Explosive volcanic eruptions can inject voluminous amounts of ash into the atmosphere

potentially crippling global infrastructure as witnessed by the recent eruptions of Eyjafjallajökull in Iceland and Puyehue in Chile. Local transport of ash in plumes and in pyroclastic density currents impacts local populations, while the fraction of eruptive products that are dispersed widely plays a role in landscape evolution. While significant improvements have been made in the development of ash dispersal models and in the conceptual understanding of processes that govern the transport of ash far from the volcanic vent, much of the uncertainty in current forecasts of ash dispersal occurs due to limited description of the proximal dynamics of volcanic eruptions. Progress in understanding and predicting dispersal is currently limited by our understanding of the volcanic processes occurring near the vent, i.e. the physical processes occurring on spatial scales of ash particles to eruptive plumes.

In this project we examined eruptive processes related to volcanic ash dispersal and detection using several different approaches. One of the primary goals of this work was to describe the aggregation physics of volcanic ash, as this plays a fundamental role in determining how long ash remains aloft and where it will be dispersed. We approached this problem with two different sets of experiments that described aggregation that results from electrostatic charging, and aggregation that results from developing a thin film of water around the ash particle. In both cases we developed a probability distribution description of aggregation potential for natural ash particles, and developed physics-based models for detailed aggregation processes. We then used a statistical mechanics approach to incorporate these relationships into large-scale numerical simultions of explosive eruptive behavior.

We developed a multiscale ash dispersal approach and applied it to the 2009 eruption of Mt. Redoubt. We have identified optimal mass flow rates, plume structure, particle size distribution, and shape factors for the separate eruptive episodes from the 2009 event. In the process, we have also developed a new protocol to evaluate the mismatch between simulations compared to depositional and satellite data. In particular, previous assessments focused on the areal extent of the ash dispersal, but we found that while a number of modeling parameters can predict the areal extent, very limited eruptive conditions (particular atmospheric flow fields) can predict the specific deposit thickness in areas of high relief. This is particularly important for the overall changes in surface albedo, and changes to ash dispersal patterns can have a significant regional radiative impact.

This project also focused on entrainment physics and characterization in eruptive flows, and in particular described the modification of entrainment due to pulsating flow conditions at the vent, and the thermal modification of collapsing jets and pyroclastic density currents that result. We found that much of the cooling of the basal regions of many pyroclastic density currents occurs in the collapse phase, with relatively little entrainment after this event in these concentrated regions of the flow, enabling them to transport material great distances without losing significant heat. The outer parts of these currents are efficient at entraining and are relatively cool, giving rise to a dichotomy of thermal conditions in many of these currents.

This project also explored the physics of the electrical charging of volcanic ash and we conducted experiments to quantify charging that results from collisional (tribocharging) processes and due to breakup in the volcanic conduit (fractocharging). In both cases we were able to measure charge distributions in natural materials. Understanding the physical basis for this process is important in understanding the interaction of these particles (and in processes like aggregation described earlier), but is also needed to better understand signals that can be monitored remotely, like volcanic lightening and electric fields, that may be used as probes of internal activity in otherwise optically opaque plumes.

Last Modified: 05/05/2018
Modified by: Josef Dufek