Around once every millenium, a large magnitude explosive eruption occurs on Earth dispersing volcanic ash across millions of square kilometers. Volcanic ash uniquely poses a wide-range of hazards to human health, infrastructure and the environment, the impacts of which are felt close to source to >1000's of km from the volcano. Importantly for large eruptions, the removal of the ash is almost impossible, which means the material remains and is remobilised in the environment, posing secondary hazards for 100's of years after the initial eruption. Here I present a study of the processes the produce, transport and deposit ash from large eruptions. I use the ~7.7 ka climactic eruption of Mount Mazama as a case study because the tephra was predominantly deposited on-land facilitating widespread data collection. I collate locations where the Mazama tephra has been recorded to produce a new isopach map and estimate of the total erupted volume (176 km³ bulk or 61 km³ Dense-Rock-Equivalent). The compilation of tephra thickness data also showed how the Mazama tephra deposit has been remobilised through time, exemplifying the uncertainties associated with field data. Remobilised deposits also provide insight into the types of secondary ash hazards that persist following large magnitude eruptions. I also investigate the physical and chemical properties of Mazama ash to provide insight into eruptive processes such as co-PDC plumes and distal ash transport. I determine from the composition of Fe-Ti oxides, that the distal ash can be attributed to the later stages of the climactic Mazama eruption. I also observe that the Grain Size Distribution (GSD) of the distal Mazama tephra is remarkably stable, a trend that is observed for other large distal deposits. This study also investigates the methods we use to analyse grain size in volcanology and outlines a new protocol for measuring the size and shape of volcanic ash using Dynamic Image Analysis (DIA). The benefits of DIA include the capacity for simultaneous particle size and shape characterisation, and the insight into particle density if used in parallel with sieve analysis. The new estimate of the total erupted volume and distal GSD of the Mazama were integrated with Ash3D, a numerical model of volcanic ash transport and deposition, to simulate the eruption and test the sensitivity of Ash3D to uncertainty in the eruption source parameters. The results stress the need to integrate radial spreading in the umbrella cloud region with advection-diffusion models when simulating the ash transport during large magnitude eruptions. Furthermore, it highlights significant knowledge gaps regarding the deposition of very fine-ash during any scale of eruption. This underscores the benefits of studying fine-ash deposition using the deposits from large eruptions where significant depositional areas and ash volumes facilitate extensive data collection and model testing.
