We present the first regional map of CO₂ Earth degassing from a large area (most of central and south Italy) derived from the carbon of deep provenance dissolved in the main springs of the region. The investigation shows that a globally significant amount of deeply derived CO₂ (10% of the estimated global CO₂ emitted from subaerial volcanoes) is released by two large areas located in western Italy. The anomalous flux of CO₂ suddenly disappears in the Apennine in correspondence to a narrow band where most of seismicity concentrates. Here, at depth, the gas accumulates in crustal traps generating CO₂ overpressurized reservoirs which induce seismicity.

During the reawaking of a volcano, magmas migrating through the shallow crust have to pass through hydrothermal fluids and rocks. The resulting magma–hydrothermal interactions are still poorly understood, which impairs the ability to interpret volcano monitoring signals and perform hazard assessments. Here we use the results of physical and volatile saturation models to demonstrate that magmatic volatiles released by decompressing magmas at a critical degassing pressure (CDP) can drive volcanic unrest towards a critical state. We show that, at the CDP, the abrupt and voluminous release of H₂O-rich magmatic gases can heat hydrothermal fluids and rocks, triggering an accelerating deformation that can ultimately culminate in rock failure and eruption. We propose that magma could be approaching the CDP at Campi Flegrei, a volcano in the metropolitan area of Naples, one of the most densely inhabited areas in the world, and where accelerating deformation and heating are currently being observed.

We present a reliable methodology to estimate the energy associated with the subaerial diffuse degassing of volcanic‐hydrothermal fluids. The fumaroles of 15 diffuse degassing structures (DDSs) located in eight volcanic systems in the world were sampled and analyzed. Furthermore, each area was measured for soil temperature gradients and for soil CO₂ fluxes. The results show that each hydrothermal or volcanic system is characterized by a typical source fluid which feeds both the fumaroles and diffuse degassing through the soil. Experimental data and the results of physical numerical modeling of the process demonstrate that the heat released by condensation of steam at depth is almost totally transferred by conduction in the uppermost part of the soil. A linear relationship is observed between the log of the steam/gas ratio measured in the fumaroles and the log of the ratio between soil thermal gradient and soil‐gas flux. The main parameter controlling this relation is the thermal conductivity of the soil (K_c). For each area, we computed the values of K_c which range from 0.4 to 2.3 W m⁻¹ °C⁻¹. Using the CO₂ soil fluxes as a tracer of the deep fluids, we estimated that the total heat released by steam condensation in the systems considered varies from 1 to 100 MW.