Energy consumption by magmatic fragmentation and pyroclast ejection during Vulcanian eruptions

Magmatic fragmentation during explosive eruptions consumes a significant amount of energy by the creation of new surface via fracturing. This process reduces the energy that can be imparted to the resulting pyroclasts in the form of kinetic energy. To date, models of Vulcanian eruptions have neglected the energy balance consumed by magmatic fragmentation. We present a 1-D model of Vulcanian eruption that considers the energy balance in rapid decompression of a pressurized magma below a caprock, followed by fragmentation and acceleration of particles. We then tested the model via decompression experiments at different temperatures (room vs 850 degrees C) and initial pressure (<20 MPa) with a fragmentation apparatus. Different series of experiments involving either fragmentation of an intact volcanic rock followed by ejection of the particles, or simply the ejection of loose particles were performed to quantify the energy consumed by fragmentation and the kinetic energy available to eject the particles. The ejection velocities observed in the fragmentation experiments of intact rocks are systematically lower than the ones with loose particles due to the energy consumed by fragmentation. This energy depends on the fragmentation threshold (i.e. the minimum pressure required to completely fragment the sample) and is inversely proportional to the fraction of pressurized pores. We propose that the effective pressure propelling the gas-particle mixtures after fragmentation corresponds to the pressure initially stored in the pore fraction minus the fragmentation threshold. We applied our caprock model to Vulcanian eruptions of Popocatepetl volcano (February 2003) and Colima volcano (10 February 1999) and obtained gas pressures at the onset of fragmentation of 11 13 MPa and 19-22 MPa, respectively. These pressures might correspond to the gas pressures required to disrupt the caprock, fragment the underlying porous magma and eject the ballistics at the observed distances. The model presented herein may help describe the dynamics of Vulcanian eruptions, and improve hazard assessment. (c) 2010 Elsevier B.V. All rights reserved.