One of the greatest remaining problems in modern volcanology is the process by which volcanic eruptions are triggered. It is generally accepted that eruptions are preceded by magma intrusion [Sigurdsson and Sparks, 1978>. The degree of interaction between previously ponded magma in a chamber and newly intruded magma determines the nature and rate of eruption and also the chemistry of erupted lavas and shallow dykes. Here, we investigate the physics of this interaction. Volcano monitoring at its most effective is a synergy between basic science and risk assessment, while hazard mitigation depends on reliable interpretation of eruption precursors. The simple and much used Mogi model relates ground deformation (Δh) to changes in magma chamber volume. Gravity changes (Δg) combined with ground deformation provide information on magma chamber mass changes. Our new models predict how the Δg/Δh gradient will evolve as a volcano develops from a state of dormancy through unrest into a state of explosive activity. Thus by simultaneous measurement of deformation and gravity at a few key stations, magma chamber processes can be identified prior to the onset of conventional eruption precursors. ¿ 2000 American Geophysical Union