The deforming bed beneath a surge-type glacier : measurement of mechanical and electrical properties

Glacier surging is a flow instability characterized by short periods of rapid glacier flow separating longer periods of normal flow. It is accepted that sustained high subglacial water pressure causes glacier surging by decoupling the glacier from its bed, but how this high subglacial water pressure is developed and sustained is the subject of debate. The current focus of glaciological research is on the interaction of subglacial processes with the subglacial drainage system. We have developed new investigative techniques for exploring two subglacial processes: basal deformation and electrical phenomena. These techniques have been applied in research undertaken on Trapridge Glacier, a small surge-type glacier in the St. Elias Mountains, Yukon, Canada; these are the first in situ measurements of deformation, electrical resistivity, and streaming potentials beneath a surge-type glacier. The development of a reliable rheological description of subglacial material required field observations of its stress—strain response; this was the motivation for our subglacial deformation experiments. We have demonstrated that no clear relationship exists between values of shear stress and effective pressure calculated using accepted methods and deformation rate; the absence of an expected relationship suggests that alternate methods for quantifying subglacial shear stress and effective pressure need to be found. Data from our subglacial electrical resistivity measurements and from the deformation measurements provide strong evidence that the distribution of normal and shear loading at the glacier bed is not even and that subglacial deformation rates can be affected by distant changes in subglacial pressure conditions. We have also observed temporal changes in electrical flow paths within subglacial sediments. We have shown that temporal variations in natural potential observed beneath Trapridge Glacier are caused by streaming potentials; streaming potentials result from cross-coupling between fluid flow and electric currents. Our data suggest that inexpensive subglacial electrode arrays may be used to supplement pressure sensors.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat