The Rheological Behavior of Ice

One focus of my research has been grain-size-sensitive flow of materials, which involves grain boundary sliding (GBS). For a given grain size and temperature, this flow behavior is dominant at lower stresses than dislocation creep, and therefore can control the rheology of materials in low-differential-stress natural environments, such as in glaciers, ice sheets, and icy planetary interiors, as well as in the interior of the Earth. The flow of ice in glaciers and ice sheets, for example, is often controlled by GBS acting in concert with dislocation motion, an ice creep mechanism I discovered with David Kohlstedt at the University of Minnesota (Goldsby and Kohlstedt, 1995; 1997; 2001).

Structural superplasticity of ice  

Recent ice creep experiments strongly suggest that the flow of glaciers, ice sheets and icy planetary interiors is often controlled by ‘structural superplastic flow’, in which GBS plays an important role (Goldsby and Kohlstedt, 1995; 1997; 2001; Peltier et al., 2000). Our experiments demonstrate that the classic Glen flow law for ice represents not a single creep mechanism, but rather transitional behavior between dislocation creep at high stresses and superplastic flow at low stresses. These conclusions already have fundamental implications for the flow of glaciers, ice sheets, and icy planetary bodies; our results have been utilized in a number of glaciological and planetary applications, including flow of the ancient Laurentide ice sheet (Peltier et al., 2000), flow of the the South Polar Ice Cap on Mars (Nye, 2000), groove formation on the surface of Ganymede (Dombard and McKinnon, 2000), impact crater retention on Ganymede (Dombard and McKinnon, 2000), convection and diapirism in Europa (Pappalardo et al., 1998; Manga, 2002), and ice-laden debris flow on Mars (Milliken et al, 2003). However, important questions remain regarding superplastic flow of ice in nature. For example, is the strong lattice preferred orientation (LPO) observed in glaciers and ice sheets consistent with the LPO produced via superplastic flow in the lab? (For a Comment/Reply discussion of this issue, click here, then here). And, what effects do impurities, present in natural ice, have on the superplastic flow rate? I am in the midst of a study funded by the NSF Office of Polar Programs-Antarctic Glaciology program to study the LPO that develops during both superplastic flow and dislocation creep of ice in the laboratory. A comparison of our laboratory results with field observations will allow extrapolation of laboratory flow laws to glaciological and planetary conditions with even more confidence. I am also studying the effects of impurities on ice flow in a new NASA-funded project.

Environmental SEM micrograph of a sample of fine-grained ice deformed to a compressive strain of 70%. The white scale bar represents 50 mm. The black lines are grain boundaries which are thermally etched in the low chamber pressure of the ESEM, allowing for accurate determination of grain size and shape. Note the lack of significant grain flattening, and the numerous occurrences of 4-grain junctions, not present in significant numbers in undeformed samples, indicative of a significant contribution of grain boundary sliding (GBS) to the creep rate. Click here to see details about sample preparation.

 

Transformation plasticity of ice  
Recent high pressure experiments demonstrate the occurrence of another type of superplastic flow mechanism, transformation superplasticity (TSP), in ice (Dunand, Schuh and Goldsby, 2000). TSP, first observed in experiments on iron by Sauveur (1924), arises from internal stresses generated by volume change during a phase transformation. TSP is typically observed by subjecting a sample to a small differential stress and cycling temperature about a phase boundary. During the phase transformation, the sample has lower viscosity (in some cases, by many orders of magnitude, depending on the transformation kinetics) than either end-member phase. We applied a small, nominally constant differential stress to an ice sample at constant temperature while cycling pressure about the ice I-II phase boundary. The viscosity of ice in the midst of the ice I-II transformation was ~7 orders of magnitude smaller than either pure ice I or pure ice II at the same temperature. Ours is the first demonstration of pressure-induced TSP for any material. TSP may result in anomalous weakening at ice phase boundaries within larger icy satellites, as well as at olivine-spinel and other phase boundaries in the Earth, and may be generally important for the internal dynamics and evolution of planetary bodies. The complex bending of subducted slabs in the Earth depicted in tomographic studies might be caused by TSP-induced slab weakening (Panasyuk and Hager, 1998). TSP might also explain low viscosity layers that appear in models of the global geoid and of glacial isostatic adjustment.