VERY STICKY STUFF: RHEOLOGICAL ADVENTURES WITHIN THE MOUNT HOOD VOLCANO MAGMATIC PLUMBING SYSTEM

The density and viscosity of magma are important when evaluating sub-volcanic processes that occur before and between volcanic eruptions. These properties can be used to calculate the rate magma rises as a diapiric body deep within the Earth’s crust, as well as magma ascent rate through near-surface volcanic conduits before an eruption event. This study investigates relationships among density, viscosity, SiO₂ weight percent, magma ascent rate and magma crystallinity in lavas erupted during Mt. Hood volcano eruptive periods over the past ~ 70,000 years at temperatures between 800 ^oC and 1200 ^oC. The partial molar volume model of Bottinga and Weill (1970) is applied to calculate magma densities and the Arrhenius model of Giordano, Russell, and Dingwell (2008) is applied to calculate magma viscosities. Ranges in magma ascent velocities are calculated using two models. The first method (Stokes Law) assumes magma to ascend as a coherent diapiric body at deep crustal levels due to difference in density between magma and the surrounding crustal rock. The second method (Hagen-Poiselle Law) assumes magma to be forced upward through a stationary cylindrical pipe due to lithostatic pressure exerted by the surrounding rock. This model approximates magma moving through dike systems from the magma chamber to the overlying volcano. Magma crystallinity is calculated using integrated areas of X-ray diffraction peaks associated with crystalline and amorphous phases. Model calculations indicate that higher diapiric ascent rates correlate with lower magma densities, while higher magma ascent rates in shallow pipes correlate with lower melt viscosity, and that weight percent SiO₂ weight is an unreliable indicator of magma crystallinity. The results are important in elucidating the relationships between magma rheology, chemical composition, and crystallinity during each of Mt. Hood’s eruptive periods.