Field and experimental characterisation of tephra sedimentation and resuspension processes

Understanding the physical processes affecting the sedimentation of volcanic fragments ejected in the atmosphere (i.e., tephra) is crucial to improve the interpretation of deposits and to develop more accurate models aiming at better forecasting the impact of volcanic ash on human activities and health. Among these processes, collective settling within descending ash-laden currents (ash fingers) is known to increase fine ash fall velocity, but is yet poorly described. After deposition, volcanic ash has the potential to be resuspended by wind, threatening communities located close to loose tephra deposits over various spatial and temporal scales. In this thesis, new field observations and laboratory experiments are used to characterise the properties of ash fingers and define conditions favouring their development. The syn-eruptive remobilisation and resuspension of tephra deposits by wind is investigated in the specific setting of Sabancaya volcano (Peru).