From plate tectonics, it is known that the major tectonic plates of the earth behave as if swimming on the earth's mantle. But when considering seismic waves or tides, the Earth's mantle behaves like a solid body. Thus, the same mantle material may react as a fluid or as a solid depending on the time scale of the applied forcing. Viscoelasticity is the mathematical description of this phenomenon: viscous flow of the material for long-time loading of larger than one Million years and elastic deformation for short-time processes (less than a few years). Glacial-isostatic adjustment with characteristic times of 100 to 100,000 yr lies between these two bounds and has to be considered in terms of viscoelaticity.
More recent investigations recommend to consider -- in addition to material compressibility -- the lateral variability of viscosity which is derived from geodynamics, in order to describe the deformational behaviour of the solid earth realistically.
See also: Density Structure of the Earth
References:
Bagge, M., Klemann, V., Steinberger, B., Latinovic, M., Thomas, M. (2021): Glacial-isostatic adjustment models using geodynamically constrained 3D Earth structures. - Geochemistry Geophysics Geosystems (G3), 22, 11, e2021GC009853.
Tanaka, Y., Hasegawa, T., Tsuruoka, H., Klemann, V., Martinec, Z. (2015): Spectral-finite element approach to post-seismic relaxation in a spherical compressible Earth: application to gravity changes due to the 2004 Sumatra-Andaman earthquake. - Geophysical Journal International, 200, p. 299-321. | GFZpublic | doi.org/10.1093/gji/ggu391 | URI | PDF |
Cambiotti, G., Klemann, V., Sabadini, R. (2013): Compressible viscoelastodynamics of a spherical body at long timescales and its isostatic equilibrium. - Geophysical Journal International, 193, 3, p. 1071-1082.| GFZpublic | doi.org/10.1093/gji/ggt026 | PDF |
Klemann, V., Martinec, Z., Ivins, E. R. (2008): Glacial isostasy and plate motion. - Journal of Geodynamics, 46, 3-5, p. 95-103. | GFZpublic | http://doi.org/10.1016/j.jog.2008.04.005 | PDF |
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