Thermal and thermochemical convection in the Earth's mantle and dynamics of magma ascent in silicic volcanoes

This dissertation consists of two parts: (1) thermal and thermochemical convection in the Earth's mantle; and (2) dynamics of magma ascent in silicic volcanoes.; Results from analog fluid dynamics experiments indicate that plate-scale mantle flow can locally suppress plume formation over large fractions of the CMB and that heat flow from the core is approximately 30% +/- 10% higher than it would be in the absence of plate-scale mantle flow. Heat flux is spatially non-uniformly distributed along the CMB and may vary by approximately a factor of two between regions where plumes are suppressed and regions of plume formation. As a consequence, the convective flow pattern of the Earth's outer core may be relatively sensitive to changes in the overlying mantle flow. Moreover, hot mantle may be focused into regions of hot lower mantle upwelling. A second set of experiments indicates that a chemically dense lower-mantle layer can persist over the lifetime of the Earth without becoming completely entrained into the overlying mantle.; A powerspectral and multifractal analysis of obsidian samples indicates that "flow banding" formed by a multiplicative deformational process of repeated (continuous) magma autobrecciation, reannealing, and viscous deformation. A numerical model of magma flow in the volcanic conduit confirms that under a wide range of explosive and effusive eruption conditions shear-induced fragmentation (autobrecciation) of magma near the conduit walls is likely to occur. Fragmentation may play an important role in reducing the likelihood of explosive behavior by facilitating open-system magma degassing. Model simulations of diffusive bubble growth, due to joint CO2 and H2O exsolution under open- and closed-system degassing conditions, were compared against observed volatile contents of obsidian from the ca. 1340 eruption of Mono Craters, California. Results indicate nonequilibrium opening-system degassing throughout the eruption, which evolved from explosive to effusive with time. These predictions are consistent with observed conditions of volcanic systems, with the degassed nature of effusive silicic lavas, and with textural observations at the outcrop to microscale.