MODELING THE CLIMATIC PHASE OF THE 2450BP PLINIAN ERUPTION OF PULULAGUA (ECUADOR)

The 2450BP climactic phase of the Plinian eruption of Pululagua volcano (Ecuador) occurred in weak atmospheric conditions resulting in a symmetrical tephra deposit that can be described by concentric isopach and isopleth maps. Grain size distributions at 53 different localities show a decrease in the median grain size (Md_Φ, together with an increase in the sorting (σ_Φ), away from the vent. Furthermore, the Md_Φ presents concentric patterns, similar to the isopach and isopleth maps, confirming the nature of calm conditions at the time of the eruption. Applying the Voronoi tessellation technique to infer the total grain size distribution of the deposit yields to a Md_Φ = -0.73 (relatively coarse) and a σ_Φ = 2.75 (poorly sorted). Poor sorting of tephra deposits is also indication of weak wind advection.

The accumulation of individual grain size classes with distance from the vent shows that the position of the plume corner (x₀) is located at about 8-10 km from the vent, in good agreement with the position of the corner inferred from the plume height (x₀ = 0.24H_t, with H_t = 35km, x₀ = 8.75km). Furthermore, our analysis shows that the transition between sedimentation from the plume margins (gas thrust and/or convective region) and the umbrella cloud occurred for clast in the size range of 32-16mm.

An inversion technique combined with the advection-diffusion model TEPHRA2 was applied on the Pululagua Plinian deposit (inversion based on both tephra thickness and grain size distribution) and generated eruption parameters in good agreement with field data (i.e. column height: 25-30 km above sea level; volume: 0.5 km³). We applied different techniques for the determination of the deposit volume (i.e. exponential and power-law methods), confirming that curve fitting techniques can be problematic above all when the proximal and/or distal data are missing.

In addition, the TEPHRA2 model reproduces a concentric isopach map with good agreement between computed and observed data in medial and distal area (> 6 km from the inferred vent).