A numerical determination of debris jet length scales and depositional structures in diatremes
Sweeney, Matthew R.
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Maar-diatreme volcanoes are the result of explosive subsurface interactions between magma and water that cut deeply into country rock. Although they constitute one of the most common volcanoes on the planet, historic observations of eruptions are extremely rare. As a result, experiments and numerical modeling are necessary to complement field work that is limited both spatially and temporally, in order to determine dynamics by which maar-diatremes grow and evolve during the course of an eruption. In this work, I present results from two-dimensional multiphase computational modeling of phenomena known as "debris jets", which are mixtures of vent-filling debris, vapor, and liquid water that form above subsurface explosion sites. Debris jets are the primary means by which mass is transported from diatremes to the surface, but many aspects of their formation, propagation, and deposition are still not fully understood. Based on field exposures and experiments, it is known that some debris jets formed during the course of an eruption do not breach the surface. Here, I show that the likelihood of a debris jet erupting is controlled by the amount of gas formed during the explosive magma-water interaction event. Unless extremely large amounts of magma and water are involved, it is unlikely that most explosions deeper than 250-300m breach the surface. Furthermore, redistribution of material within a diatreme is primarily driven by subsidence following debris jet passage and entrainment plays a noticeable, but small role in vertical transport. Lastly, debris jet deposits are defined by high concentrations (>33%) of material that is sourced at or directly adjacent to the explosion site, which implies that the composition of a deposit can be used to estimate its source location.