A QUARRYING EXPERIMENT BENEATH A THICK VALLEY GLACIER: ROLE OF TRANSIENT WATER PRESSURE
To study quarrying in real time, we installed a 0.3 m high granite step under 210 m of ice at the bed of Engabreen, an outlet glacier of the Svartisen Ice Cap in Norway. The step, which protruded upward into the ice, contained an acoustic emission system to monitor fracture events and other sensors to measure normal stress, water pressure, and cavity height in the lee of the step. Water pressure and normal stress increased as the glacier began to flow around the step. At that time, low-level acoustic emissions recorded clasts abrading the stoss side of the step. After 2.5 days water was pumped under pressure to the downstream side of the step, causing ice-bed separation. Within 70 minutes, a leeward cavity, 40 mm in height, formed and extended, to some extent, to the step's stoss surface. The pump was then shut down. Two hours later closure of the cavity and associated reloading of the step by ice caused acoustic emissions to increase dramatically for about five hours. These acoustic emissions likely resulted from growth of pre-existing fractures in the step. Post-experimental inspection of the step showed that its lee surface had been quarried.
Our results indicate that transients in basal water pressure can accentuate deviatoric stresses in subglacial bedrock, accelerating crack growth. Thus, long-term quarrying rates may be driven, or at least enhanced, by fluctuations in subglacial water pressure and may not be well represented by steady-state erosion rules.