The ascent and eruption of arc magmas: A physical examination of the genesis, rates, and dynamics of silicic volcanism

The ascent and eruption of magmas in arcs determines the thermal and compositional structure of the crust and shapes the surface landscape. The first section of this study examines the intrusion of mafic mantle melts into the lower crust and thermal and dynamic response of the crust to this input. Distinct melting and mixing environments are predicted as a result of the crustal thickness and age of the arc system. The residence time of arc magmas and their rate of ascent from depth can be constrained by the decay of naturally occurring isotopes of uranium and its daughter products. 226Ra-excesses in arc magmas have been interpreted to result from flux melting of the mantle above subducting slabs and subsequent fast ascent rates from slab to surface, up to 1000 m/yr and higher. However, in this work it is demonstrated that incongruent melting of the lower crust can either maintain or augment mantle-derived 226Ra-excesses and so reduce inferred vertical transport rates.; The second half of this work addresses the explosive volcanic eruption of silicic magmas. Like many other processes nature, volcanic eruptions involve multiple, mechanically distinct phases: for instance, ash particles interacting with a turbulent gas phase in an explosive volcanic eruption. A multiphase numerical model is developed and applied to conduit conditions and pyroclastic flow transport. To elucidate the role of particle collisions in redistributing momentum after fragmentation in a volcanic conduit, a numerical study was performed comparing the behavior of an inviscid, or collision-less, granular material with a granular material whose viscosity and pressure were modeled using kinetic theory.; The final chapter of this work examines the degree to which particle transport in pyroclastic flows results for bed-load transport or suspended load transport. Numerical leaky and saltating boundaries are developed to examine both over-land and over-water transport of pyroclastic flows. The leaky boundary permits the examination of suspended load in isolation from bed-load transport. A unifying concept is energy dissipation due to particle-boundary interaction: leaky boundaries dissipate energy more efficiently at the boundary than their saltating counterparts and have smaller run-out distance.