Pressurization of Shallow Magma Reservoir Preceding Basaltic Eruptions at an Open‐Vent Volcano

Abstract

Open‐vent volcanoes are characterized by the persistent activity of outgassing, lava effusion and mild explosions, which is, however, occasionally interrupted by more violent explosive activity (paroxysm), for example, Stromboli (Italy). In spite of the different (two orders of magnitude) explosive intensities and different petrological magma source, regular explosive activity and paroxysms share the same inflation pattern of ground deformation. We show that this similar ground deformation pattern is also reflecting a common pressurized shallow reservoir of gas‐poor, crystal‐rich stagnant magma source located at 280 m depth beneath the eruptive vent. We demonstrate that this source can also explain the ground deformation recorded during flank effusive eruptions, suggesting that both explosive and effusive dynamics are involving the same shallow magmatic system. We propose a fluid mechanical model of a basaltic conduit that correlates a geodetically‐derived pressure change of the shallow reservoir and eruptive phenomena at the vent. The pressure changes corresponding to the ground inflation before explosive eruptions (regular “Strombolian” explosion and paroxysm) and deflation during lava effusion are compatible with the magma volume discharged during each eruption, whereas a dynamic pressure caused by viscous magma flow in the conduit can be used to infer the final inflation velocity of the magma surface before the paroxysm.