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HAL Id: jpa-00226465

https://hal.archives-ouvertes.fr/jpa-00226465

Submitted on 1 Jan 1987

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PERMEABILITY, WETNESS AND CREEP OF TEMPERATE ICE

L. Lliboutry

To cite this version:

L. Lliboutry. PERMEABILITY, WETNESS AND CREEP OF TEMPERATE ICE. Journal de Physique Colloques, 1987, 48 (C1), pp.C1-663-C1-663. �10.1051/jphyscol:1987199�. �jpa-00226465�

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JOURNAL DE PHYSIQUE

Colloque C1, supplement au n o 3, Tome 48, mars 1 9 8 7

PERMEABILITY, WETNESS AND CREEP OF TEMPERATE ICE

L. LLIBOUTRY

L a b o r a t o i r e de G l a c i o l o g i e , C . N . R . S . a n d U n i v e r s i t e d e G r e n o b l e I, B.P. 96, F-38402 St-Martin-d'Heres C e d e x , France

Abstract : Ice is said to be temperate when it is in local equilibrium with water veins and water inclusions ( 1 , Z ) . Temperate ice remains a poorly understood material offering a large field for investigation.

Pressure solution of air in percolatin3 water explains why, in temper- ate glaciers, ice becomes progressively bubble-free between 100 and 200 m in depth

( 3 ) . Emptying of large air bubbles (formed by coalescence of several hundred of

small ones) through capillary veins is not excluded, however.

Permeability of glacier ice suffering strain, grain boundary migration and recrystallization does not follow Darcy's law, and evolves with time, as shown by laboratory and field observations (4,5). Moreover, the temperature field depends on the local stress, rather than on Fourier's law. Therefore, the classical theory of elacier sliding ( 6 ) , which assumes ice to be dry and impermeable, is unrealistic.

Most water veins and lenses should be more or less normal to the principal direction corresponding to the highest compressive stress. With this model, tentative gover- ning equations for temperature and for water flux are suggested. They yield

surprising results (7); if ice were Newtonian viscous, the flux of cold reaching the interface would be much higher than needed for refreezing all the exuding water.

Field measurements have shown that the liquid water content in a temperate Alpine glacier fluctuates widely at the decinetre scale, without general trend from surface to bottom ( 8 ) . The variations are completely uncorrelated with grain size or salinity. An explanation is offered, which considers water lenses at grain boundaries emptying into the water veins when they have grown enough.

Tertiary creep of temperate ice, with a multimaxima fabric, is assumed to follow an isotropic third-power viscous law, even when it is wet. The lowering of viscosity for an increasing water content (9) is understandable if processes at the grain boundaries are thought to be the main factor controlling creep, contrarily to usual theory. Grain boundaries with water lenses should no longer control creep, the adjustment of strains in neighbouring crystals being facilitated by melting-refreezing. As a test for the theory, it should be investigated whether transient reversible creep (Duval's pseudo-elasticity ) , which is mainly due to the misfit of grains, diminishes when the water content increases.

REFERENCES

(1) L.Lliboutry. J. Glaciology, 10, (1971), 15-29.

(2) C . F . Raymond and W.D. Harrison. J. Glaciology, 14,(1975), 81-101.

(3) W. Berner,B. Stauffer and H.Oeschger. Z.Gletscherkde. Glazialgeol.,l3,(1977), 209-21 7.

(4) J.F. Nye and S. Mae. J. Glaciology,ll, (1972), 81-101.

(5) D.Hantz and L.Lliboutry. J. Glaciology, 29,(1983),227-239.

(6) J , F.Nye. Proc. R. Soc. A, 311,(1969), 445-467.

(7) L.Lliboutry (in press).

(8) L.Lliboutry and P.Duva1. Ann. Geophysicae, 3 ,(1985), 207-224.

(9) P. Duval . IAHS Publ. 118, (19771, 29-33.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987199

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