The hidden activity of volcanoes: magma degassing events

The ratio between fumarole gas species of predominant magmatic origin (i.e., CO₂, He) and CH₄, which is typically produced in the hydrothermal environment, is a powerful indicator of the upflow of magmatic fluids towards the surface. The analysis of time series of fumarolic composition from different dormant volcanoes (e.g., Campi Flegrei, Vesuvius, Vulcano, Panarea, Nisyros, Mammoth Mt) reveals similar anomalous peaks of the CO₂/H₂O, CO₂/CH₄, and He/CH₄ ratio during unrests, suggesting recurrent events of magma degassing. The sudden upflow of deep magmatic fluids causes pressure buildup and heating of the hydrothermal systems, earthquakes and ground deformations, which precede the fumarole gas compositional anomalies. These anomalies, once normalized to the time and to the amplitude of the curve, show the same shape: a rapid increase in the geochemical indicator followed by an exponential decrease.

An intriguing consideration based on these examples (almost all the volcanoes for which a long time series of fumarolic compositions are available) is that episodes of magma degassing probably reflect the normal background activity of volcanoes, making the interpretation of volcanic unrests challenging. Concurrently with magma degassing events, a significant increase in the CO₂ degassing process was either measured or qualitatively observed in these systems. A better knowledge of these degassing episodes is needed to improve our understanding of the volcanic behavior and to better constrain the release of CO₂ from quiescent volcanoes.