Evolution of basal crevasses links ice shelf stability to ocean forcing

Iceberg calving is one of the primary mechanisms responsible for transferring ice from the Antarctic ice shelves to the ocean, but remains poorly understood. Previous theories of calving have sought to explain the calving process as a brittle phenomenon that occurs rapidly when surface or basal crevasses penetrate the entire ice thickness. Here we show that the strain-rate-weakening nature of ice permits initially narrow basal crevasses to seed an instability that gives rise to locally enhanced ductile deformation and thinning over length scales that are large compared to the ice thickness. This ductile failure process, called necking, amplifies long wavelength features of bottom topography and allows basal crevasses to penetrate an increasing fraction of the ice thickness as they advect downstream. Application of the model to the four largest Antarctic ice shelves shows that necking occurs downstream of pinning points and sharp protrusions in the ice shelf embayment where stress is highly concentrated. However, model predictions are sensitive to assumptions about basal melting and refreezing within crevasses, suggesting that the combination of mechanical instability and ice–ocean interaction on the scale of an individual crevasse may play a leading role in controlling ice shelf stability. •Iceberg calving from ice shelves is controlled by ductile failure in a process called plastic necking.•Plastic necking widens narrow basal crevasses and allows them to penetrate an increasing fraction of the ice thickness as they advect downstream.•Basal melting and refreezing within crevasses alters crevasse penetration depths providing a link between ice shelf stability and ocean forcing.