What can Pyroclastic Deposits really tell us?

Abstract:

Explosive volcanic eruptions are inherently dynamic, likely responsible for high variability of the boundary conditions acting during magma break-up, transport and deposition. Eruptive processes cannot be observed directly because of the hostile nature close to the eruptive vent(s) and the opaqueness of the volcanic edifice and suspended ash. Detailed observation of on-going eruptions and thorough descriptions of the related deposits have allowed for empirical relationships between eruption and deposit characteristics.

Magma evolves during its ascent as a response to horizontal and vertical gradients of stress and temperature. This commonly results in strong vertical and horizontal gradients in crystallinity and porosity, potentially accentuated by heterogeneous strain distribution during flow. As a result, the ratio of dense to porous magma and the geometry of the gradient are largely unknown. Magma disintegration is largely controlled by its textural properties and the applied stress, leading to considerable variability in the fragmentation timescales and efficiency.

Many deposits are investigated primarily through the analysis of key properties, selected grain size classes and small sample numbers. As flow deposits tend to show a significant textural variability, most quantitative analysis of explosive eruptions is done via the analysis of fall deposits. Their appearance (massive or stratified, degree of sorting) is a result from a combination of eruptive, transport and deposition processes and should therefore not straightforwardly be takes as a proxy for any of these. We show how magma textures and eruption condition may both affect the contribution to a certain granulometry class. For a qualitative reassembling of the magma column and a better understanding of the observed eruption dynamics, the described magma heterogeneity and the related response during an eruption has to be taken into account.