Strain-rate estimates for crevasse formation at an alpine ice divide: Mount Hunter, Alaska Seth CAMPBELL, 1,2 Samuel ROY, 1 Karl KREUTZ, 1 Steven A. ARCONE, 2 Erich C. OSTERBERG, 3 Peter KOONS 1 1 University of Maine, Orono, ME, USA E-mail: seth.campbell@umit.maine.edu 2 US Army Cold Regions Research and Engineering Laboratory (CRREL), Hanover, NH, USA 3 Department of Earth Sciences, Dartmouth College, Hanover, NH, USA ABSTRACT. Crevasse initiation is linked to strain rates that range over three orders of magnitude (0.001 and 0.163 a –1 ) as a result of the temperature-dependent nonlinear rheological properties of ice and from water and debris inclusions. Here we discuss a small cold glacier that contains buried crevasses at and near an ice divide. Surface-conformable stratigraphy, the glacier’s small size, and cold temperatures argue for limited rheological variability at this site. Surface ice-flow velocities of (1.2–15.5)  0.472 m a –1 imply classic saddle flow surrounding the ice divide. Numerical models that incorporate field-observed boundary conditions suggest extensional strain rates of 0.003–0.015 a –1 , which fall within the published estimates required for crevasse initiation. The occurrence of one crevasse beginning at 50m depth that appears to penetrate close to the bed suggests that it formed at depth. Field data and numerical models indicate that a higher interior stress at this crevasse location may be associated with steep convex bed topography; however, the dynamics that caused its formation are not entirely clear. INTRODUCTION Because of the significant information crevasses can provide regarding glacier flow dynamics, an increased understand- ing of their evolution is important. However, crevasse dynamics are difficult to quantify due to numerous variables which affect their formation. Crevasses form when a yield tensile stress or critical strain rate is reached. Early research suggested critical strain rates of 0.01 a –1 are required in extending flow to form surface crevasses in temperate ice (Holdsworth, 1969), while Meier (1958) suggested that crevasses in temperate ice and firn seem to form at lower strain rates than those required in cold firn. Hambrey and Mu ¨ller (1978) found new crevasses opening over strain rates ranging from 0.004 to 0.163 a –1 . Other studies (Kehle, 1964; Vaughan, 1993) used the concept of a principal stress at the ice surface exceeding a critical value to cause crevasse formation. Consistent with Meier’s suggestion, Vaughan (1993) found that 90–320kPa are needed to cause crevasses in both cold and temperate glaciers while Forster and others (1999) found 169–224kPa are needed in temperate ice. However, Vaughan (1993) found no systematic relationship between firn temperatures and critical stresses, contrary to Meier’s (1958) original suggestion that cold firn or ice has a greater tensile strength than warmer firn and ice. Tensile strength, along with elastic properties of ice, are heavily temperature-dependent; however, Vaughan (1993) sug- gested that tensile strength also depends on density, bed and surface topography, ice depth, impurities (e.g. water or debris) and crystallography. The range of stresses, strains, nonlinear rheological properties, thermal regimes, geomet- ric boundary conditions and the physical condition of firn and ice challenge the efficacy of crevasse evolution models in complex glaciers. Here we contribute to the published strain-rate estimates required for crevasse formation within polar (cold) glaciers using numerical models constrained by field-measured surface strain rates of a relatively simple glacier. We consider a small, cold, alpine glacier located high in the Alaska Range, USA. Its extent is well defined, it contains minimal debris and water and it likely exhibits relatively constant annual velocities, alleviating some material prop- erty and associated modeling complexities. The lack of debris and water content was determined from observations made during two field seasons that included shallow firn-core samples collected in 2010 and snow-pit samples collected in 2011 (Campbell and others, 2012a). Our observations and GPS data confirmed this was an ice divide, while ground-penetrating radar (GPR) profiles revealed minimal deformation within surface-conformable stratigraphy (SCS). Most importantly the GPR profiles revealed buried crevasses at and immediately adjacent to the ice divide, which eliminated the possibility that the crevasses formed up- glacier. In addition, we expected minimal lateral variability in ice rheology due to the small dimensions of the site. This small saddle-shaped glacier is bounded by severe icefalls only 500 m from the divide, and the ice is likely frozen to the bed because of the low annual temperatures. Given all these constraints, our objectives were to determine the strain rates required to cause these crevasses and to find the controls on their formation. If a finite-element model with reasonable assumptions for rheological control (e.g. temperature) can reproduce field-observed velocities then it may be used to explore factors that control strain rate. We used a three-dimensional (3-D) finite-element numerical model with material properties and boundary conditions from field data to produce strain-rate and stress distributions that may lead to crevasse formation. We used a surface DEM and GPR profiles collected over the glacier to constrain model geometry (e.g. surface and basal topography). A unique feature of this site is that our GPR data also reveal that the ice-divide crevasse is buried at 50 m depth and may reach the bed. This depth suggests that these particular crevasses did not form at the surface, and so indicates maximum tensile stresses or strain rates within the Annals of Glaciology 54(63) 2013 doi: 10.3189/2013AoG63A266 200 https://doi.org/10.3189/2013AoG63A266 Published online by Cambridge University Press