JOINTING AROUND MAGMATIC DIKES AS A PRECURSOR TO THE DEVELOPMENT OF VOLCANIC PLUGS

Hawaiian-style eruptions commonly follow a characteristic sequence of events, from fissure eruptions fed by planar dikes, to eruptions through discrete vents, until the flow is sustained through a single vent underlain by a volcanic plug. Existing theories to explain flow localization focus on the relationship between non-uniform dike thickness and variations in magma flow rates. Here we propose a mechanism for conduit widening that precedes flow localization, and we present geologic evidence from an Oligocene-aged dike-plug system at Ship Rock, New Mexico. We document and characterize a systematic set of dike-perpendicular joints found in the sedimentary rock adjacent to the intrusions. This joint set, along with a set of dike-parallel joints and the bedding planes, divide the strata into rectangular blocks that subsequently can be entrained in the flowing magma to widen the dike. Using field and lab data and a stress analysis, we show that the dike-perpendicular joints are intimately associated with the emplacement of magma and thermal pressurization of pore fluids in the host rock. This fracturing process is a precursor to entrainment of the host rock and sustained flow through discrete vents. Our work contributes to a broader understanding of the characteristics and dynamics of Hawaiian-style volcanoes.