143-156 - International Glaciological Society
143-156 - International Glaciological Society 143-156 - International Glaciological Society
JOURNAL OF GLACIOLOGY I fJO f +f If 9 f x-- --- -- -- -- -- -- Cogs Fig. 13. Observed shear in the April 1973 experiment; distilled-water ice deformed by 5.30 revolutions. Fig. 14. lee fabric jrom the April 1973 experimmt; distilled-water ice deformedfor 365 h. Experiment of April [973 The velocity in this experiment was 143 m/year for a total duration of 365 h (5.30 revolutions). On this occasion the ice was made from distilled water instead of the tap water used in previous experiments. The same layer of blue ice about 3 cm thick was formed, but within it the shear strain was substantially less, as shown in Figure 14. On the other hand, "Tb was 1.66 times' higher. Figure 14 shows the much larger blue-ice crystals which are obtained with distilled water. This indicates that the ions present in tap water enhance ice recrystallization and creep. CONCLUSIONS Although they have certain errors, the experiments carried out in 1970 and 1973 with the viscometer indicate that important facts are ignored in the present theories. The delayed elasticity of bubbly ice, which is much more important than the pseudo-elasticity of Duval ([1977]) and which is of the Kelvin not of the Maxwell type, cannot be ignored; it increases the drag. The progressive elimination of air bubbles from the bottom ice of glaciers would
SIMULATED GLACIER SLIDING appear to be an important process. A better simulation of sliding processes would occur if bubble-free ice were used. Kamb's (1970) statement that the layer of blue ice found at the bottom of glaciers is caused by regelation may be questioned; in the ice tested here it is not the case. This is consistent with the findings of Souchez and others (1978) who have shown that the ionic composition of base glacier ice cannot arise as a result of a classical melting- . refreezing process. Creep-rates in ice are considerably altered by the presence of ions. This makes it very difficult to reproduce the exact mode of basal deformation over an obstacle. At best we may determine the friction law over a sine profile for different known ionic contents. A new viscometer, a development of Pen elope, is now ready to work in Grenoble. Nevertheless, Penelope will still be useful because the flow in it is two-dimensional and very high confining pressures can be attained; this will not be possible with the new viscometer. ACKNOWLEDGEMENTS My thanks are due to Alsthom-Atlantique who provided time for the present work, to Professor Lliboutry for his help in interpreting the results and in writing this paper, to Miss M. 1. Clement for her assistance with the experimental work, to Mr A. Chaillou and Mr L. Philippe for their electronic and mechanical expertise, and to CNRS for the provision of funds. The profile of the "bedrock" is given by the boundary conditions are for z = h, (I) forward velocity u = V, a constant; (2) transverse velocity w = 0; for z :::: :: 0, (3) shear strain-rate APPENDIX ": = a cos wx, ou ow )lxz = az + ox = 0 ; (4) to second order, q(x, z) denotes the strain function, q = 0 gives z = a cos wx. A general solution of the biharmonic equation y4q = 0 is q = f(z) cos wx+g (z) sin wx+C,z+C,z'+CJz" f(z) and g(z) being of the form (A+Bwz) exp (wz)+ (C+Dwz) exp (-wz). The boundary conditions are then (I) f'(h) = 0, g' (h) = 0, C,+2C,h+3CJh' = V, (2) f(h) = 0, g(h) = 0, (3) f" (o)+w'f (o) = 0, g" (o)+w'g (o) = 0, C, = 0, (4) f(o) = -Va, g(o) = o. The four relations g(h) = g' (h) = g(o) = g" (o) = 0 imply that g(z) = o. In order not to have a constant term for C, = 0 is needed. Hence, ca", OTxz Tx -az-' C=-A-Va, D=B-Va, } A (exp (2wh)-I)+Bwh (exp (2wh)+I) = . Va (l+wh), 2A exp (2wh)+B[ (1+2Wh) exp (2wh)+I] = Va. (A-I)
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JOURNAL OF GLACIOLOGY<br />
I<br />
fJO<br />
f<br />
+f<br />
If<br />
9<br />
f<br />
x-- ---<br />
-- -- -- -- --<br />
Cogs<br />
Fig. 13. Observed shear in the April 1973 experiment; distilled-water ice deformed by 5.30 revolutions.<br />
Fig. 14. lee fabric jrom the April 1973 experimmt; distilled-water ice deformedfor 365 h.<br />
Experiment of April [973<br />
The velocity in this experiment was <strong>143</strong> m/year for a total duration of 365 h (5.30 revolutions).<br />
On this occasion the ice was made from distilled water instead of the tap water used<br />
in previous experiments. The same layer of blue ice about 3 cm thick was formed, but within<br />
it the shear strain was substantially less, as shown in Figure 14. On the other hand, "Tb was<br />
1.66 times' higher. Figure 14 shows the much larger blue-ice crystals which are obtained with<br />
distilled water. This indicates that the ions present in tap water enhance ice recrystallization<br />
and creep.<br />
CONCLUSIONS<br />
Although they have certain errors, the experiments carried out in 1970 and 1973 with<br />
the viscometer indicate that important facts are ignored in the present theories. The delayed<br />
elasticity of bubbly ice, which is much more important than the pseudo-elasticity of Duval<br />
([1977]) and which is of the Kelvin not of the Maxwell type, cannot be ignored; it increases<br />
the drag. The progressive elimination of air bubbles from the bottom ice of glaciers would
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