4D gravity-geodesy – An integral part of comprehensive monitoring and modelling of large silicic magmatic systems

Prior to a volcanic eruption, volatile-rich magma and gases can accumulate deep within the volcano and migrate upward interacting with associated hydrothermal systems. This movement can lead to pressurisation and fracturing within the volcanic plumbing system, generating characteristic precursory seismic signals and volumetric changes as the volcanic edifice inflates. However, in the case of large silicic magmatic systems such as Campi Flegrei, Yellowstone or Laguna del Maule, these processes can occur on time scales of 102-106 years with repeated episodes of unrest interspersed by long periods of quiescence. Constraining the physicochemical processes responsible for periods of non-eruptive unrest (intruding magma vs. hydrothermal fluids) and determining whether their signals indicate pre-eruptive conditions is particularly problematic. However, by integrating 4D (spatial and temporal) gravity and precise geodetic (e.g. InSAR, continuous GNSS) observations, the large density contrast between magma, volatiles, hydrothermal fluids and the surrounding country rock can greatly improve characterisation of the location, amount and rate of subsurface magma movement especially when interpreted together with seismicity or other eruption precursors. For example, 4D gravity-geodesy studies have been able to place constraints on the dimensions, depth, and melt fraction of magma bodies in large silicic systems (Laguna del Maule). These studies have also enabled quantification of mass and/or density changes for modelling (3D inverse, analytical, numerical) shallow magma movement on temporal (months to years) and spatial (10s km) scales within large silicic magmatic systems (Campi Flegrei). While continuous temporal gravity networks with a limited number of instruments operates at a few basaltic systems (Hawaii), the logistical difficulties (temporal variations, spatial scale, instrument costs) have been a barrier for monitoring large silicic systems. The recent development of MEMS gravity meters (with orders of magnitude lower instrument costs) may bring continuous gravity on par with seismic monitoring. Instrumentation and advanced modelling codes are thus now at a stage that 4D gravity-geodesy could be used as an integral component of any comprehensive study of large silicic magmatic systems.