Quantifying flow and stress in ice mélange, the world's largest granular material

Tidewater glacier fjords are often filled with a collection of calved icebergs, brash ice, and sea ice. For glaciers with high calving rates, this "mélange" of ice can be jam-packed, so that the flow of ice fragments is mostly determined by granular interactions. In the jammed state, ice mélange has been hypothesized to influence iceberg calving and capsize, dispersion and attenuation of ocean waves, injection of freshwater into fjords, and fjord circulation. However, detailed measurements of ice mélange are lacking due to difficulties in instrumenting remote, ice-choked fjords. Here we characterize the flow and associated stress in ice mélange, using a combination of terrestrial radar data, laboratory experiments, and numerical simulations. We find that, during periods of terminus quiescence, ice mélange experiences laminar flow over timescales of hours to days. The uniform flow fields are bounded by shear margins along fjord walls where force chains between granular icebergs terminate. In addition, the average force per unit width that is transmitted to the glacier terminus, which can exceed 10⁷ N/m, increases exponentially with the mélange length-to-width ratio. These "buttressing" forces are sufficiently high to inhibit the initiation of large-scale calving events, supporting the notion that ice mélange can be viewed as a weak granular ice shelf that transmits stresses from fjord walls back to glacier termini.