Cauldron Collapse Provides Natural Laboratory to Constrain Ice-Fracture Thresholds

Glaciers and ice shelves can lose large volumes of ice rapidly through iceberg calving. The appropriate mathematical description of the ice fracture mechanisms that precede calving remains open to investigation. One approach describes fracture that occurs after stress (or strain rate) within the ice exceeds some threshold, but the threshold values are uncertain and may vary with ice conditions. We use the 2015 drainage of a subglacial lake at the Eastern Skaftá Cauldron, Vatnajökull Ice Cap, Iceland, as a geophysical-scale natural laboratory in which to study fracture thresholds of undamaged ice. The ice around the Eastern Skaftá Cauldron floated on a subglacial lake and rose slowly over a several-year period as the lake filled, then in 2015 collapsed during a large jökulhlaup. Aerial images, ArcticDEM elevation models and satellite altimetry profiles of the area taken before and almost immediately after the lake drainage show cauldron collapse and a resultant pattern of concentric ring fractures. In situ GPS data from the center of the cauldron provides a time-variable rate of subsidence with temporal resolution of minutes during the drainage event. We implement a visco-elastic ice model constrained by these data to determine time-variable stress and strain rate fields for the Eastern Skaftá Cauldron, and we find upper and lower bounds on thresholds of stress and strain rate that match the observed fracture pattern.