Characterization of source unsteadiness and entrainment into explosive eruptions using laboratory- and field-based methods

Abstract:

Unsteady explosive eruptions are defined by changes in (source conditions) mass flux at the vent occurring on time scales shorter than the rise time of the ash plume. Unsteady eruptions pose daily risks for air traffic and local communities, however, their plume dynamics and spreading heights are poorly understood and represent a critical knowledge gap in volcanology. Development of a classification scheme for the full range of unsteady eruption behavior requires quantification of source unsteadiness and subsequent effects on the entrainment rate, α_e, of ambient atmosphere into unsteady ash plumes. Accordingly, we introduce the source Pulsation number (Pu₀ = τ_p/τ_e) where τ_p is the period between pulses of eruption source mass flux and τ_e the eddy overturn time of the mixture at the source. We expect the full range of explosive eruption plume behavior to occur between endmember “buoyant plumes”, with Pu0 << 1, and “buoyant thermals”, with Pu0 > 1; these regimes are separated by a transitional “connected thermals” regime, with Pu₀ ≈ 1. To test our hypotheses, we deployed a novel portable ground-based L band (3.2 mm wavelength) Doppler radar instrument at Sabancaya Volcano, Peru in 2018 to monitor Vulcanian eruptions over two weeks with unprecedented spatial (12.5 m) and temporal (0.5 s) resolution. We also conduct experiments injecting unsteady fresh water plumes into a density stratified saltwater layer by varying 0 < Pu₀ < 1.6 among experiments and estimating α_e. Our experimental results confirm that endmember buoyant plume and thermal Pu₀ regimes have α_e values approaching those found in previous studies for buoyant plumes and thermals, respectively (Figure 1). The transitional “connected thermal” regime occurs where Pu₀ ≈ 0.5 and has an α_e value between the values of the endmember regimes. Radar data from Sabancaya’s Vulcanian eruptions reveal Pu₀ 0.8 for eruption pulses detected during semi-continuous eruptive periods. Ongoing work aims to estimate the full range of Pu₀ and α_e for all eruptions in the radar dataset for comparison with our analog experiment dataset. These results suggest that Pu₀ can be used to classify all explosive eruptions, with steady and unsteady source conditions, on one regime diagram.