Afluid filled cylindrical cavern of circular cross section in a homogeneous infinite salt formation under a hydrostatic stress is set under internal pressure that differs from the confining pressure. The fluid in the cavern and in the mixture is treated as ideal and the solid as elastic. The initial state of stress is a consequence of the outside pressure and the cavern pressure. Perturbing the cavern pressure induces small changes in the solid and fluid densities.We compute these fields as functions of the radial distance from the cavern centre and show that depending on the relative stress levels the (salt) formation experiences either a dilatation or a compaction that is concentrated in a boundary layer near the cavern wall. © 2005 Taylor & Francis Group, London.