Continuous gravity measurements across the June – July 2021 series of lava fountains at Mt. Etna volcano

The gravity time series from two iGrav superconducting gravimeters and an AQG-B absolute quantum gravimeter, installed at Mt. Etna volcano, reveal a marked gravity decrease during early-June to late-July 2021, a period when more than 20 short-lasting and strongly explosive eruptions (the so-called lava fountain events) took place from one of the summit craters of the volcano. GNSS data indicate that this phase of gravity decrease was associated to deflation of the volcano edifice. 
We performed a joint inversion of the gravity data and deformation field to define the parameters of the common mass/pressure source. The optimal source is located beneath the summit crater area of Mt. Etna, at a depth between 2 and 3 km below the sea level. Results of the joint modeling also point to a residual mass change that is largely in excess of the corresponding volume change, for any reasonable density of the material extracted from the source. 
We propose that the observed gravity decrease was mostly driven by a decompression-related density change, i.e., an increase in the proportion of exsolved gas to magma in the source reservoir. This hypothesis is checked through comparing the results of the geophysical data inversion with independent estimates of the change in exsolved gas content due to pressure decrease in the source reservoir.