Along the lateral margins of floating ice shelves in Greenland and Antarctica, ice flow past confining margins and pinning points is often accompanied by extensive rifting. Rifts in zones of marginal decoupling (“detachment zones’) typically propagate inward from the margins and result in many of Earth’s largest calving events. Velocity maps of detachment zones indicate that flow through these regions is spatially transitioning from confined to unconfined shelf flow. We employ the software package \textit{icepack} to demonstrate that longitudinally decreasing marginal resistance reproduces observed transitions in flow regime, and we show that these spatial transitions are accompanied by near-margin tension sufficient to explain full-thickness rifts. Thus, we suggest that zones of progressive decoupling are a primary control on ice shelf calving. The steadiness of detachment zone positions may be a good indicator of ice shelf vulnerability, with migratory or thinning detachment zones indicating shelves at risk of dynamic speedup and increased fracture.

Changes in glacier terminus position have been implicated as one of the primary drivers of the rapid changes in glacier dynamics observed across the globe in the last two decades. Iceberg calving exerts a critical control on the terminus position of the vast majority of marine-terminating glaciers, yet calving is relatively poorly understood due to the inherent difficulties in collecting observations of a stochastic process in a dangerous setting. Time-lapse camera and satellite observations suggest that the style of iceberg calving can vary tremendously in both space and time depending on the physical properties of the terminus, ranging from the detachment of giant tabular icebergs every few decades from Antarctic’s floating ice shelves to the growlers produced nearly daily from serac topples along Alaska’s coast. Here we extract quantitative metrics on the relative importance of calving driven by branching and uncorrelated fractures through application of fragmentation theory to iceberg size distributions extracted from high-resolution digital elevation models for 17 fjords around Greenland. We find that iceberg size distributions typically deviate from the widely-assumed power-law form for icebergs with surface areas >0.05 km^2, with fewer icebergs than predicted by the power-law for larger sizes. Icebergs larger than ~0.1 km^2 primarily calve as the result of full-thickness penetration of uncorrelated fractures (i.e., as tabular icebergs). Although the dataset is temporally sparse for the majority of the study sites, the data suggest that iceberg formation via branching fractures reaches a seasonal peak in summer, when icebergs up to ~0.1 km^2 follow power-law distributions. These data provide a novel means to assess the accuracy of iceberg calving models and potentially to constrain the physical characteristics of termini susceptible to the marine ice cliff instability mechanism.