Modeling particle motion and near-vent deposition in explosive volcanic eruptions
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Models to predict tephra deposition from explosive volcanic eruptions have historically relied on the Advection-Diffusion-Sedimentation (ADS) methodology. While models using this approach have been shown to accurately predict tephra dispersion patterns in the medial and distal parts of the fall deposit where tephra transport is dominated by the ambient wind field, they are very limited in their ability to adequately describe near-vent deposition. This limitation is due to the reduction of the modeled eruption column to a parameterized source space, such as a line, and causes ADS models to be incapable of describing the complex relationship between the eruption column and the ambient wind. The internal structure of the eruption column, coupled with the effects of the ambient wind, have a significant effect on near-vent tephra dispersal. The limitations of existing deposit inversion methods were explored using the four 600 BP subplinian tephra fall deposits from the Inyo volcanoes in eastern California, USA. The fall deposits were mapped and characterized using field and laboratory measurements, and the resulting isopleth maps were inverted to determine maximum column height and ambient wind velocity at the time of deposition. For the two most elongate deposits, the deposit dimensions plotted outside of the inversion nomogram space, indicating extremely high ambient wind velocities. To address the limitations in existing tephra deposition models, a Lagrangian particle tracking model has been written as an extension to the fluid dynamics based Active Tracer High-resolution Atmospheric Model (ATHAM), a large-eddy simulation of volcanic eruptions. The new deposition model, referred to as the Large Particle Module (LPM), tracks individual tephra clasts through the complex wind field generated by the interaction between the eruption column and the ambient wind. The combined model is referred to comprehensively as ATHAM-LPM and can be run in 2D or 3D, covering spatial areas up to 50 km high and 100 km wide and temporal periods up to several hours. Results from ATHAM-LPM modeling of a 2D, time-steady eruption indicate that there are four primary modes of tephra transport within an eruption column: ballistic, column, corner, and umbrella. With the exception of ballistics, transport trajectories are named on the basis of where the clasts decouple from the eruption column; thus, a corner trajectory describes the path of a clast that sediments from the corner of the column, where flow transitions from the vertically rising convective column to the laterally spreading umbrella region. In addition, ATHAM-LPM clasts indicate that both the settling speed and horizontal inertia play an important role in determining the maximum depositional range for specific clast sizes and densities; the new equivalency for pumice and lithic clast sizes suggested by the model output is supported by field observations for various fall deposits, including the Inyo eruptions studied in this work. ATHAM-LPM was used in 2D, time-varying mode to model two radially symmetric fall deposits: Fogo A (Agua de Pau volcano, Azores) and Pululagua BF (Pululagua volcano, Ecuador). Model output of grain size distributions closely matches recorded field observations, both with regard to maximum grain size dispersal and relative abundances of clasts at given distances from the vent. The maximum column heights predicted using ATHAM-LPM are within the low end of the range of previously proposed heights associated with each eruption. When run in 3D, time-varying mode, ATHAM-LPM illustrates the complex nature of the interaction between the eruption column and the ambient wind. Ambient winds can enhance air entrainment in the rising column, thereby stabilizing eruption columns that might otherwise collapse. Conversely, very high winds relative to the mass eruption rate can destabilize a column by overpowering the upward momentum of the column material, thereby initiating column collapse. Ambient winds decrease ash concentrations in the eruption cloud and decrease maximum column height, though in all simulated eruptions, column material reached the tropopause. Within the eruption columns, tephra clasts remain largely homogenously mixed until reaching the height of neutral buoyancy or collapse, at which point they segregate on the basis of diameter and density, with smaller, lighter clasts traveling higher and further than their more massive counterparts.