Superplastic deformation of ice: Experimental observations

Abstract

Abstract. Creep experiments on fine-grained ice reveal the existence of three creep regimes: (1) a dislocation creep regime, (2) a superplastic flow regime in which grain boundary sliding is an important deformation process, and (3) a basal slip creep regime in which the strain rate is limited by basal slip. Dislocation creep in ice is likely climblimited, is characterized by a stress exponent of 4.0, and is independent of grain size. Superplastic flow is characterized by a stress exponent of 1.8 and depends inversely on grain size to the 1.4 power. Basal slip limited creep is characterized by a stress exponent of 2.4 and is independent of grain size. A fourth creep mechanism, diffusional flow, which usually occurs at very low stresses, is inaccessible at practical laboratory strain rates even for our finest grain sizes of--3 •m. A constitutive equation based on these experimental results that includes flow laws for these four creep mechanisms is described. This equation is in excellent agreement with published laboratory creep data for coarse-grained samples at high temperatures. Superplastic flow of ice is the rate-limiting creep mechanism over a wide range of temperatures and grain sizes at stresses • 0.1 MPa, conditions which overlap those occurring in glaciers, ice sheets, and icy planetary interiors.

Comment on “Superplastic deformation of ice: Experimental observations” by D. L. Goldsby and D. L. Kohlstedt

“…[4] As indicated by Goldsby and Kohlstedt [2001], the creep regime of glacier ice with a stress exponent of 1.8 is found at deviatoric stresses lower than 0.1 MPa. As at high stresses, strain rates are significantly lower than those of single crystals oriented for basal slip and deformed in the same conditions.…”

“…[2] Creep experiments on fine-grained ice (grain size lower than 200 mm) performed by Goldsby and Kohlstedt [1997] have made it possible to identify a grain size dependent creep regime with a stress exponent of $1.8 and an activation energy of 49 kJ mol À1 for 215 T 236 K. According to Goldsby and Kohlstedt [1997, 2001, grain boundary sliding (GBS) significantly contributes to deformation in this creep regime. Microstructures data obtained from scanning electron microscope measurements also support a deformation mode dominated by GBS similar to the deformation mode of superplastic materials.…”

The stability against freezing of an internal liquid-water ocean in Callisto

“…The stability against convection of a layer (in this case, Callisto's outer ice shell) can be estimated by means of the Rayleigh number de®ned at the layer base, Ra base ; for non-newtonian viscosity, this is These regimes are associated with premelting effects 18 , or with recrystallization under stress 30 , and have still higher values 18 of Q; they dominate at temperatures above ,240±260 K (different reports give different temperatures in this range 18,29 given by 7 Ra base agrh n 2 =n DT k 1=n b 1=n exp v=n 1 where a is the volumetric thermal expansion coef®cient, g is the acceleration due to gravity (taken in general as the surface value, 1.24 m s -2 for Callisto), r is the density (930 kg m -3 for water ice I), h is the effective layer thickness, DT ( T base 2 T s ) is the temperature difference between the base and the surface of the layer, k is the thermal diffusion coef®cient, b is a parameter that depends on creep mechanism and temperature, and n is a constant that depends on creep mechanism. v ( QDT=RT 2 i ) is the Frank±Kamenetskii parameter (which is related to the viscosity contrast through the layer caused by temperature differences), where Q is the activation enthalpy of creep deformation (which, because of experimental uncertainty, can be taken as the activation energy), R is the gas constant, and T i is the adiabatic temperature (approximately constant) in the case of convection.…”

Effects of ice deformation on Röthlisberger channels and implications for transitions in subglacial hydrology

“…Depending on the creep mechanism and the stress magnitude, values from n = 1 to n = 4 are appropriate (Steinemann, 1954;Durham and others, 1997;Goldsby and Kohlstedt, 2001). We take n = 3 following Glen (1955), as is common in glaciology.…”