We study the propagation of a buoyant liquid-filled fissure from a reservoir under constant pressure within the framework of linear elastic fracture mechanics. We conducted laboratory experiments by injecting aqueous solutions into gelatin solid: an analogue for elastic and brittle crustal rocks. Fissure velocity and injection rate of liquid were measured rather than being imposed. Our experimental results allow evaluation of how the different driving and resistive pressures evolved during fissure propagation and highlight the influence of the fracture resistance of the host solid. In an initial transient propagation regime, elastic pressure generated by the fissure is balanced by the fracture pressure; the fissure propagates radially with decreasing velocity and increasing injection rate, controlled by the source conditions. Subsequently, buoyancy overcomes the source pressure as the driving force, and vertical steady state propagation is established. The fissure develops a bulbous head and propagation is controlled by the balance in this head, between buoyancy pressure and fracture pressure. Even after this transition, the constant values of velocity, flux, and strain energy release rate reflect the source conditions. Our model suggests that greater horizontal dyke cross section reflects larger source pressure and that mafic dykes propagating from shallow magma chambers are unlikely to attain steady state. Moreover, our experiments place constraints on the mechanics of time-dependent failure of the solid as a process that resists fissure propagation: propagation velocity scales with the square of the height of the fissure head, and fracture toughness of rocks would be length scale dependent rather than a material property.