As glaciers move over the landscape, sediment entrained within basal ice grinds against the underlying bedrock, gradually wearing it away. This process shapes the landscape through erosion, forming various characteristic glacial landforms. The grinding action of the basal sediment against the bed rock is called abrasion. An “abrasion law” relates glaciological properties, such as sliding speed, shear stress, or ice flow speed, to the abrasion rate. Developing an abrasion law that accurately reflects physical processes while remaining simple enough for use in landscape evolution models is challenging. Field data alone is insufficient to derive this physical relation because, while paleo records can provide estimates of eroded volumes, they lack information about the glaciological conditions that caused the erosion. In contrast, modern glaciers allow measurements of glaciological properties, but accurately determining eroded volumes remains difficult, limiting the ability to establish a precise abrasion law. Laboratory experiments offer controlled environments where relevant glaciological factors can be measured over weeks or months, but accurately measuring the miniscule eroded volume is logistically challenging. Here we have addressed this problem by using highly controlled laboratory experiments in a novel cryogenic ring shear device along with a high precision instrument capable of measuring the miniscule amounts of erosion that occurred in those experiments, ultimately allowing us to quantitively constrain the abrasion law.

Specifically, we used a cryogenic ring shear device (Fig. 1) to slide debris-ice over rock beds of various lithologies to create striations (Fig. 2)—the wear marks created through abrasion. The volume of material abraded away in the striations was measured using a white light profilometer. For comparison, we conducted simpler experiments with a direct shear apparatus, which resulted in only a limited amount of abrasion. In the ring shear experiments, both flat (planar) and non-flat (stepped) rock beds were eroded. Our findings indicate that a power-based abrasion law accurately represents the physical processes driving abrasion (Fig. 3). This approach is not only consistent with the physical mechanisms involved but also simple to incorporate into landscape evolution models. By adopting this power-based approach to abrasion, landscape evolution models can more effectively simulate how landscapes evolve under glaciers, reducing uncertainties associated with unknown aspects of the underlying physical processes.

As a part of this research, two summer interns from the College of Menominee Nation participated in a full-time, 12-week geoscience intern program. They assisted the team and prepared posters and presentations based on their work.

The major outcomes of the work to date are:

1. Development of a power-based abrasion law: This law is recommended for use in landscape evolution models, where abrasion scales with basal shear stress and slip velocity.
2. Revised energy expenditure estimate for abrasion: Our findings show that abrasion accounts for approximately 1% of the energy expenditure of glaciers, significantly lower than the previously hypothesized 30%.
3. Characterization of striation types: The most abundant striation types (1, 2, or 3) vary with the extent of the striation and exhibit distinct morphologies related to drag forces.
4. Training and educational impact: This research provided training opportunities for 2 PhD students, 2 University of Washington undergraduates, and 2 College of Menominee Nation undergraduate students.

Last Modified: 10/22/2024
Modified by: Lucas K Zoet