Abstract
How can deep-ocean high-pressure eruptions produce explosions and volcanic ash? Molten-fuel coolant interaction (MFCI), a thermohydraulic mechanism that leads to powerful explosions and to the fragmentation of magma, requires a pre-mix of magma with entrapped water in a setting where both components are separated by an insulating vapour-film. The latter is sensitive to the ambient pressure, which means that MFCI processes are suppressed in submarine settings at depths larger than 100 m. Recent studies have identified, however, a pressure-insensitive type of explosive interaction between liquid water and hot magma, termed induced fuel-coolant interaction (IFCI). This thermohydraulic mechanism is initiated by the formation of cracks in cooling magma into which the water coolant can infiltrate, driving explosive fragmentation and thus "boosting" the production of fine ash. IFCI was identified as the main process that generated notable amounts of fine ash in the kilometre-deep 2012 submarine eruption of Havre volcano, Kermadec arc, New Zealand. We show results from both numerical models and laboratory experiments with Havre melt that mimic deep-sea explosive eruptions and examine the consequences of our findings for deep-sea eruption scenarios in terms of energy release and resulting products. For example, at surface pressures the force produced during dry gas-driven fragmentation increases by 50% with IFCI, and the proportion of extremely fine ash doubles. We demonstrate that IFCI between magma and water can occur in a wide range of wet environments regardless of pressure, from subaerial to the deep sea, suggesting that induced fuel-coolant interaction might play an unappreciated role in deep submarine eruptions and probably in other eruptions involving water. We discuss differences and similarities between IFCI and MFCI explosions and present a set of indicators that can help volcanologists to identify IFCI processes as main drivers of explosive eruptions.