Quasi-Static Granular Flow of Ice Melange

We use Landsat 8 imagery to generate ice melange velocity fields at Greenland's three most productive outlet glaciers: Jakobshavn Isbrae, Helheim Glacier, and Kangerdlugssuaq Glacier. Winter velocity fields are generally steady and highly uniform. Summer velocity fields, on the other hand, tend to be much more variable and can be uniform, compressional, or extensional. We rarely observe compressional flow at Jakobshavn Isbrae or extensional flow at Helheim Glacier, while both are observed at Kangerdlugssuaq Glacier. Transverse velocity profiles from all three locations are suggestive of viscoplastic flow, in which deformation occurs primarily in shear zones along the fjord walls. We analyze the transverse profiles in the context of quasi-static flow using continuum rheologies for granular materials and find that the force per unit width that ice melange exerts on glacier termini increases exponentially with the ice melange length-to-width ratio and the effective coefficient of friction. Our estimates of ice melange resistance are consistent with other independent estimates and suggest that ice melange may be capable of inhibiting iceberg calving events, especially during winter. Moreover, our results provide geophysical-scale support for constitutive relationships for granular materials and suggest a potential avenue for modeling ice melange dynamics with continuum models.