CO₂/CH₄ ratio in fumaroles a powerful tool to detect magma degassing episodes at quiescent volcanoes

Long time series of fumarolic compositions at Campi Flegrei (Italy), Mammoth Mountain (California), Panarea (Italy) and Nisyros (Greece) show rapid increases, up to orders of magnitude, of the CO 2 /CH 4 ratio systematically with the occurrence of volcanic unrest periods. These easily detected anomalies originate with the arrival of CH 4 ‐poor magmatic fluids in the shallower levels of the volcanoes. The data suggest that volcanoes are characterized by magmatic activity at depth also in periods of apparent quiescence. The activity is constituted by the pulsing release of large amount of fluids which either cause unrest periods (seismicity and ground deformation) or possibly could precede volcanic eruption. This type of volcanic activity can be monitored trough the classical geophysical techniques together with the systematic sampling and analysis of fumaroles.