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Steam‐driven eruptions, both phreatic and hydrothermal, expel exclusively fragments of non‐juvenile rocks disintegrated by the expansion of water as liquid or gas phase. As their violence is related to the magnitude of the decompression work that can be performed by fluid expansion, these eruptions may occur with variable degrees of explosivity. In this study we investigate the influence of liquid fraction and rock petrophysical properties on the steam‐driven explosive energy. A series of fine‐grained heterogeneous tuffs from the Campi Flegrei caldera were investigated for their petrophysical properties. The rapid depressurization of various amounts of liquid water within the rock pore space can yield highly variable fragmentation and ejection behaviors for the investigated tuffs. Our results suggest that the pore liquid fraction controls the stored explosive energy with an increasing liquid fraction within the pore space increasing the explosive energy. Overall, the energy released by steam flashing can be estimated to be 1 order of magnitude higher than for simple (Argon) gas expansion and may produce a higher amount of fine material even under partially saturated conditions. The energy surplus in the presence of steam flashing leads to a faster fragmentation with respect to gas expansion and to higher ejection velocities imparted to the fragmented particles. Moreover, weak and low permeability rocks yield a maximum fine fraction. Using experiments to unravel the energetics of steam‐driven eruptions has yielded estimates for several parameters controlling their explosivity. These findings should be considered for both modeling and evaluation of the hazards associated with steam‐driven eruptions. Experimental studies to unravel the steam‐driven eruption energeticsInfluence of liquid fraction and rock petrophysical properties on the explosive energySteam flashing results in a faster fragmentation with respect to gas expansion and into higher ejection velocities of fragmented material

Steam‐driven eruptions are explosive events that are fueled by pressurized water and steam trapped within rock and sediments. We show how rock properties modulate explosion size, dynamics, and hazard footprint based on examples from Lake Okaro (New Zealand). Laboratory decompression experiments demonstrate that fragmentation of strong/unaltered host rocks comes with a high energy cost (∼10%–11% of bulk explosion energy). Consequently a low energy fraction (∼7%–8%) remains for kinetic energy and thus particle ejection. In contrast, disaggregation of unconsolidated sediments requires little energy (<2%–7%), allowing higher outputs of kinetic energy (22%–25%), and more efficient debris dispersion. Experimental estimates of bulk explosive energies are consistent with both field observations and empirical models applied to Lake Okaro crater dimensions. This integration of experimental methods, field observations, and empirical modeling underscores the dominant role of alteration state and host rock lithology when estimating crater‐forming and ballistic hazards in volcanic/geothermal areas. Steam‐driven eruptions are explosions that frequently occur in volcanic and geothermal areas. They are powered by the sudden release and expansion of steam and liquid water trapped under high pressure within the pore spaces of host rocks. Here we have experimentally studied how the strength of rock hosting steam and liquid controls the nature of explosions. Specifically, we used experiments to estimate the relative amounts of energy that goes into breaking rock up, versus that required for ejecting particles upwards and outwards. We used natural rock samples collected from well‐studied explosion craters at Lake Okaro (New Zealand). Experiments recreated pressures and temperatures of the geothermal system and allowed sudden decompression of water saturated rock. We demonstrated that the porosity, permeability, and strength of rocks is well reflected in different experimental behaviors. Experiment data was scaled to field settings and natural examples. This showed that stronger rocks require much energy to break, hence, if they are the dominant host rocks, less energy is available for particle ejection. This implies a smaller hazard footprint. Future hazard assessment for steam‐driven eruptions should take these findings into account. Experimental calculations can be scaled to crater sizes yielding estimates of energy partitioning for fragmentation and ejection processesUnconsolidated and weak lithologies disaggregate with low‐energy, producing larger craters and ejecta volumes with greater hazard footprintsHard‐rock requires more energy to fragment, which results in smaller craters, lower ejecta volumes and smaller hazard footprints Experimental calculations can be scaled to crater sizes yielding estimates of energy partitioning for fragmentation and ejection processes Unconsolidated and weak lithologies disaggregate with low‐energy, producing larger craters and ejecta volumes with greater hazard footprints Hard‐rock requires more energy to fragment, which results in smaller craters, lower ejecta volumes and smaller hazard footprints