The calving of icebergs from large Antarctic ice shelves is controlled mainly by the formation and propagation of rifts originating from the side margins of the ice shelf and local areas of grounding. Using InSAR, we observe the evolution of rifts along Hemmen Ice Rise, on Ronne Ice Shelf, Antarctica prior to the large calving event of October 1998. We couple these observations with a computer model combining the viscous flow of an ice shelf with a linear elastic fracture mechanics description of the propagation of rifts. The model reveals that the ice melange trapped in between the rifts exerts a major control on the propagation of rifts, and in turn on ice shelf stability. Melting of the ice melange from oceanic or atmospheric warming would significantly increase the propagation rate of rifts and threaten the ice-shelf stability.