that persist for decades placement of velocity markers. The methods for calculating and are transported tens of kilometers. Yet, open crevasses the principal axes used here are an adaptation of this certainly h ave a finite lifetime; as ice moves into a changing approach. A total of 110 point measurements of velocity are stress field existing crevasses become relict, while new ones interpolated to form a ve locity-fi eld grid. Gradients in the open. Rel ict crevasses may p ersist in the new stress regime, velocity fi eld are then used to calculate components of the may disappear by ablation or infilling with snow and refro­ strain-rate tensor at each of the grid nodes. The velocity gra­ zen meltwater, or may close by shear a nd compressional dient method h as the ad\'a ntage of rapid data coll ection and strains. The ques tions that rem ain about the life cycle of cre­ process ing, which enabl es a large sampling of the fl ow field. vasses m ay a t least in part be due to a very limited number of However, the a rea sampled by this meth od is less certain compa ri sons of crevasse fields with high spatial resolution than with techniques for direct measurement of deforma­ measurem ents of the flow field. tion wit hin "stra in elements". In this study the problem is addressed with a large obse r­ vational data set defining the surface flow fi eld at a short Velocities length scale (10- 20 m). This allows the strain-rate tensor to Surface \"C locities were measured during two summers by be calculated on a grid of densel y spaced points with hori­ repeated surveys of an array of stakes using a tota l station zontal resolution approximately equal to one-fifth of the ice theodoli te. In the first year, 46 sta kes were pl aced in the depth.
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