WATER-RICH silicic magmas may erupt explosively, giving rise to massive columns of fragmented ash(1-5), or effusively as viscous, bubbly lavas to form lava domes(6-8). Complex cycles of explosive and effusive eruptions are often observed(8-12). Although these two eruption mechanisms have been modelled independently(1,8), the conditions under which a volcano exhibits a particular style of suption are not known and the transitions between eruption styles are poorly understood. By modelling the ascent of magma along a permeable conduit, we show here that, for small magma-chamber overpressures, the style of eruption-explosive or effusive-is dependent on the magma flow rate. We also identify a critical overpressure above which only explosive eruptions occur. Complex eruption sequences follow naturally. For example, if a shallow magmatic system is supplied with magma at an intermediate flow rate, the eruption will be slow and effusive until the chamber over-pressure becomes too large. An explosive eruption then relieves the overpressure and an effusive eruption style resumes. We suggest that monitoring of temporal variations in chamber overpressure (for example, by measuring ground deformation) can be used to assess whether a passively effusing volcano has the potential to erupt explosively.