All known propagating rifts (PR's) are growing away from shallow ridge axis seafloor. We examine a model for rift propagation in which the ridge axis conduit, like a crack in the lithosphere, propagates in response to a stress concentration at its tip. The stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity-spreading stresses associated with anomalously shallow ridge axis along the growing ridge. This stress concentration near the tip is characterized by a stress intensity factor K, which must exceed a threshold value to cause failure of the plate. A remotely applied tensile stress would make a positive contribution to K but would act equally on both ridge segments and not favor the propagation of either. The pressure in the crack may, depending on its distribution, make either a positive or negative contribution to K. For a high enough propagation rate, viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the PR tip would make K<0. This simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well-studied 95.5 ¿W PR of the Galapagos spreading center. Analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the PR axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 Pa s (1020 P). We also examine a more general model of a ridge-transform-ridge system on the basis of energy conservation. In terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis. However, the more general model also includes mechanisms that may stop ridge propagation.

Propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis, i.e., if the stress intensity factors for both ridge axes are similar. Both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 ¿W in the Galapagos. Finally, in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation.