Time‐Resolved Trigger Processes Leading to the Plinian Eruptions at Sakurajima Volcano, Japan

Mafic magma recharge of crustal reservoirs and subsequent magma mixing has been considered a direct trigger of volcanic eruptions. However, although recharge frequently occurs in many active volcanoes, it rarely leads to an eruption immediately, making its role as a trigger ambiguous. Sakurajima volcano, Japan, has vigorously erupted three times since the 15th century following a common process; mixed magmas after recharge were once stored in a shallow, thick conduit before each eruption (conduit pre‐charge). We reconstructed the magma migration with a high time resolution by diffusion modeling on orthopyroxene and magnetite. Orthopyroxene phenocrysts recorded prolonged diffusive re‐equilibration timescales of years or more after recharge‐and‐mixing. Magnetite, which has the fastest elemental diffusivity among the phenocrysts examined, predominantly lacks zoning. This demonstrates that the mineral phase was re‐equilibrated with surrounding magma and homogenized via elemental diffusion after the final magmatic perturbation, implying the final repose of the shallow pre‐charged magma body for more than several tens of days. After this shallow stagnation period, the Plinian magmas began to ascend and reached the surface within 55 hr. Mass balance calculations show that crystallization‐driven vesiculation upon pre‐charge can produce overpressure sufficient to cause an eruption. The Sakurajima cases demonstrate the hierarchical timescales of trigger processes leading to the explosive eruptions. Plain Language Summary Understanding how volcanic eruptions are triggered is an important scientific challenge that directly contributes to studying volcanic hazard. Volcanic ejecta often shows evidence of the injection of hot magma into magma reservoirs, with the injection often considered an eruption trigger. However, such injection is frequently observed in many active volcanoes, while rarely leads to an eruption immediately. Sakurajima volcano, Japan, is an excellent test field to understand the trigger mechanism because three large‐scale eruptions occurred in historical time following a similar sequence. A previous study has revealed that the Sakurajima magmas once stagnated in a shallow, thick conduit before each of the eruptions. In this study, we conducted elemental diffusion simulation of phenocrysts and constrained the duration of the shallow stagnation to be more than tens of days. We also found that magma injection occurred repeatedly but did not immediately l