Creep Behavior of Ice in Polar Ice Sheets

Abstract

Major advances in the understanding of the plasticity of ice Ih have been made with the development of studies of the flow of glaciers and, recently, with the analysis of deep ice cores in Antarctica and Greenland. Evidence is presented that indicates deformation, in glaciers and ice sheets, is essentially produced by basal slip. Other deformation modes are required for compatibility reasons: non-basal slip, dislocation climb, cross-slip and/or grain boundary sliding. Diffusion processes cannot be invoked as a significant deformation mode, even at low stresses. A grain size effect is expected for secondary creep, as soon as the stress exponent of the flow law is lower than 2. A threshold stress could explain the apparent high stress exponent found at the ice divides in polar ice sheets. It is shown that the presence of a liquid phase at grain boundaries or around particles significantly reduces the ice viscosity. Dynamic recrystallization is very active in glaciers and ice sheets. Grain boundary migration associated with normal grain growth and dynamic recrystallization is considered as an efficient accommodation process of basal slip. Migration recrystallization, which is associated with tertiary creep and with the rapid migration of grain boundaries between dislocation-free nuclei and deformed grains, produces an interlocking grain structure and grains well oriented for basal slip.