Numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle-derived magma at mid-ocean ridges. Our model includes the following effects: melting due to decompression, magma migration by percolation, and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge. The distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material. The calculation of densities is based on a simple petrological model in which garnet-spinel herzolite melts to form a basaltic liquid and a harzburgite residue. We find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis, faster than the plate velocity, is confined laterally by stably stratified residual material beneath the newly formed plates. This effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included. Our results provide a simple explanation for the narrowness of the zone of crustal formation at mid-ocean ridges. The model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced, perhaps giving rise to along-axis variations in the character of seafloor spreading. The narrow, rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus. This may allow the liquid at depth to segregate into macroscopic channels, which would account for the consensus from experimental petrology that the liquids parental to MORB are derived from well below the base of the crust. ¿ American Geophysical Union 1989