Jointing around Magmatic Dikes as a Precursor to Conduit Geometry Evolution
Monday, 15 December 2014
Basaltic volcanic eruptions commonly follow a characteristic sequence of events. In the upper few kilometers of the crust, magma rises in planar dikes, which result in fissure eruptions when they intersect the surface. In some cases, the fissures do not feed significant eruptions and die out within hours. In other cases, magma stops flowing in much of the fissure, but continues to erupt from discrete vents, until finally the flow is sustained through a single vent underlain by a volcanic plug that is able to carry much greater volumes of magma to the surface. Ancient volcanic systems that have been exposed by erosion provide a unique opportunity to directly investigate a volcanic plumbing system and gain insight on the physical mechanisms responsible for conduit geometry evolution. Here we present field data from an Oligocene-aged dike-plug system at Ship Rock, New Mexico, collected from a DEM and overlaying orthophotograph made using new techniques from structure from motion. We document and characterize a systematic set of over 200 dike-perpendicular joints found in the sedimentary host rock adjacent to the intrusions. This joint set, along with a set of dike-parallel joints and the bedding planes in the host rock, allows the host rock to be broken into individual blocks that can be subsequently entrained in the flowing magma to widen the dike. We use elastic crack models and solutions for coupled heat and fluid flow in porous media to understand the conditions necessary for host-rock fracturing to occur. We show that this fracturing is intimately associated with the interaction of magma and groundwater and is a precursor to erosion of the host rock and sustained flow through discrete vents.