We consider instability of a two layered solid body of a denser material on
top of a lighter one. This problem is widely known to geoscientist in
sediment-salt migration as salt diapirism. In the literature, this problem has
often been treated as Raleigh-Taylor instability in viscous fluids instead of
solid bodies. In this paper, we propose a successive linear approximation
method for large deformation in viscoelastic solids as a model for salt
migration.