Fundamental questions persist regarding volcanic conduit erosion during sustained Plinian events, despite the clear control of conduit geometry on eruptive behaviour. This study has addressed this gap through a combination of detailed field work on the Avellino Plinian eruption at Vesuvius with a focus on lithic components, and modelling of stresses in wall rocks during steady eruptions. Together, the two approaches enable the interpretation of the processes and sources of lithic generation. The Avellino deposits provide a natural laboratory with a well-characterized subsurface from which lithic source depths are inferred. The lithic abundance, componentry and grain size distribution vary significantly with distance from the vent and between Plinian phases with different eruption rates. Conduit implosion is inferred to be the key mechanism involved in conduit erosion during the Avellino eruption, with secondary roles played by vent spalling/collapse and viscous shear below magmatic fragmentation producing xenoliths. The variations in component Total Grain Size Distributions (TGSDs) are interpreted to be controlled by both pre-existing weaknesses and fragmentation mechanism. Based on the lithic volumes and subsurface stratigraphy the upper, lava-hosted conduit radius increased by 167%, and the lower, carbonate-hosted conduit increased sharply at the fragmentation depth. The pressures predicted by conduit flow models mean that the wall-rock surrounding the conduit is fractured during the eruption, which would significantly reduce its strength and increase its permeability. The amount of conduit implosion predicted from analytical and Finite Element models can be many times more voluminous than the initial conduit and increases with deeper magmatic fragmentation, which increases conduit underpressure, and with conduit size. The modelled volume of failed conduit wall is greater for elliptical than cylindrical geometries, and is reduced considerably for 'hybrid' models, which transition from elliptical at depth to a cylindrical geometry. The combined, field and modelling work constrains how the conduit evolved during the Avellino eruption, and has broad implications for our understanding of eruption dynamics.