Numerical experiments of passive continental extension with decompressive mantle melting have been conducted to investigate controls on the development of end-member, volcanic and magma-poor, rifted margins. A prediction of end-member margin morphology is made by comparing the relative timing of continental breakup and start of magmatic emplacement. Volcanic margins are interpreted to form when magmatic emplacement begins prior to the full thinning of the continental crust, while magma-poor margins are predicted to form when continental breakup precedes any magmatic emplacement. Systematic investigations of potential influencing variables demonstrate that a variety of factors may influence this relative timing, with model results producing a spectrum of magmatic character. Of the investigated factors, the initial lithosphere geotherm and crustal thickness appear to be the most significant influences on margin morphology. Independent variation of either variable is capable of altering the predicted end-member morphology between volcanic and magma poor. Variations in mantle potential temperature, extension rate, and crustal rheology demonstrate an ability to influence passive margin magmatic character, but are unable to independently induce development of a magma-poor margin. In aggregate, model results suggest that mantle exhumation and formation of a magma-poor margin are encouraged by: a depressed lithosphere geotherm, thin continental crust, rapid extension rates, low mantle potential temperature, and a strong crustal rheology. Relatively early magmatic emplacement and formation of a volcanic margin is predicted for the majority of modeled conditions, and appears bolstered by: an elevated geotherm, thick continental crust, slow extension, high potential temperature, and a weak crustal rheology.