Conduit stability effects on intensity and steadiness of explosive eruptions

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.

Effusion Rate Evolution During Small‐Volume Basaltic Eruptions: Insights From Numerical Modeling

“…It is worth stressing that the adopted numerical model assumes that the evacuation of magma stops when the effusion rate drops below a critical effusion rate ( q c ), which may occur either before or after reaching the lithostatic pressure (Aravena, Cioni, de' Michieli Vitturi, Pistolesi, et al, 2018). However, considering that a reservoir pressure far below the lithostatic value may induce instability conditions, collapse processes, and conduit closure (Aravena et al, 2017; Aravena, Cioni, de' Michieli Vitturi, & Neri, 2018; Macedonio et al, 1994), the eruption shutdown can be otherwise defined with the application of a critical reservoir pressure ( p c ) (Marti et al, 2000). This modification would introduce some variations in the resulting effusion rate curves (Figure 5).…”

Evolution of Conduit Geometry and Eruptive Parameters During Effusive Events

“…For numerical modeling, we use a 1‐D steady‐state model currently available on line (https://github.com/demichie/MAMMA) which considers the most important processes experienced by magmas during ascent (Aravena et al, , ; La Spina et al, ). The model developments related to this work are associated to the adoption of a depth‐dependent dyke‐like conduit geometry (already available on line) and the inclusion of appropriate criteria for studying its temporal evolution (section 2.2).…”

“…Although some formulations have been proposed for describing the controlling factors of such erosive mechanisms, it is difficult to quantify their relative importance (Aravena et al, ; Macedonio et al, ). Conduit collapse can only occur in the presence of a large pressure difference between country rocks and magma in the conduit, and it is not expected to occur during effusive eruptions (Aravena et al, ; Macedonio et al, ). Accordingly, basaltic effusive eruptions present favorable conditions for addressing fluid shear stress, which is controlled by magma viscosity, velocity, and country rock mechanical properties.…”

Mechanisms of Ash Generation at Basaltic Volcanoes: The Case of Mount Etna, Italy

“…Potential consequences deriving from this series of events are sudden changes in the magma fragmentation depth, longer magma residence time in the conduit, increasing proportion of cooled, crystallised (tachylitelike) magma vs. vesiculating (sideromelane-like) magma, and variations in the proportion of sideromelane vs. tachylite ash particles in the deposited fragmented magma (section "Analysis of magma residence time in conduits"). Ultimately, the pressure decrease associated with the lack of a sustained magma column inside the conduit, occurring during the initial and waning phase of an eruption, can favour the collapse of conduit walls and the production of lithics (Aravena et al, 2018). Similarly, pressure profiles associated with smaller conduits favour conduit walls instability and thus eruptions with a higher proportion of lithic fragments (Aravena et al, 2017).…”