Slug ascent and associated stresses during strombolian activity with non‐Newtonian rheology

Abstract

Strombolian activity is generally assumed to be driven by overpressurized gas slugs that rise through the magma‐filled volcanic conduit and burst at the surface. We develop an analytical model for this process that incorporates a generic, depth‐dependent, and non‐Newtonian magma rheology. Our model also describes the film draining after the burst and allows for the computation of the stresses exerted on the conduit walls using a new analytical solution for the thickness of an annular film flow. Using Stromboli volcano, Italy, as reference, it was evaluated with a specifically designed, non‐Newtonian rheological model based on petrochemical data. The results show the importance of using a realistic magma viscosity model when modeling the slug ascent: A 100 kg gas slug attains its maximum overpressure of 2.1 bars about 40 s before it rapidly expands and finally bursts at only 1.4 bars. This preburst pressure drop is not reproduced by constant viscosity models. The normal stresses on the conduit wall are generally dilatational above and adjacent to the slug and constrictive below it and after the burst. In both cases their maximum values exceed 3 bars, whereas the shear stresses stay negligible. Furthermore, it was found that the slug needs to start deeper than 25 m to build up its full burst pressure. At greater depth, the burst pressure decreases because of the adiabatic slug gas. Finally, a viscous plug at the top of the conduit is shown to increase the explosive potential of the volcano significantly.

The implications of gas slug ascent in a stratified magma for acoustic and ground deformation source mechanisms in Strombolian eruptions

“…Lieth and Hort (2016) numerically investigated slug ascent through a magma with variable viscosity filling the conduit. Their results agreed with the experimental and numerical results from Del Bello et al (2015) and Capponi et al (2016a) showing that, for plugged conduits, higher plug viscosity caused i) slower ascent of a shorter slug, and ii) higher burst overpressure (Lieth and Hort, 2016), and further highlighting the need of a better integration of experimental and numerical methods with field observations linking the eruptive dynamics to the source processes. Although Capponi et al (2016a) illustrated how a plug may affect the flow organisation, a detailed link between flow processes and pressure variations was missing.…”

Violent expansion of a rising Taylor bubble

“…The criterion that they propose is very similar to the one shown below in Eqs. (11) and (13), which demonstrates in very simple terms the importance of the initial bubble depth and of the pressure above the liquid free surface.…”

“…Another process in which the phenomenon studied here plays an important role is volcanic eruptions of the so-called Strombolian type (see, e.g., Refs. [8][9][10][11][12][13]). Citing from Ref.…”

Gas flow dynamics in the conduit of Strombolian explosions inferred from seismo-acoustic observations at Aso volcano, Japan

“…This layer makes the rising gas flow more complex than a single slug (Del Bello et al 2015;Capponi et al 2016Capponi et al , 2017. However, the existence of this layer cannot contribute to the acceleration of the gas ascent velocity in the calculation (Lieth and Hort 2016). On the other hand, Goto et al (2014) proposed that the slug might consist of numerous microbubbles that subsequently burst during an explosion.…”