Browsing by Author "Gonnermann, Helge M"

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    Controls on magma supply from depth at Kīlauea Volcano, Hawai'i
    (2016-06-02) Blaser, Austin P; Gonnermann, Helge M
    Magma supply rate from depth to the summit at Kīlauea volcano, Hawai'i is not constant through time. Even during the current Pu'u 'Ō'ō-Kupaianaha eruption (1983-present), a surge in supply occurred from 2003-2007. This surge was accompanied by summit inflation, increased CO2 emissions, and finally culminated in a dike intrusion and surface eruption. Recent work has suggested that a surge in supply is a result of an increase in dissolved volatiles in the melt zone. Alternatively, the surge could come from a mechanical change in the melt zone, perhaps as an increase in permeability or melt production. I develop a numerical model of two-phase flow between the melt zone and summit at Kīlauea to explore the role of volatile composition, conduit geometry, and pressure in the melt zone on supply rate. Results suggest that volatiles have a limited role in increasing supply, a dike-shaped conduit is likely, and pressure increases in the melt zone drive changes in supply. Additionally, a dike-shaped conduit yields a decoupling of conditions at the surface from those at depth, which impacts how conditions in the mantle below Kīlauea are inferred from surface deformation and summit gas emissions.
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    Effects of bubble coalescence, permeability, and degassing process on the transition from explosive to effusive eruption of rhyolitic magma
    (2016-04-26) Nguyen, Chinh Thuc; Gonnermann, Helge M
    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.
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    The rheology of particle-liquid suspensions, the shape and connectivity of vesicles in pyroclasts and implications for the Plinian eruption of basaltic magma
    (2015-12-01) Moitra, Pranabendu; Gonnermann, Helge M; Lee, Cin-Ty A; Lenardic, Adrian; Hirasaki, George J
    This thesis consists of three projects based on magma ascent dynamics during volcanic eruptions. In the first project, I quantified vesicle shapes in pyroclasts, from different styles of volcanic eruptions, using a dimensionless shape factor. I found that this shape factor can be related to a dimensionless Capillary number, estimated from coupled bubble growth and magma ascent modeling and thus, to the eruption styles. My second project dealt with understanding the effect of crystals on the rheological properties of magma from dynamically similar analog laboratory experiments. I found that the rheological properties of particulate suspensions depend on the applied shear rate and maximum packing fraction of a particulate system, which is a function of particle size- and shape-modality. Using empirical formulations, I showed that non-Newtonian rheology of crystalline magma may cause large changes in magma discharge rates for small changes in driving pressure gradient and/or crystal shape- and size-modality. In the third project, I measured permeability of pyroclasts from the Plinian style eruptions of basaltic magma at Mt. Etna (122 BCE) and Mt. Tarawera (1886) and found that the permeability of these pyroclasts are 1-2 orders of magnitude larger than that of the pyroclasts from Plinian style eruptions of silicic magmas. Using numerical modeling I found that the permeability thresholds are approximately at 35% of magma porosity and formulated the porosity-permeability relationships for pyroclasts from both the studied eruptions.