HR: 13:40h
AN: C12B-01 INVITED     [PDF]
TI: What lies beneath? The question of sub-surface crevasses
AU: * Vaughan, D G
EM: d.vaughan@bas.ac.uk
AF: British Antarctic Survey, Madingley Road, Cambridge, CB3 0ET United Kingdom
AU: Nath, C
EM: pcna@bas.ac.uk
AF: British Antarctic Survey, Madingley Road, Cambridge, CB3 0ET United Kingdom
AB: The process of fracture is a major control on the form of the Antarctic ice sheet, both through the process of iceberg calving and through the isolation of streaming flow by the formation of crevassed shear margins. Furthermore, the presence of crevasses is a valuable diagnostic of stress within the ice sheet. Their persistent visibility after burial provides our primary tool for mapping changes in ice stream activity. However, the processes of fracture initiation and growth are poorly understood. Hitherto it has been a fundamental assumption that crevasses always initiate at the surface and propagate to some limiting depth. We revisit the linear elastic fracture mechanics (LEFM) description of the initiation and critical-growth of near-surface crevasses to examine this assumption. We assume growth of an isolated crevasse from a small "starter crack", driven by tensile stress resulting from glacial deformation and opposed by weight-induced lithostatic stress. The LEFM approach allows us to estimate the minimum length of "starter crack" required for growth to occur. In this formulation the dynamic tensile stress varies substantially with depth, such that the tensile strain rate remains constant. We show that under these conditions crevasse initiation may be from centimetre-scale starter cracks located either at the surface or at depths of 10-30 m. At present, the formulation of a reliable predictive model is limited by an incomplete knowledge of the mechanical properties of firn and a lack of material evidence to support the existence of "starter cracks". Ground penetrating radar data acquired on Rutford Ice Stream, Antarctica show crevasses at depths of several metres beneath the ice surface that do not appear to have been bridged in the normal way, and which must have been formed either at depth, or at the surface in areas where no surface crevassing is currently visible. The theoretical treatment together with apparent observations of sub-surface crevassing calls for a reinvestigation of the processes of fracture to re-establish the validity of assuming that the depth of a crevasse is a reliable indication of its age.
DE: 1827 Glaciology (1863)
DE: 1863 Snow and ice (1827)
DE: 4540 Ice mechanics and air/sea/ice exchange processes
DE: 5104 Fracture and flow
DE: 6020 Ice
SC: Cryosphere [C]
MN: 2003 Fall Meeting