A magma chamber may be considered as an open, non adiabatic chemical reactor that continuously receives new liquid, crystallizes, and erupts lava. Heat is transferred to the walls, and crystals settle on the chamber floor. Considering, for simplicity, the chamber magma as a thermally and chemically homogeneous two-component liquid in which the solidification rate is a bell-shaped function of temperature and composition, the system is governed by two coupled differential equations for the evolution of temperature and composition with time. For constant external factors, such as fresh-magma input flow or heat loss flux, the magma chamber tends to a steady state. For a certain domain of external factor values, there are two stable steady state solutions instead of one. When the external factors vary slowly, the magma chamber state also drifts continuously, except when the external factors leave the bistability domain. Then all characteristics of the volcanic system undergo a sudden jump, for example, from basic to felsic composition and from high to low lava output flow when the fresh-magma input flow of a waning volcano declines. The jump takes place over a time of the order of the magma residence time and is thus short compared to the lifetime of the chamber. Such a magmatic catastrophe, which may occur for a small and continuous evolution of the external conditions, results in a radical change in eruption style and volcanic products and may trigger an exceptionally large or violent eruption. This kind of event may explain the widespread observation of compositional gaps in lava series (e.g., the Daly gap), a number of radical changes in eruptive style, and certain eruptions which appear unique in the lifetime of some composite volcanoes. ¿ 1999 American Geophysical Union