Transport and mixing dynamics from explosions in debris-filled volcanic conduits: Numerical results and implications for maar-diatreme volcanoes

Abstract

Most volcanoes experience some degree of phreatomagmatism during their lifetime. However, the current understanding of such processes remains limited relative to their magmatic counterparts. Maar-diatremes are a common volcano type that form primarily from phreatomagmatic explosions and are an ideal candidate to further our knowledge of deposits and processes resulting from explosive magma–water interaction due to their abundance as well as their variable levels of field exposure, which allows for detailed mapping and componentry. Two conceptual models of maar-diatreme volcanoes explain the growth and evolution of the crater (maar) and subsurface vent (diatreme) through repeated explosions caused by the interaction of magma and groundwater. One model predicts progressively deepening explosions as water is used up by phreatomagmatic explosions while the other allows for explosions at any level in the diatreme, provided adequate hydrologic conditions are present. In the former, deep-seated lithics in the diatreme are directly ejected and their presence in tephra rings is often taken as a proxy for the depth at which that particular explosion occurred. In the latter, deep-seated lithics are incrementally transported toward the surface via upward directed debris jets. Here we present a novel application of multiphase numerical modeling to assess the controls on length scales of debris jets and their role in upward transport of intra-diatreme material to determine the validity of the two models. The volume of gas generated during a phreatomagmatic explosion is a first order control on the vertical distance a debris jet travels. Unless extremely large amounts of magma and water are involved, it is unlikely that most explosions deeper than ∼250 m breach the surface. Other factors such as pressure and temperature have lesser effects on the length scales assuming they are within realistic ranges. Redistribution of material within a diatreme is primarily driven by subsidence following debris jet passage. Natural debris jet deposits often contain wall rock material that originated above the explosion site; likely this material was first transported downward by subsidence associated with preceding explosions and debris jets. The length scales of debris jets decrease with increasing depth, all other factors staying equal. The characteristics of the domains formed from just one explosion along with the difficulty of erupting deep-seated material indicate that erupted material at maar-diatreme volcanoes has a complex history before it is erupted and is mostly sourced from the upper-most part of the diatreme.