Magma reservoir evolution during the build up to and recovery from caldera-forming eruptions : A generalizable model?

Silicic calderas globally tend to record a cyclic magmatic, structural, and eruptive evolutionary progression. Some calderas are polycyclic, involving multiple catastrophic collapses in the same approximate location. Here we discuss five examples from well-studied, geologically-young and active magmatic systems: The Kos-Nisyros Volcanic Complex (Greece), Long Valley (USA), Campi Flegrei (Southern Italy), Rabaul (Papua New Guinea), and Okataina (New Zealand) in order to gain insights on the inner workings of caldera systems during the build up to and recovery from large explosive eruptions. We show that the sub-caldera magmatic system evolves through a series of processes, here collectively termed “caldera cycle”, that are common to monocyclic and polycyclic calderas. In the case of polycyclic calderas, they accompany the transition from one caldera-forming eruption to the next. The caldera cycle comprises (1) the period of pre-collapse activity (incubation, maturation, widespread presence of a magmatic volatile phase), (2) the catastrophic caldera-forming (CCF) eruption, and (3) post-collapse recovery (resurgence, renewed eruptions, subsequent maturation) or the possible cessation of the cycle. The incubation phase corresponds to a period of thermal maturation of the crust, during which eruptions are frequent and of small volume due to the limited capability of reservoirs to grow. During the maturation phase, magma reservoirs gradually grow, coalesce, homogenize, magmas differentiate, and eruption frequency decreases. The system transitions into the fermentation phase once an exsolved magmatic volatile phase is continuously present in the reservoir, thereby increasing the compressibility of the magma and instigating a period of runaway growth of the reservoir. A CCF eruption at the end of the fermentation phase could be the concatenated result of multiple magmatic processes (including magma recharge, volatile exsolution, and crystal mush remelting) pressurizing the reservoir, while external factors (e.g., tectonic processes) can also play a role. Postcaldera eruptions, subvolcanic intrusions, and hydrothermal activity typically continue, even if the magma supply wanes. If, however, magma supply at depth remains substantial, the system may recover, initially erupting the remobilized remains of the CCF reservoir and/or new recharging magmas until a shallow reservoir starts to grow and mature again. Placing other calderas worldwide within this framework would enable to test the robustness of the proposed framework, deepen the understanding of what controls the duration of a cycle and its individual phases, and refine the petrologic, geophysical, and unrest symptoms that are characteristic of the state of a system.