Host Rock Variability Powers the Diversity of Steam‐Driven Eruptions.

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]