Dept. of Geosciences Colloquium: Ice rifts and the fingering of radially-spreading extensional flows

Abstract:

Ice sheets have the capacity to induce a significant change in sea level and in global climate. At present the Antarctic ice sheet, which holds about 90% of the total ice mass, is being eroded mostly along its ice shelves, where the ice interface with the ocean, through two major mechanisms: melting and ice-berg calving. These processes can be enhanced through the emergence of ice rifts, massive fractures that extend the ice/ocean interface prone to melting and may trigger calving. 

We investigate how ice rifts form and evolve by modelling the deformation of ice shelves as the spreading of a thin film of viscous fluid that displaces a denser and relatively inviscid fluid.  The circular front of such a displacement by a more viscous fluid is known to be stable in sheardominated
flows. This is the case when the viscous fluid is confined between two parallel plates (Hele-Shaw geometry), and when it has a free surface as in viscous gravity currents propagating on horizontal planes.

In a set of laboratory experiments we show that a similar front of a gravity current of strainrate- softening fluid that is predominantly extensional can become unstable and evolve tearing patterns consisting of rifts and tongues. These patterns, which do not emerge in shear-dominated flows, tend to coarsen over time through the closure of some rifts and the merging of adjacent tongues. We model the emergence of these patterns as a fluid mechanical instability, which predicts that the number of rifts and tongues declines with time and is selected by competition between interfacial hoop stress, geometric stretching, momentum dissipation and spatial curvature. We find that the model predictions are consistent with some of the experimental observations. We also show that such an instability can persist on spherical surfaces, which has potential implications to ice sheets of a planetary scale such as those present during snowball epochs and on icy moons.

Event Organizers: Dr. Roy Barkan and Dr. Asaf Inbal