Magma dynamics and collapse mechanisms during four historic caldera‐forming events

Abstract

Four historic caldera‐forming events were studied to understand the relationship of magma withdrawal processes and caldera subsidence mechanisms. Two calderas are silicic (Katmai in 1912 and Pinatubo in 1991), and two are basaltic (Fernandina in 1968 and Miyakejima in 2000). All events have sufficient geophysical, geologic, and petrologic data with which to examine and model magma withdrawal and caldera collapse. The data reveal that the magmas erupted at Katmai and Pinatubo were in a bubbly state in the reservoir immediately before and during caldera collapse. The bubbly magma allowed for its efficient extraction from the reservoir, causing significant underpressures to develop rapidly, particularly in the case of Katmai where the erupted rhyolite was voluminous and nearly aphyric and has very low viscosity. The rapidly developing underpressures at Katmai and Pinatubo caused sudden en masse caldera collapse halfway through the climactic eruptions, thereby liberating large amounts of seismic energy. At Fernandina and Miyakejima, by contrast, caldera collapse was initiated early and continued for an extended period of time from weeks to months, consisting of a series of discrete subsidence events manifested by large earthquakes at Fernandina and by very long period (VLP) signals at Miyakejima. Systematic changes in earthquake magnitudes and quiescent intervals at both volcanoes reveal changes in friction, as collapse took place during extended time intervals.