  In modern geodynamics, isostasy can be viewed either as the static
equilibrium of the crust that minimizes deviatoric stresses, or as a dynamic
process resulting from the viscous relaxation of the non-hydrostatic crustal
shape. Paper I gave a general formulation of Airy isostasy as an elastic
loading problem solved with Love numbers, and applied it to the case of minimum
stress isostasy. In this sequel, the same framework is used to study Airy
isostasy as the long-time evolution of a viscoelastic shell submitted to
surface and internal loads. Isostatic ratios are defined in terms of
time-dependent deviatoric Love numbers. Dynamic isostasy depends on the loading
history, two examples of which are the constant load applied on the surface in
the far past and the constant shape maintained by addition or removal of
material at the compensation depth. The former model results in a shape
decreasing exponentially with time and has no elastic analog, whereas the
latter (stationary) model is equivalent to a form of elastic isostasy.
Viscoelastic and viscous approaches are completely equivalent. If both load and
shape vary slowly with time, isostatic ratios look like those of the stationary
model. Isostatic models thus belong to two independent groups: the
elastic/stationary approaches and the time-dependent approaches. If the shell
is homogeneous, all models predict a similar compensation of large-scale
gravity perturbations. If the shell rheology depends on depth, stationary
models predict more compensation at long wavelengths, whereas time-dependent
models result in negligible compensation. Mathematica and Fortran codes are
available for computing the isostatic ratios of an incompressible body with
three homogeneous layers.
