This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
Explosive volcanic eruptions are driven by the rapid growth of gas bubbles in magma that burst in the eruption column as they rapidly expand. By measuring the size of these bubbles just before they burst, it is possible to explore the processes that culminate in dangerous, explosive eruptions. However, it is impossible to monitor the interior of an exploding eruption column at the level of detail required, and by the time the final volcanic product of ash is observable, the bubbles that drove the eruption have already burst. Consequently, it is necessary to examine the surfaces of individual ash particles to analyze the imprints that these exploding bubbles have left on ash surfaces. This project is directed to developing a technique to determine the size distribution of these bubbles, and thus shed light on the processes of vesiculation and disruption immediately prior to fragmentation during explosion.
Bubble sizes can be measured on the basis of the radius of curvature of concavities on the surfaces of ash particles. These ash particles would have been the remnants of inter-bubble films and Plateau borders that existed prior to the fragmentation of the erupting magmatic foam, so should preserve information about the bubbles that separated them. Bubble fragment curvature can be constructed using digital elevation maps for individual ash fragments derived from Stereo-Scanning Electron Microscopy (SSEM). 3D shapes and bubble volumes can then be reconstructed from the imprinted curvatures in orthogonal cross sections of bubble "craters." This way, it is possible to determine bubble size, even when the expanding bubbles have burst during eruption (fragmentation). The technique developed in this project will be applied to natural ashes from recent eruptions of Fuego, Mt. Spurr, and Mt. St. Helens volcanoes, and the relations between bubble size distributions (BSD), bubble number densities (BND), and ash particle size distributions (PSD) will be quantified. This will make it possible to test a number of scientific hypotheses that can determine the relations between magma vesiculation dynamics of energetic volcanic eruptions and their products. It is expected that this study will result in new volcanic product analysis tools available for the broader community of volcanologists for application to, among others, pre-fragmentation vesiculation in ashes from highly energetic volcanic eruptions; hazard assessment of past and future eruptions; and stratospheric ash and its climate implications.
