An assessment of surge‐induced crevassing and the formation of crevasse squeeze ridges

Evidence for extensive crevassing is preserved on the deglaciated forelands of many surging glaciers as crevasse squeeze ridges (CSRs). At some point these crevasses make direct connection with the bed in order to become sediment filled, and full‐depth connections have been inferred from turbid water up‐wellings in crevasses and the formation of concertina eskers. The dynamics of seven surging glaciers are assessed, using a linear elastic fracture mechanics approach, to determine the likely directions of fracture and controlling parameters for Mode I crevasses. Extensional surface strain rates are insufficient to promote top‐down full‐depth penetration. For small crevasse spacing (<5 m), surface strain rates are sufficient for top‐down crevassing to depths of 4–12 m, explaining the extensive surface crevassing associated with glacier surging. As has been shown in other settings, top‐down, full‐depth crevassing is only possible when water is added and approaches 97% of the crevasse depth. The provision of sufficient meltwater to facilitate this is problematic due to the extensive surface crevassing, unless water can move along connected crevasses to a dominant water capturing crevasse. For ice thicknesses greater than ∼200 m, basal water pressures in excess of 80–90% of flotation are required for full‐depth, bottom‐up crevassing. Field evidence suggests that this is the default for surging glaciers and that, on occasion, water pressures may even become artesian. CSRs, found across many surging glacier forelands and ice margins, most likely result from the infilling of basal crevasses, driven for the most part, bottom‐up, by high basal water pressures.