Reconciling observations of volcanic deformation and degassing from basaltic volcanoes

Abstract:

Recent syntheses of multi-sensor and multi-satellite datasets for global volcano monitoring have provided insights into different phases of volcanic eruptive cycles. Here we use thermodynamic models to explore how different initial magma volatile contents affect the physical properties of basaltic magmas such as density and compressibility; and assess the consequent impact on co-eruptive observations of volcanic deformation and degassing. We define V̅ as the observed reservoir volume change, and S̅ as the observed SO₂ output, both normalised by the volume of magma erupted. An exploration of parameter space shows that basalts with high initial H₂O content or high initial gas content have high S̅ and high compressibility, hence during eruptions V̅ is small. As an illustration, increasing the initial magmatic H₂O content from 1.0 to 3.0 wt% and the initial gas content from 0.01 to 1.0 wt% increases S̅ up to 41% and 144% respectively, and decreases V̅ up to 62% and 38% respectively. Varying magmatic oxygen fugacity, which affects how sulfur partitions into the hydrous exsolved volatile phase, from NNO−1 to NNO+1 increases S̅ up to 112% but decreases V̅ by only <10%. In contrast, we find that varying the magma’s CO₂ content has little effect on both S̅ and V̅. These results may be used to understand patterns of volcano deformation and degassing observed globally. Arc basalts have higher initial volatile contents than ocean island basalts, which leads to <400% higher S̅ and a lower V̅ by <80%. Thus for the same eruption volume, the vertical displacement observed during basaltic arc eruptions (normalised by volume erupted) may be 90% less than that observed for ocean island eruptions for magma stored at depths of 2 km prior to eruption. Consistent with our conceptual models, we observe that deformation has been detected at 91% of oceanic island basalts volcanoes (10/11) which have erupted during the satellite era (2003–2020), but only 31% of arc basalt volcanoes (15/48). Finally, we compare the model predictions with eruptions compiled from global deformation and degassing catalogues and the Global Volcanism Project. We find that there is often a mismatch between our model predictions and observations. This suggests that the gas separated from its source magma can accumulate or degas, which has a large influence on surface observations.