Fig. 5: Hazards of explosive eruptions for communities surrounding the volcano, aviation and global cooling effect of Earth's surface where volcanic umbrella ash-gas clouds spread in the stratosphere.
Fig. 6: Eruption column regimes for relatively large explosive eruptions. a-c) Images of explosive eruptions occurring in the Buoyant Plume (a), Partial Collapse (b), and Total Collapse (c) regimes. d-f) Conceptual model sketches each regime describing key regions where erupted mass is partitioned between spreading as an umbrella cloud in the atmosphere at a level of neutral buoyancy or spreading along Earth's surface as pyroclastic density currents (PDC) with phoenix clouds rising and spreading above. e) In the Partial Collapse regime mass is partitioned simultaneously between umbrella clouds and pyroclastic density currents.
Fig. 7: Regime diagram for classifying multiphase jets studied in the laboratory that are analogs for eruption columns. The upward momentum, or jet strength, is on y-axis in log-space and the concentration of particles, or particle volume fraction, is on the x-axis in linear-space. Transitions among multiphase jet regimes are smooth and the Partial Collapse regime occupies a larger parameter space than that defined by previous studies due to the effects of inertial particles that have a complex two-way momentum transfer coupling with the carrying fluid. The eruption source parameter space for the 1980 eruption of Mt. St. Helens, WA, USA is outlined with an orange rectangle, indicating that it occurred in the Partial Collapse and Total Collapse regimes, which agrees with direct observations of the eruption column and inferences from the eruption deposit.
The volcanology community has decided that current eruption classifications are limited in their ability to (Bonadonna et al. 2016; Manga et al. 2017):
Consistently classify all styles of explosive eruptions ranging from small puffs of ash to the largest catastrophic caldera-forming eruptions
Identify features of eruption deposits that are diagnostic of the eruption style and, in turn, quantitatively constrain eruption source parameters
Distinguish eruption styles with relatively constant or time-varying source parameters during the eruption
Provide quantitative constraints on the mass of erupted mixtures delivered to spreading ash clouds in the atmosphere and pyroclastic density currents during an eruption
Communicate the wide diversity of eruption styles and associated hazards to the public
Accordingly, the community has called for the development of new eruption classifications that can achieve these goals.
My research on mass partitioning in large eruption columns has built a foundation for establishing a new eruption classification scheme where eruption styles are classified on the basis of their strength (initial upward momentum) and the concentration of rocks, pumice and ash in the erupted mixture (particle volume fraction; Fig. 7). However, this foundation is missing a keystone required to classify all explosive eruption styles: a metric for distinguishing eruptions with relatively constant (steady) versus time-varying (unsteady) source parameters. The large eruption styles I have studied are relatively easy to model with analog experiments and computer simulations because their source parameters can be considered steady for the main phase of the eruption. Most eruptions occurring daily on Earth are much smaller and have unsteady source parameters that can change before the erupted mixture reaches its spreading height in the atmosphere or before it collapses back to Earth. The highly time-dependent nature of their source parameters makes them difficult to model with analog experiments and computer simulations. Thus, it is unclear whether these small unsteady eruptions occur in the same regimes and exhibit the same behavior as better understood large eruptions for similar ranges of average eruption source parameter values.
Despite their small size, unsteady eruptions still contribute to Earth's climate and present hazards to aviation and surrounding communities, thus our inability to predict their behavior and associated hazards represents a major knowledge gap in volcanology. I began addressing this knowledge gap during my PhD by discovering a new eruption source parameter metric, the source Pulsation number, that can predict the effects of steady and unsteady eruption source parameters on entrainment and, in turn, mass partitioning between spreading ash clouds and pyroclastic density currents. Currently, I'm working on using this Pulsation number metric with the eruption strength and particle volume fraction metrics, which I used to classify large eruptions, to put forth a new eruption classification scheme that achieves the goals outlined by the volcanology community.