Many large calderas result from the eruption of substantial volumes (tens or hundreds of km3) of silicic pyroclastics. Such events often begin with an airfall phase and progress to the generation of voluminous ignimbrites1-3. We propose here that many such eruptions involve two well-defined stages, based on a simple analysis of magma chamber pressure variations during an eruption. The first stage begins when an overpressured magma chamber fractures the country rock and forms a conduit to the surface. The chamber pressure decreases rapidly to values less than lithostatic pressure. We show that only small to moderate volumes of magma, representing a small fraction of the total chamber, can be erupted during this stage. In the second stage, caldera collapse results from a further decrease in magma pressure, which causes the chamber roof to fracture catastrophically and deform. Subsidence of the roof attempts to re-establish lithostatic pressures within the chamber and can drive substantial volumes of magma to the surface. Geological relationships in pyroclastic deposits associated with large caldera eruptions provide independent evidence for this model. ?? 1984 Nature Publishing Group.