Erosion processes in volcanic conduits and application to the AD 79 eruption of Vesuvius

Abstract

The flow of gas, magma and pyroclasts through a volcanic conduit produces erosion of the conduit wall. Erosion may be produced by the impact of pyroclasts on the conduit wall, fluid shear stress at the wall, conduit wall collapse, and volcanic tremor. Using a two-phase flow non-equilibrium model of magma ascent along the volcanic conduits demonstrated that the erosion due to the impact of particles on the wall can occur only above the magma fragmentation level of the conduit where the particles or pyroclasts remove the wall material by an abrasion process. This abrasion process was found to be the largest near the conduit exit where the gas-magma velocities are the largest. The erosion due to the fluid shear stress at the wall can be produced along the entire length of a conduit, depending on the wall roughness and yield strength of wall rocks. This shear stress is the largest near the magma fragmentation level where the gas-magma viscosity and velocity gradients are very large. The collapse of conduit wall due to the difference between the gas-magma and lithostatic pressures can occur below and above the magma fragmentation level, causing the production of lithics directly when the wall collapses inward, and indirectly when the wall collapses outward. The effectiveness of different erosion mechanisms was tested with the magma characteristics, conduit geometry, and wall rock properties pertaining to the AD 79 eruption of Vesuvius. It was found that during the white and gray magma plinian eruption phases the lithics should had come from the deep as well as from the shallow regions of the conduit. The conclusions from erosion modeling are also consistent with the limited field data whereby the gray magma phase deposits are associated with larger lithic content and larger proportion of deep limestone fragments.