Magmatic processes associated with the development of large silicic calderas

Large caldera-forming eruptions are among the greatest hazards faced by mankind. Eruptive sequences from caldera-forming eruptions at Ossipee ring complex, New Hampshire, and Lake City caldera, Colorado, reveal that magmas underwent crystal fractionation to produce a zoned silicic magma chamber in each case before caldera formation. At Ossipee, magmatic inclusions with resorbed phenocrysts and positive Eu anomalies demonstrate that magma replenishment caused chamber rejuvenation and the initiation of caldera collapse. At Lake City, two ignimbrite units associated with episodes of caldera collapse have mafic-rich bases; this indicates that these collapse events were initiated by mafic magma replenishment. A numerical model is developed that describes how magma chamber rejuvenation causes a reduction in crystal content, a decrease in gas pressure, and hence caldera collapse. During caldera formation at Ossipee and Lake City, magmas from progressively deeper levels in the chambers were erupted and intruded to shallower levels. Periods of reverse compositional zonation and lateral compositional variation at the top of ignimbrite units and in post-collapse intrusions indicate that a period of accelerated magma interaction occurred in the chambers towards the end of subsidence. Scaled analogue experiments of caldera collapse into density-stratified magma chambers reveal a similar period of reverse zonation and dynamic fluid interaction as the subsiding block approaches the chamber base. Additionally, the experiments indicate that piecemeal or asymmetric collapse cause vortices to develop in the chamber which efficiently disrupt stratification. After caldera collapse at Ossipee and Lake City, replenishing magma caused mixed residual magma to intrude along caldera faults to shallow levels and drive resurgence. The work illustrates how particular magmatic processes influence caldera collapse, and in turn, how caldera collapse affects particular magmatic processes.