Recent advances in our understanding of arc magmatic systems indicate that melt is stored for long periods in low‐melt fraction crystal mushes, and that eruptible magma reservoirs are short‐lived and are assembled rapidly by buoyancy‐induced instabilities and draining of the crystal mush. Many aspects of their architecture remain unclear, particularly in relation to their geometry and shallow melt distribution. We investigate the storage of melt below the active Soufrière Hills Volcano (SHV), Montserrat, using joint geophysical inversion combined with a quantitative interpretation approach based on rock physics. We jointly inverted active‐source P‐wave traveltimes and gravity anomalies to derive coincident 3D models of P‐wave velocity and density to a depth of 8 km. Comparative analysis of the active SHV and extinct Centre Hills volcano and effective elastic medium computations allow us to constrain temperature, melt fraction and melt geometry. A continuous column of partial melt is inferred beneath SHV, at 4‐8 km depth. Melt fraction is ~6% (ranging from 3% to 13% depending on melt geometry) and is maximum at 5‐6 km depth. When under‐recovery of the low‐vP volume is taken into account, the melt fraction is revised to ~17% (ranging from 11 to 28%). Analysis of vP/density cross plots indicates that the melt distribution is best represented by low‐aspect ratio geometries. These likely span a multi‐scale spectrum ranging from grain‐scale inclusions and fractures to 100‐meter‐scale dykes and sills. Our results confirm the concept of vertically extensive crystal mush including one or multiple more melt‐rich layers.

Plain Language Summary

We use data from the Soufrière Hills Volcano on Montserrat to study the storage of magma in the plumbing system of an active volcano. By measuring the time it takes for seismic waves to travel from artificial sound sources to recording instruments located on the island and on the seabed around it, we can determine the speed of sound in the rocks. By measuring small variations in the gravitational attraction, we can determine the density distribution. We then jointly analyze the speed of sound and density and calculate that the degree of melting of the rocks is between 6% and 17% and the total volume of molten rocks is 1.5 km3 to 4.5 km3. The partially molten region extends from 4 to at least 8 km depth. Some more thoroughly molten layers are likely present but are too small to be imaged with our method. Our findings support a model of the magma storage system comprised mostly of solid crystals with a relatively small amount of molten rocks.

Key Points

Seismic and gravity data are jointly inverted to produce models of vP and density beneath an active volcano including the magma reservoir

Temperature and melt distribution are estimated jointly from vP and are consistent with a mush zone with 6‐17% melt fraction

Analysis of vP/density cross‐plot indicates low melt fraction and low‐aspect ratio melt inclusion geometry