Effects of bubble coalescence, permeability, and degassing process on the transition from explosive to effusive eruption of rhyolitic magma
Nguyen, Chinh Thuc
Gonnermann, Helge M
Doctor of Philosophy
This thesis consists of two parts that address bubble coalescence and the degassing process inside the magma conduit during volcanic eruptions. The first part focuses on analog fluid experiments and resultant scalings for bubble coalescence, due to either gravitational and capillary forces, with application to magmatic systems. I find that film drainage is due to capillary forces at dimensionless Bond numbers, Bo < 0.25, whereas gravitational forces result in film thinning at Bo > 0.25. The film drainage time scale is given by t ~ C ln(α) τ, and is orders of magnitude faster than previously assumed for magmatic systems. Here C ~ 10 is an empirical constant, α is the ratio of initial film thickness to film thickness at the time of rupture, and τ is the characteristic capillary or buoyancy time scale at values of Bo < 0.25 and Bo > 0.25, respectively. These scalings could be used to estimate the time for bubble coalescence under static conditions, such as in a magma chamber or post-fragmentation in pyroclasts. The second part of the thesis focuses on pyroclast permeability, a consequence of bubble coalescence during volcanic eruptions. I analyze porosity and permeability of rhyolitic pyroclasts from four different Plinian eruptions. One of these is the A.D. 1912 eruption of Novarupta volcano, Alaska, which is comprised of five different episodes ranging from explosive to effusive activity. For this eruption, I find that the degree of interconnectivity, measured as the ratio of connected to total porosity, decreases with phenocryst content and with increasing eruption intensity. Through numerical modeling of diffusive bubble growth during eruptive magma ascent, I conclude that magma permeability is unlikely a sufficient condition for the transition from explosive to effusive activity. Instead, it is likely that a decrease in magma ascent leads to the transition from explosive to effusive activity during the waning stages of the eruption. Subsequently, I compare and contrast the porosity and permeability data from all four Plinian eruptions investigated: the A.D. 181 Taupo eruption, New Zealand; the A.D. 1060 Glass Mountain eruption, California; the A.D. 1314 Kaharoa eruption; and the A.D. 1912 Novarupta eruption, Alaska. I find that the Kaharoa samples have the lowest values of porosity and permeability of these four Plinian eruptions. Porosity and permeability, as well as pyroclastic textures of Kaharoa samples closely resemble those of the effusive Episode V of the 1912 Novarupta eruption. I hypothesize that the Kaharoa eruption might have undergone a high degree of open-system degassing.