ONSET OF CALDERA COLLAPSE DURING IGNIMBRITE ERUPTIONS

Abstract

The rapid depressurisation and discharge of volatile rich silicic magma chamber and the onset of caldera collapse associated with ignimbrite eruption is investigated using combined theoretical and experimental approaches. We first present a scaling analysis of the force balance on the roof of a depressurising magma chamber. This provides a failure criterion for the chamber roof and shows that the chamber underpressure required to trigger coherent (piston) collapse along a vertical or steeply outward dipping reverse ring fault increases with the roof aspect ratio (R=thickness/width). The failure criterion is validated by a series of experiments that reveal a transition, with increasing roof aspect ratio, from piston collapse to either (1) no collapse (low reservoir underpressure) or (2) non-coherent collapse (high reservoir underpressure). Then, taking into account the failure criterion obtained with the scaling analysis, a theoretical model allows calculation of the erupted chamber volume fraction required to trigger caldera collapse. We consider a two stage model, with an initially overpressured chamber that becomes progressively underpressured as the eruption proceeds. The main input parameters are the chamber depth, vertical extent and ellipticity in plan view, the magma water content, the cohesion and coefficient of internal friction of the roof, and the ring fault dip. In the case of a mechanically homogeneous roof and a single, large and rapid eruption, the erupted volume fraction required to trigger piston collapse increases with the roof aspect ratio until the chamber is totally emptied (before initiation of any collapse) at R~1-1.4, the exact value depending mainly on the ring fault dip and to a lesser extent on the coefficient of internal friction of the roof. As shown by experiments, collapse into reservoirs with R>1-1.4 probably cannot collapse as a piston and, if a surface caldera forms at all, collapse is more likely to occur non-coherently. Data for the onset of caldera collapse during seven well documented major ignimbrite eruptions are in broad agreement with the model.