Dynamic jamming fronts in iceberg-choked fjords

During summertime at the glacier terminus at Jakobshavn Isbr\ae, Greenland, calving events are followed by rapid motion in the ice m\'elange in front of the terminus. Understanding the dynamics of ice m\'elange is important because it acts as a resisting force to calving events. We analyze this motion using time-lapse photography and terrestrial radar images. Large calving events last for approximately 5 minutes, during which $\sim10^{14}$ J of potential energy is released. Motion in the ice m\'elange quickly spreads out over at least 16 km down the fjord, and relaxes in about 1 hour. The ice m\'elange can be viewed as a dense granular system, which is packed close to the jamming point. A jammed ice m\'elange resists expansion of the glacier terminus much more strongly and reduces iceberg calving, which may therefore play a significant role in glacier evolution. In our images, we observe dynamic jamming fronts, which propagate one order of magnitude faster than the instantaneous speed of the calving iceberg. From the ratio between the speed of the front and the calving iceberg we calculate a compaction that agrees with estimated compaction that we observe directly.