Meltwater that collects on the surface of a glacier often carves straight through the ice in deep, vertical shafts called glacial moulins. The inside of a moulin is dynamic and dangerous to explore, so little is known about the typical structure of these features. Now Roeoesli et al. present the first demonstration that seismic waves produced by water falling into a moulin could help reveal its geometry.
Water in a moulin usually drains to the bottom of the ice, where it lubricates the glacier’s path and accelerates its flow. For example, see how meltwater flows into a moulin on Greenland’s Russell Glacier in this video.
A better understanding of moulin structure would help clarify meltwater drainage processes and improve predictions of global sea level rise.
In July and August of 2011, Roeoesli’s team collected data from seismometers installed near a large moulin that pierces the western Greenland Ice Sheet. Some of the seismometers sat on the surface of the ice, and some were lowered into boreholes. In addition, a pressure sensor installed 163 meters deep inside the shaft monitored the surface level of the water that had collected in the moulin.
On most days the seismometers detected tremors that lasted from 4 to 16 hours. According to seismometer data, these events made the moulin’s ice walls resonate in patterns similar to those experienced by an organ pipe with one end plugged. Just as organ pipe resonance depends on the length of the tube, the observed moulin patterns appeared to depend on the level of the collected water.
On the basis of this similarity, the scientists built a model of moulin tremor production. In the model, free-falling meltwater causes tremors when it strikes the surface of the water deep inside a moulin. The impact sends acoustic pressure waves into the walls and base of the shaft and through the ice. Resonance patterns picked up by seismometers can be interpreted to reveal the moulin’s size, water level, and water depth.
Other scientists have previously used seismic data to examine the flow of glacial meltwater, but this marks the first time they have been used to model a glacial moulin. However, this model approximates a moulin as a simple cylinder, and scientists suspect that many moulins actually consist of an alternating series of shafts and wide pools. Future research could refine seismic techniques to detect more complex moulin structures and examine how moulins affect glacial movement. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2015JB012786, 2016)
—Sarah Stanley, Freelance Writer