Transient and unsteady eruptions at Sabancaya Volcano, Peru : for how long and how high are oscillations in vent mass flux remembered in the evolving plume?
Abstract
Volcanic eruptions have unsteady source conditions where variations in source mass flux are comparable in magnitude to the mean mass flux, occur over timescales equal to or less than plume-rise times and can vary spatially around the crater. Numerical and experimental simulations of eruptions commonly assume constant source mass flux, but it is unclear to what extent unsteadiness influences entrainment of ambient atmosphere, turbulent mixing of entrained atmosphere and, in turn, the delivery height of volcanic ash and gas. We examine high-resolution, ground-based thermal infrared imagery of a range of unsteady explosive behavior at Sabancaya Volcano, Peru, to assess how unsteady source conditions influence turbulent mixing and entrainment. Studied events include: (1) a large, transient explosion dominated by a single initial pulse with an e-fold time in peak temperature of ~30 s, (2) two "emergent explosions" (~2-3 minutes duration) with quasi-periodic emissions at 12-20 s intervals, and (3) a long-lived (~4 hours duration) sustained plume with quasi-periodic pulses at 20-30 s intervals. Dominant source oscillation periods were measured from time-evolution of the thermal imagery, using peak temperatures near the crater rim as a proxy for source mass flux. Time-series of temperature fluctuations at the source correlate with those up to a finite height above the crater rim, between about 200 to 600 meters and varying with source velocity and temperature. We characterise this height as the thermal mixing height for each event, above which the individual source pulses become indistinct and the plume effectively loses "memory" of the initial unsteadiness. A possible implication of this work is that steady plume models are valid at a finite height above the vent for unsteady eruptions. Ongoing work further aims to further explore the dominant source conditions governing the mixing height, including source velocity and temperature, entrainment rates, and pulsation period.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V31A..08R
- Keywords:
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- 4314 Mathematical and computer modeling;
- NATURAL HAZARDS;
- 8414 Eruption mechanisms and flow emplacement;
- VOLCANOLOGY;
- 8428 Explosive volcanism;
- VOLCANOLOGY;
- 8445 Experimental volcanism;
- VOLCANOLOGY