All Is Not Lost: The Transition from Order to Disorder in Greenland’s Glaciers

Abstract:

Iceberg calving is a major process involved in the removal of large volumes of ice from ice sheets to the oceans, but remains relatively poorly understood. The part of the puzzle that remains the least clear is the dynamics involved in the transition between intact glacier ice and rapid fragmentation during calving events. Because calving is a sudden rapid event, it is often the case that what gets captured by satellite observation is the before and after, rather than the exact moment of failure. Here we exploit this fact and use a statistical approach using a collapse model to investigate whether or not there exists a quantifiable critical fracture density, or "critical mass" of closely-spaced fractures, beyond which dynamic fragmentation of the glacier occurs. To do this we study the size distribution of fragments in proglacial mélange, using this information to infer physical properties of the pre-collapsed ice, such as material strength and the energy necessary to create a fragmentation event using methods widely applied in civil and even weapons engineering, but not previously applied in a glaciological context. Characterizing fracture density at different locations and quantifying a critical factor to describe the transition from highly-fractured to collapse/fragmentation region enable us to understand the distribution of observable surface fracture patterns and underlying differences within regions of individual glaciers and between glaciers. This investigation is well-served by the only-recently-available high resolution data over Greenland’s glaciers by LandSat, MODIS and Operation IceBridge, among others. While many studies have focused on the propagation of crevasses in the glacier ice as a means of predicting calving, the main goal of this work is to consider the physical transition between the two phases of collapse. As such, our work focuses (1) on the pre-collapse state as characterized by crevasse pattern formation in the glacier, or level of damage present; and (2) on the post-collapse state as characterized by proglacial mélange properties. Application of these techniques to glaciers yield insight into the processes leading up to collapse and enable us to determine critical parameters such as critical fracture density, antecedent to collapse events.