Causes of variability of icefacies deformation rates

A range of laboratory studies have attempted to determine the effect of the physical presence of debris on ice rheology, by creating ice in the laboratory with embedded debris in carefully controlled conditions. These studies generally have found that the presence of debris decreases deformation rates, and that strain rates decrease with increasing debris-content (Fig. 63.4). For example, Hooke et al. (1972) found that the strain rate of ice decreased exponentially with the addition of debris for debris concentrations greater than 10%, although at lower concentrations, results were inconsistent. They attributed this strengthening to crystal-scale processes and, specifically, the inhibition of dislocation motion by the physical presence of debris (cf. Weiss et al., 2002). Nickling & Bennett (1984) similarly found that

Solids content (vol %)

Figure 63.4 The results of various studies showing the effect of the presence of solid impurities on the creep rate of ice, in relation to the creep rate of debris-free ice. (From Budd & Jacka, 1989.)

Solids content (vol %)

Figure 63.4 The results of various studies showing the effect of the presence of solid impurities on the creep rate of ice, in relation to the creep rate of debris-free ice. (From Budd & Jacka, 1989.)

the strength of debris-ice mixes increased with debris concentration until debris reached 75% by volume, and accounted for this pattern in terms of changes in internal friction and cohesion. Jacka et al. (2003), however, found that there was little impact of debris concentration in laboratory ice on minimum strain rates.

Studies of the rheology of debris-laden ice sampled from real glaciers, on the other hand, indicate fairly consistently that the presence of solid impurities leads to an increased flow rate. The results of those few studies, in keeping with the general structural and dynamic observations for relative rheological behaviour of glacier ice in situ as outlined above, have found that debris-laden basal ice was weaker than contiguous clean ice sampled from the same glacier. At near zero temperatures, Lawson (1996) found that samples of stratified facies debris-laden basal ice (5-20% debris) from the Taylor Glacier had a mean peak strength only 40% of that of overlying englacial ice. Fisher & Koerner (1986) found enhanced flow rates in basal ice with solid impurities that they correlated with the solid impurity content, rather than any other possible enhancement factor.

The findings for laboratory ice, then, contrast with the various kinds of evidence from more field oriented situations for higher flow rates in ice with solid impurities. This supports the suggestion that the physical presence of debris does not in itself have a substantial effect on the strength of ice (Budd & Jacka, 1989; Durham et al., 1992), although Budd & Jacka suggest that an exception to this may be near the melting point, and we investigate the effect of temperature later in this chapter. In general terms, however, it seems likely that it is indirect effects associated with the presence of debris, for example, in terms of chemical and water content, that cause the observed higher flow rates in debris-laden ice facies.

Basal ice facies in all environments typically have enhanced concentrations of soluble impurities (Cuffey et al., 1999; Hooker et al., 1999; Fitzsimons et al., 2000; Hubbard et al., 2000a, 2004), and the presence of soluble impurities in ice tends to increase the flow rate (e.g. Jones & Glen, 1969; Nakamura & Jones, 1973). Nakamura & Jones, for example, found that a concentration of just 28ppm of dopant hydrogen fluoride increased the flow rate by a factor of five. The soluble impurities are likely to be acquired during the process of regelation, because solute acquisition takes place rapidly when meltwater interacts with solid impurities (Brown etal., 2001). Soluble impurities may also be concentrated in liquid water found in veins at triple grain junctions and interfacial films (Cuffey etal, 1999).

The rheological effect of bulk soluble impurity content is hard to isolate, however, because it is closely and causally related to bulk water content, such that

0 0

Post a comment