Tephra: field, theory and application

In this work we briefly introduced the current state of the art for plume dynamics and plume modelling (chapters 1 and 2). From these, it was found that several questions remained unanswered. One of them what about adding some quantitative methodology to tephra identification when using geochemistry. Using discontinuous two tephra layers discovered at Burney Spring Mountain, northern California, this aspect was explored. Stratigraphic relationships suggest that they are two distinct tephras. Binary plots and standard similarity coefficients of electron probe microanalysis data have been supplemented with principal component analysis in log-ratio transformed data to correlate the two tephra layers to known regional tephras. Using principal component analysis, we are furthermore able to bound our uncertainty in the correlation of the two tephra layers (chapter 3). After removal of outliers, within the 95% prediction interval, we can say that one tephra layer is likely the Rockland tephra, aged 565-610 ka, and the second layer is likely from Mt Mazama, the Trego Hot Springs tephra, aged ~29 ka. Using cluster analysis on several vectors of chemical elements another quantitative methodology was explored (chapter 4). It was found that in most cases, geochemical analysis of a tephra layer will be assign to a single cluster, however in some cases the analysis are spread over several clusters. This spreading is a direct result of mixing and reworking happening in the tephra layer.;The dynamics of volcanic plumes were also investigated. We introduce a new method to estimate mass eruption rate (MER) and mass loading from the growth of a volcanic umbrella cloud or downwind plume using satellite images, or photographs where ground-based observations are available with a gravity current model (chapter 5). The results show a more fully characterised MER as a function of time than do the results given by pre-existing methods, and allow a faster, remote assessment of the mass eruption rate, even for volcanoes that are difficult to study. A new gravity current model for umbrella cloud was tested which allows to transition from one regime to another against measured data from several eruptions (chapter 6). Once the model was proved to be accurate, the different variables were tested to observed their impact on the spreading of the umbrella cloud. As a result it was found that the evolution of the radius changes not only in power-law with time but also indicates transitions in regimes. Chapter 7 is an end-to-end framework to probabilistic forecasting of volcanic ash transport and improved eruption source parameters.;In summary, this dissertation demonstrates four main contributions to volcanology: 1. The importance of bringing quantitative methods to tephra identification and how these methods can help in characterization of tephra. 2. The importance of the spread of volcanic cloud in the atmosphere as a gravity current. Particularly for prediction of the ash dispersal since the spreading as a gravity current happens over large distances from the volcano and even upwind. But also because it can help in getting a first and fast estimation of the mass eruption rate of an eruption which can be followed with time. 3. The importance of studying the structures and features on volcanic plume as they can reveal information about the dynamics of spreading and improve the estimation of regime transitions. 4. The need for the different communities working on tephra to communicate and understand each others approach fo better collaboration and multi-approach work.