Large calderas are formed by collapse of the crust into magma chambers during the eruption of vast quantities of silicic magma. Here I show that magmatic buoyancy can play a key role in attaining the conditions necessary for these eruptions to occur. During an eruption, magma buoyancy allows the simultaneous conditions of the chamber becoming underpressurized, whilst the conduit remains open, consequently the eruption continues and reduces chamber pressures further. The maximum chamber underpressures attained, and the associated stresses imposed on the surrounding crust, are dependent on the magma-crust density contrast and chamber size. Sufficiently large imposed stresses cause the crust surrounding the chamber to collapse, stimulating a voluminous eruption and caldera formation. The increase in magma buoyancy associated with the compositional evolution of a magma body accounts for the common transition from initial small or moderate sized intermediate composition eruptions, to ultimately large silicic caldera collapse events. ¿ 1999 American Geophysical Union