Conduit stability effects on intensity and steadiness of explosive eruptions

Álvaro Aravena,Mattia De’Michieli Vitturi,Raffaello Cioni,Augusto Neri

doi:10.1038/s41598-018-22539-8

Álvaro Aravena, Mattia De’Michieli Vitturi + Show 2 more

Open Access

https://doi.org/10.1038/s41598-018-22539-8

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Abstract

Conduit geometry affects magma ascent dynamics and, consequently, the style and evolution of volcanic eruptions. However, despite geological evidences support the occurrence of conduit widening during most volcanic eruptions, the factors controlling conduit enlargement are still unclear, and the effects of syn-eruptive variations of conduit geometry have not been investigated in depth yet. Based on numerical modeling and the application of appropriate stability criteria, we found out a strong relationship between magma rheology and conduit stability, with significant effects on eruptive dynamics. Indeed, in order to be stable, conduits feeding dacitic/rhyolitic eruptions need larger diameters respect to their phonolitic/trachytic counterparts, resulting in the higher eruption rates commonly observed in dacitic/rhyolitic explosive events. Thus, in addition to magma source conditions and viscosity-dependent efficiency for outgassing, we suggest that typical eruption rates for different magma types are also controlled by conduit stability. Results are consistent with a compilation of volcanological data and selected case studies. As stability conditions are not uniform along the conduit, widening is expected to vary in depth, and three axisymmetric geometries with depth-dependent radii were investigated. They are able to produce major modifications in eruptive parameters, suggesting that eruptive dynamics is influenced by syn-eruptive changes in conduit geometry.

Highlights

Magma ascent dynamics during explosive eruptions is mainly controlled by the relation between magma supply and discharge rate[1,2], which are strongly related to magma chamber conditions and volcanic conduit geometry[3]
Modeling of magma ascent through axisymmetric volcanic conduits with fixed and depth-dependent radii suggests that magma composition and rheology play a leading role in defining the conditions of conduit stability, and in controlling conduit dimensions, evolution of mass discharge rate and the amount of lithic fragments found in pyroclastic deposits
In particular: (a) Dacitic/rhyolitic magmas need conduits several times wider than phonolitic/trachytic magmas for developing mechanically stable conduits, which could explain the high mass discharge rates commonly observed in sustained dacitic/rhyolitic eruptions, whereas phonolitic/trachytic magmas would be capable of producing sustained explosive eruptions with relatively low mass discharge rates

Summary

Introduction

Magma ascent dynamics during explosive eruptions is mainly controlled by the relation between magma supply and discharge rate[1,2], which are strongly related to magma chamber conditions and volcanic conduit geometry[3]. Conditions for syn-eruptive mechanical stability of volcanic conduits have never been addressed in detail, nor has been studied the effect of magma rheology on them. Aravena et al.[8] proposed that conduit dimensions control its mechanical stability, and a minimum radius (and a minimum mass flow rate) for reaching stable conditions can be computed. Under these assumptions, axisymmetric conduits with fixed radius represent a mechanically stable configuration only for a narrow range of conditions, and widening processes probably lead to geometries characterized by depth-dependent dimensions. Data from selected case studies were analyzed, for a semi-quantitative validation of modeling results

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