Eruption dynamics during Plinian eruptions : insights from the stratigraphic variations of deposit structures and pumice textures of the Minoan eruption (Santorini, Greece) and the Laacher See eruption (East Eifel, Germany)

The in situ observation of conduit processes as well as eruption dynamics during major Plinian eruptions is still impossible, however, understanding these processes is essential to enhance hazard mitigation and risk reduction near active volcanoes. Here we set out to reconstruct the conditions in the conduit during Plinian eruptions through a detailed analysis of the fall deposits of two major Plinian eruptions (Minoan eruption as the chemically homogeneous and the Plinian phases of the Laacher See eruption as chemically heterogeneous endmember) focussing on the stratigraphic variation of deposit structures (granulometry, modal composition, clast shape parameters) and pumice textures (clast density, permeability, vesicle size distribution) in combination with chemical data and intensive parameters. Changes in the aforementioned parameters reflect changing transport conditions like the granulometric data of the fall deposit being an indicator for fragmentation and eruption intensity or the acceleration of the melt being controlled by degassing that is mirrored in the pumice density and vesicle size distribution. The data collected by means of various methods and their simultaneous interpretation demonstrates that deposit structure and pumice texture are indeed a powerful tool to constrain and reconstruct the temporal variation of conduit flow and eruption dynamics during major Plinian eruptions. It was found that the chemically homogeneous deposits are much easier to invert for conduit flow processes and eruptions dynamics because viscosity is a major factor controlling transport properties of the melt. With the viscosity being fixed parameters like conduit geometry, or shifting of the fragmentation horizon can be constrained much better.