Ice mélange and the rheology of floating granular materials*

Ice mélange is the Earth's largest granular material. Formed by broken icebergs created by glacial calving events in narrow straits or fjords, ice mélange can influence the dynamics of tidewater glaciers and thus the mass loss rate of surface ice into the oceans. Fundamentally, ice mélange is a floating granular material that jams as it is pushed through a frictional channel. Our scaled-down laboratory experiments examine its rheology, transverse velocity profiles, and thickness profiles. Using polypropylene (which has a density close to that of ice), we tested spheres, rectangular prisms, and collections of irregular shaped solids. We placed them in a narrow water tank with rough walls to simulate a fjord. The particles are slowly pushed by a "glacier terminus" composed of a suspended acrylic plate attached to force sensors. Using two cameras (one above and one on the side), we can calculate the surface velocity field using particle image velocimetry. The force on the terminus sharply increases when friction between particles and the tank walls causes the mélange to jam and buckle. Our preliminary results demonstrate that the force on a glacier terminus can vary by 10-50% between jamming events.