Magma compressibility matters: a key to decoding multiparameter datasets from active volcanoes

The joint analysis and interpretation of multiparameter datasets from active volcanoes may lead to misleading conclusions, if important factors are not appropriately considered. Among these, magma compressibility, which is mainly controlled by the volume fraction of exsolved gas in the magma, may play a key role.
Past studies showed that the intrusion of new magma in a shallow reservoir may lead to significant mass increase without the expected volume change, since magma compressibility buffers most of the chamber expansion. Similarly, the magma chamber volume reduction during an eruptive phase may be much lower than the volume of erupted material, due to pressure-driven gas exsolution and expansion, compensating the withdrawal of magma, thus buffering the contraction of the reservoir.
Here, we introduce a theoretical study on how the different compressibility of the magma at different depths (variable amount of exsolved volatiles in equilibrium with the silicate melt) may influence the patterns of deformation and gravity changes observed at the surface. Magma intruding a volcano’s plumbing system may induce heterogeneous responses across different depths. At deeper levels, where magma compressibility is lowest, volume change may be substantial and control most of the observed ground deformation. Conversely, at shallower levels, where magma compressibility is highest, important mass changes may develop with only minor volume changes, accounting for most of the gravity changes observed at the surface. 
An important broader implication is that ground deformation and gravity data may not be suitably modelled by assuming a single, uniform source. Rather, a vertically distributed and mechanically heterogeneous magma system may need to be considered. This underscores the need for a joint interpretation of deformation, gravity, and volatile content data when investigating volcanic processes.