The influence of conduit geometry on the dynamics of caldera-forming eruptions

During caldera collapse events, new conduits may open and dramatically modify the style of the associated eruption. In order to investigate the effect of conduit geometry on the dynamics of caldera-forming eruptions, we performed numerical simulations of magma ascent in singlevent, cylindrical conduits and ring-fissure conduits. The results show that, for a given volume of conduit, the discharge rate is an order of magnitude smaller in ring-fissure conduits due to the higher friction. Furthermore, the transition from a sustained Plinian column to a collapsing fountain feeding pyroclastic flows occurs at a discharge rate an order of magnitude higher for ring-fissure conduits, due to the higher rate of air entrainment in a curtain jet than in a cylindrical jet. The production of pyroclastic flows from ring-fissure conduits therefore requires a much larger conduit volume than from a cylindrical conduit. We argue that this should be correlated with a much larger volume of lithics in the deposit. We confront these theoretical considerations with two geological examples. The Taupo ignimbrite has been emplaced by a high-discharge rate pyroclastic flow and contains a small volume of lithics. This is consistent with its emplacement during a single-vent eruptive phase. The Bishop Tuff has been emplaced by a lower discharge rate pyroclastic flow and yet contains much more lithics. This is consistent with an eruption through successive vents migrating along a ring fissure. The lithic content of an ignimbrite and the eruption discharge rate can therefore give some insights into the mechanism of caldera collapse and the role of ring fissures as magma conduits. A conclusion of our analysis is that perfectly annular ring-fissure conduits do not favour the formation of pyroclastic flows. The occurrence of pyroclastic flows in virtually all calderaforming eruptions suggests that magma ascent is mainly localised in more restricted conduits. A corollary conclusion is that widespread ignimbrites, which record high-discharge rate eruptions, are not necessarily the result of ring fissure opening during caldera collapse.