The influence of moisture on pyroclastic density current dynamics and deposits: implications for understanding volcanic stratigraphy

Pyroclastic Density Currents (PDCs) are hazardous, multiphase currents of heterogeneous volcanic material and fluid. Moisture can enter a PDC system through internal (e.g., pre-eruptive) or through external processes during transport. Prior to this thesis, the impact of moisture on PDC systems has been largely overlooked, and the role of moisture on PDC behaviour, properties of pyroclastic materials and the resulting depositional architecture formed during PDC-forming eruptions has not been addressed. The work outlined in this thesis utilises geomechanical, geotechnical and analogue models alongside fieldwork observations to gain a better insight into the role and behaviour of moisture in PDC systems.This work demonstrates that the addition of moisture into pyroclastic material alters the angle of repose, cohesion, and shear strength. This changes material properties from flowable to non-flowable and impacts the fluidisation profiles and gas escape structures formed. Further investigation of pyroclastic material using geotechnical equipment has shown that shear strength increases at low moisture contents and shear thickening behaviours occur at high moisture contents. This indicates that moisture-rich layers are more likely to resist shearing and be preserved in volcanic successions.Characterisation of PDC deposits in Tenerife have revealed distinct erosional and remobilisation behaviours of flow-unit ash layers that are associated with varying moisture conditions. Additionally, experiments utilising a fluidised flume, and the sectioning of a ballotini current and substrate in gelatine, have allowed internal features to be observed. The addition of moisture to the substrate effectively suppressed erosion, remobilisation, and the formation of various features within the substrate.These results are crucial for understanding flow unit architecture and estimating the frequency of PDC emplacement units during explosive eruptions. A greater understanding of PDC deposits and the processes that formed them is essential for improving our interpretation of current conditions and deposit formation which isessential for future hazard assessments.