Advisor: Dr. Neal Iverson
Glaciers move under their own weight, either by deforming or sliding. Seasonal advance and retreat is natural, but most modern glaciers have fallen out of equilibrium so that they are retreating more than they are advancing.
Accelerated glacier sliding makes glaciers more vulnerable because ice moves more quickly to lower (warmer) elevations where it melts faster, or because it is delivered more quickly to the ocean where ice breaks off as ice bergs. It also increases weathering rates, which not only control landscape evolution, but also carbon flux.
Shrinking glaciers are concerning because they are an important source of drinking water, irrigation, and power to many communities, and the meltwater and ice bergs contribute to sea-level rise.
The illustrations to the right were drawn by Rudolf Reschreiter in 1911, and depict Vernagtferner glacier as a fearsome beast advancing and retreating up the valley. I first found them at this fun Columbia University blog.
Using a ring-shear device at Iowa State University, I studied the mechanics of subglacial debris-bed friction. My experiments tested and expanded upon Bernard Hallet's theory (1979, 1981), which states that debris-bed friction is a product of bed-normal ice velocity and is independent of total normal stress applied by the ice. These experiments are the foundation of my master's thesis.
Modeling the speeds of sliding glaciers reveals major uncertainty to estimates of sea-level rise and landscape evolution. In sliding models, friction between ice-entrained debris and the bed is often overlooked. For the common case of sparse debris in basal ice, theories developed in the 1970s by G.S. Boulton and B. Hallet included contradictory treatments of the forces that push particles against the bed. Boulton assumed that these forces scale with effective pressure—the difference between ice pressure and water pressure in cavities beneath particles—whereas Hallet assumed these forces depend on the rate of ice convergence toward the bed from melting and bed-parallel stretching of ice on stoss surfaces. The resultant bed-normal drag on particles depends on movement of ice past them by regelation and enhanced creep of ice.
To test these contrasting hypotheses, a large ring-shear device was used to slide temperate ice with sparse debris over a smooth rock bed. Isolated gravel-sized till particles in contact with the bed were built into an ice ring (outer diameter = 0.9 m, width = 0.20 m, thickness = 0.24 m) that rotated at a steady speed. A fluid, with its temperature controlled to the nearest 0.01oC, surrounded the ice chamber to keep the ice at its pressure-melting temperature. Meltwater drained to atmospheric pressure from the edges of the bed. During experiments, either the ice convergence rate or total bed-normal stress was incremented, and shear stress was measured until a steady value was attained. In separate rate-controlled tests without ice, the dynamic friction coefficient between the particles and the rock bed was measured.
Results indicate that friction between particles and the bed depends on convergence rate. In contrast, total normal stress has no effect on bed shear stress, in agreement with Hallet’s model. However, water-filled cavities formed beneath particles rather than the regelation ice expected from Hallet’s model. These observations can be explained by an adjusted model that appeals to mass conservation in melt films that exist everywhere at ice-rock boundaries. While ice converges with the bed, melting at the tops of particles creates pressure gradients and flow within melt films that push particles against the bed. Higher convergence rates generate more melt that steepens pressure gradients. Film thicknesses are sufficient to neglect intermolecular interactions associated with premelting. Finally, by incorporating observed particle rotation, the adjusted model is made consistent with the experimental data and observations.
Thompson, A. C., Iverson, N. R., Zoet, L. K., An experimental study of subglacial debris-bed friction during basal slip. Oral presentation at: American Geophysical Society Meeting; 2019 Dec 13; San Francisco, CA. abstract
Thompson, A. C., Iverson, N. R., Zoet, L. K., An experimental study of debris-bed friction during basal slip. Oral presentation at: International Glaciological Society Symposium on Glacial Erosion and Sedimentation; 2019 May 12; Madison, WI. abstract
Most of these are my photos, but photos of me are from the PolarTREC blogs.
I spent two field seasons in 2017 and 2018 on a team surveying glacier forefields (the rock surface in front of a glacier) using LiDAR and Structure-from-Motion. These data were used to create digital elevation models (DEMs) of the rock surface to be statistically analysed and fed into glacier sliding models.
Limestone beds at Castleguard forefield, in Banff National Park, created a stepped surface for the ice to slide over. As a result, the ice seperated from the rock as it slid over the steps, creating subglacial water-filled cavities. The ice has since retreated, but there are several physical features on the rock surface that reveal the extent of the cavities.
In the field, I measured step geometry, cavity size, and striation orientation to analyze the geometry of subglacial cavities. I compared those geometries to theory () and have been using ArcGIS.10.x. to identifiy spatial patterns.
Thompson, A. C., Iverson, N. R., Zoet, L. K., A relationship between sliding speed and effective pressure based on observations of deglaciated bedrock. Poster session presented at: Geological Society of America North-Central Section Meeting; 2018 April 17; Ames, IA. abstract
*Awarded best graduate student poster!
A PolarTREC educator, Lauren Adamo, joined us in Switzerland to document our work for the public and to give presentations at schools and libraries.
I visited the Swiss Alps in August 2018 with an ISU class taught by Jacqueline Reber to learn more about their formation and to see some of the evidence in person. Though I had to miss a good portion of the fieldtrip to do the fieldwork described above, I was able to observe evidence of large-scale compression, subduction, and glacially carved landscapes at Gemmipass and the Findelgletcher Valley above Zermatt.
At Findelgletcher Valley, I led a day of the fieldtrip devoted to an introduction to glaciers. We talked about mass balance while looking down on the glacier, glacial landscapes while looking at the matterhorn and surrounding peaks and valleys, deposition while walking down the moraine, erosional features while standing on the rock in front of the ice, and drainage while standing by the outlet at the ice margin.
It was a fun challenge to plan a fieldtrip to a place I had never seen from accross an ocean, but it was a successful day, and I discovered the amazing website Swiss Topo in the meantime!
Map of a full day fieldtrip at Findelgletcher above Zermatt, Switzerland. Created using Swiss Topo.