In situ monitoring and physical modelling of sub-glacial deformation at Skalafellsjokull

The controls on glacial movement remain one of the most poorly understood elements of the glacial system, largely due to the inaccessibility of the subglacial environment. Here a geotechnical study is presented that investigates the behaviour of till under pore pressure controlled decreases in normal effective stress. This is supplemented with field data from subglacial wireless probes, dGPS, GPR and a UAV survey of the glacial foreland to provide a broad view of subglacial deformation at Skalafellsjökull, IcelandThe geotechnical laboratory investigation was undertaken on till sampled from an ice marginal location at Skalafellsjökull, close to the probe deployment site. This showed that the till adhered to the Mohr-Coulomb model, behaving plastically. In addition, a back pressure shear box was used to model pore pressure controlled reductions in normal effective stress. Linearly reducing normal effective stress by increasing back pressure resulted in episodic increases in horizontal strain rates. Small reductions in shear stress resulted in large strain rate reductions but dilation hardening did not occur. Dilation hardening has previously been suggested as a causal factor in stick-slip glacial motion and a reason for the apparent scale dependence of till rheology, but these experiments suggest it may not be as significant as previously thought.High spatial variability was found throughout the field investigation in probe, dGPS and mapping data from the UAV survey. dGPS deployed above in situ subglacial probes recorded hourly ice movement indicative of stick-slip motion but it was not possible to link this motion to subglacial processes via probe data. Pore pressure data suggested spatially variable coupling to the subglacial hydrological system and inconsistent forcing by surface melt. Tilt data showed consistent movement patterns and lacked regular up-glacier movements previously thought to indicate ice-bed decoupling and sliding. The lack of evidence for forcing of velocity variability from the in situ data suggests a global forcing mechanism at this site, probably longitudinal or lateral coupling to higher velocity areas of the glacier. The spatial variability of the subglacial data has similarities to the metre scale spacing of flutes on the foreland mapped with the UAV imagery, suggesting flute formation has potential to result in complex probe behaviour