Abstract
Eruption source parameters (in particular erupted volume and column height) are used by volcanologists to inform volcanic hazard assessments and to classify explosive volcanic eruptions. Estimations of source parameters are associated with large uncertainties due to various factors, including complex tephra sedimentation patterns from gravitationally spreading umbrella clouds. We modify an advection-diffusion model to investigate this effect. Using this model, source parameters for the climactic phase of the 2450 BP eruption of Pululagua, Ecuador, are different with respect to previous estimates (erupted mass: 1.5–5 × 1011 kg, umbrella cloud radius: 10–14 km, plume height: 20–30 km). We suggest large explosive eruptions are better classified by volume and umbrella cloud radius instead of volume or column height alone. Volume and umbrella cloud radius can be successfully estimated from deposit data using one numerical model when direct observations (e.g., satellite images) are not available.
Highlights
Eruption source parameters are used by volcanologists to inform volcanic hazard assessments and to classify explosive volcanic eruptions
We find that an erupted mass between 1.5–5 × 1011 kg, umbrella cloud radius between 10 and 14 km, and eruption column height of 20–30 km can describe the thinning of the Pululagua deposit with distance from the vent
We propose that the Volcanic Explosivity Index (VEI) scale be updated to include the umbrella cloud radius as a metric to characterize large explosive eruptions, acknowledging that any single eruption source parameters (ESPs) cannot completely categorize the nature of large explosive eruptions
Summary
Eruption source parameters (in particular erupted volume and column height) are used by volcanologists to inform volcanic hazard assessments and to classify explosive volcanic eruptions. Deposit data (i.e., mass per unit area, thickness, local grain-size distribution) are used to estimate the erupted mass, plume height, and total grain-size distribution[5,18,19,23,24,25,26] One advantage of these models is that they can better estimate ESPs with uncertainty quantification[25,27]. Model assumptions, again either statistical or numerical, may lead to biased estimates of ESPs. In particular, many numerical models assume that tephra is released from either a point, a vertical line atop the volcano or as diffusion along a vertical line[18,19,20,21,30,31] and that the deposit thins monotonically from the vent, with the exception of secondary maxima associated with ash aggregation or local topography and low atmosphere wind fields[7,32,33,34,35,36,37]. A laterally spreading cloud transports a large volume of tephra rapidly away from the volcano in all directions, reaching radii of 10–100 s of kilometers and markedly changing the distribution of tephra on the ground
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