EROSION PROCESSES IN VOLCANIC CONDUITS AND APPLICATION TO THE AD 79 ERUPTION OF VESUVIUS

The flow of gas, magma and pyroclasts through a volcanic conduit produ ces erosion of the conduit wall. Erosion may be produced by the impact of pyroclasts on the conduit wall, fluid shear stress at the wall, co nduit wall collapse, and volcanic tremor. Using a two-phase flow non-e quilibrium model of magma ascent along the volcanic conduits demonstra ted that the erosion due to the impact of particles on the wall can oc cur only above the magma fragmentation level of the conduit where the particles or pyroclasts remove the wall material by an abrasion proces s. This abrasion process was found to be the largest near the conduit exit where the gas-magma velocities are the largest. The erosion due t o the fluid shear stress at the wall can be produced along the entire length of a conduit, depending on the wall roughness and yield strengt h of wall rocks. This shear stress is the largest near the magma fragm entation level where the gas-magma viscosity and velocity gradients ar e very large. The collapse of conduit wall due to the difference betwe en the gas-magma and lithostatic pressures can occur below and above t he magma fragmentation level, causing the production of lithics direct ly when the wall collapses inward, and indirectly when the wall collap ses outward. The effectiveness of different erosion mechanisms was tes ted with the magma characteristics, conduit geometry, and wall rock pr operties pertaining to the AD 79 eruption of Vesuvius. It was found th at 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 l imestone fragments.