Transient effective stress variations forced by changes in conduit pressure beneath glaciers and ice sheets

The resistance to sliding beneath soft-bedded glaciers depends on the effective stress, N, which is controlled by the subglacial hydrological system. While large-scale water transport is primarily through conduits, the effective stress profile beneath most of the glacier base is expected to be controlled by seepage flows through the permeable sediments. Models of the response to forced changes of the nearconduit effective stress demonstrate that perturbations in N decay over a characteristic lateral distance that is typically only a few times the sediment thickness. This implies that only relatively small changes to the spatially averaged effective stress, N , can be produced if conduit spacing is comparatively large and the glacier and sediments remain in contact. To produce larger changes to N , it is likely that flotation must be achieved beneath a significant portion of the glacier base. At higher values, spatial variations in N can produce gradients in the thickness, h, of a fringe of ice-infiltrated sediments immediately adjacent to the glacier base; this has implications for the development of glacial landforms when sliding causes sediment transport.