Studies of pyroclastic fall transport and deposition

Abstract

Deposits of explosive eruptions provide insight to volcanic parameters and processes that serve in hazard assessment of potential future eruptions. This dissertation research aims to assess and improve current methodologies to estimate ejecta volumes and ash dispersion and is based on two well-documented eruptions, the 1959 Kilauea Iki eruption in Hawaii, USA, and the 17 June 1996 Ruapehu eruption in New Zealand. The first project (Chapter 2 and 3) investigates the variability in volume calculations that arises from the three steps involved in estimating ejecta volume from measurements of deposit thickness: the spacing and number of deposit measurements, the hand-contouring of spatial data, and the method used to fit and integrate deposit volumes from a contour map. A group of 101 volcanologists from 11 countries independently contoured isopachs for pairs of maps with different numbers of deposit thickness measurements of the 1959 Kilauea Iki eruption deposit. We find an uncertainty in isopach area of 7% across the well sampled deposit that increases to over 30% for isopachs of the largest and smallest thickness measurements. After excluding the most proximal deposit for which no measurements are available, we find surprisingly consistent volume estimates across the large variety of returned isopach maps (s = 7.3% to 13.5%), small variations between maps of different sampling densities, and an uncertainty of volume estimates due to different methodologies of s = 11.6% on average. The standard deviation across the total volume (including interpolated thicknesses for the proximal deposit) amounts to s = 35%. Volume uncertainties are largest for the most proximal and the most distal field (s = 58% and 59%), and lies at s = 8% across the densely sampled deposit. We furthermore find that the deposit beyond the 5 cm isopach contains only 1% of the total eruption volume, whereas the extrapolated near-source deposit contains 61% and the intermediate deposit 38% of the total volume. Therefore, the relative uncertainty within each zone impacts total volume estimates differently. We propose a new convention of stating all three partial volume estimates: one for the deposit that is constrained by measurements extending to the largest and smallest reasonable isopachs, one for the extrapolated deposit above the largest isopach and one for the extrapolated distal deposit beyond the smallest isopach. The second project (Chapter 4 and 5) develops a new inversion approach that couples tephra isomass measurements with an ash dispersion model for the Ruapehu eruption on 17 June 1996 in New Zealand. The windadvected plume is discretized by point sources that release tephra into the atmosphere, which then disperse through the atmosphere and accumulate on the ground. Particle size dependent measurements of tephra isomass and inversion of the advection-diffusion process then yields the tephra distribution along the plume trajectory. We use synthetic data to arrive at efficient point source spacing and to test our model and assess challenges that arise from data gaps and noise. We find best results if we implement a second-order Tikhonov regularization, and use bootstrapping to estimate uncertainty of mass released along the plume trajectory. Comparison of particle fall times and time of sampling collection, as well as observations during the eruption, reveal that particles smaller than 250µm likely settled as aggregates. We apply a simplified model for the settling of fines < 250µm and recover distinct fallout trends for aggregates, which differ from those that fell as single particles. In addition, we compute the resulting particle size distribution within the plume and find that the particle mode shifts from an initial 1ɸ mode to a 2:5ɸ mode 10 km from the vent, and is dominated by a 2.5 to 3ɸ mode beyond 10 km from the vent. The computed particle distributions inside the plume provide new constraints on the mass transport processes within weak plumes and improve previous models. The distinct decay trends between single particle and aggregate settling may serve as a new tool to identify particle sizes that fell as aggregates for other eruptions.