Magma Dynamics and Conduit Growth Mechanisms Inferred From Exposed Volcano Conduits At The San Rafael Subvolcanic Field, Utah

The dynamics of volcanic eruptions are the result of the coupling between magma dynamics and the geometry of the volcano conduit. Conduit geometry partly determines the velocity distribution of the magma, but the velocity also affects conduit geometry through different erosion mechanisms. We study Pleistocene volcano conduits exposed at 500 - 2000 m depth in the San Rafael subvolcanic field, Utah, to explore velocity distributions across real volcano conduits and conduit growth mechanisms. Three basic types of conduits are observed; (1) dike bulges up to 2 m, (2) erosion dominated brecciated conduits with irregular, often elongate shape, lengths of about 100 m and width of tens of meters, and (3) clean conduits of about 6 m diameter in which sustained flow is localized. Detailed geological maps of different conduits and studies of the xenoliths entrained from the wall-rock, and the wall-rock itself, reveal different growth mechanisms of the volcano conduit. In some conduits viscous flow textures in the xenoliths and the wall-rock indicate high rates of heat transfer to the wall-rock compared to growth rate of the volcano conduit. In other conduits no evidence of high heat transfer rates is present, and the growth mechanism is thought to be purely mechanical. Xenolith, crystal and vesicle contents are measured along different profiles across the volcano conduits. This information along with whole-rock chemical analysis is used to calculate the variation of magma viscosity along different profiles through the conduits. These viscosity profiles reveal information about changes in relative velocity and hence magma dynamics across the volcano conduits.