Large‐Scale Drainage of a Complex Magmatic System: Observations From the 2018 Eruption of Ambrym Volcano, Vanuatu

In late 2018, Ambrym volcano, Vanuatu, erupted for the first time in 3 years. We show that the eruption was the result of a 6.5‐m‐wide, ∼0.7‐km3 dike intrusion which propagated for more than 20 km into the eastern rift zone. The eruptive sequence began with a small shallow dike within the main caldera but ultimately triggered a much larger intrusion, draining both the shallow (1–2 km) and deeper (∼5 km) magmatic systems. Extension of ∼3 m is estimated across the eastern rift zone with more than 2 m of subsidence within the caldera. Modeling suggests that the stress changes induced by the initial dike emplacement helped trigger and guide the propagation of the flank intrusion. Despite the large volume of material removed from beneath the caldera (∼0.7 km3), limited slip is observed along bounding faults supporting the hypothesis that the long‐term formation of the caldera at Ambrym is a result of repeated moderate‐sized events. Key Points The 2018 eruption at Ambrym was associated with more than 2 m of caldera subsidence and the intrusion of a ∼26‐km‐long dike To explain the complex deformation pattern requires that the intrusion was fed by multiple magma sources beneath the caldera Comparison between slip along caldera faults and eruption volume suggests Ambrym's caldera forms through repeated moderate‐sized eruptions