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

Abstract

The interpretation of geophysical measurements at active volcanoes is vital for hazard assessment and for understanding fundamental processes such as magma degassing. For Strombolian activity, interpretations are currently underpinned by first-order fluid dynamic models which give relatively straightforward relationships between geophysical signals and gas and magma flow. However, recent petrological and high-speed video evidence has indicated the importance of rheological stratification within the conduit and, here, we show that under these conditions, the straightforward relationships break down. Using laboratory analogue experiments to represent a rheologically-stratified conduit we characterise the distinct variations in the shear stress exerted on the upper sections of the flow tube and in the gas pressures measured above the liquid surface, during different degassing flow configurations. These signals, generated by varying styles of gas ascent, expansion and burst, can reflect field infrasonic measurements and ground motion proximal to a vent. The shear stress signals exhibit timescales and trends in qualitative agreement with the near-vent inflation-deflation cycles identified at Stromboli. Therefore, shear stress along the uppermost conduit may represent a plausible source of nearvent tilt, and conduit shear contributions should be considered in the interpretation of ground deformation, which is usually attributed to pressure sources only. The same range of flow processes can produce different experimental infrasonic waveforms, even for similar masses of gas escape. The experimental data resembled infrasonic waveforms acquired from different vents at Stromboli associated with different eruptive styles. Accurate interpretation of near-vent ground deformation, infrasonic signal and eruptive style therefore requires detailed understanding of: a) spatiotemporal magma rheology in the shallow conduit, and b) shallow conduit geometry, as well as bubble overpressure and volume.

Ground deformation reveals the scale-invariant conduit dynamics driving explosive basaltic eruptions

“…However, the differences in the gas and crystal contents (and, hence, in the viscosity) between the two magma types are not explicitly taken into account in our model. We nevertheless argue that the low viscosity of mafic, gas-rich magma rising from depth favours its rapid upward migration and interaction with denser, degassed magmas residing in the shallow conduit, which can act as a viscous plug promoting resistance to conduit flow 42,43 . .…”

Analogue experiments on the rise of large bubbles through a solids-rich suspension: A “weak plug” model for Strombolian eruptions

“…Scaled analogue experiments with gas slugs ascending from a low to high viscosity Newtonian liquid have explored the effects of a near-surface viscous transition on slug flow in a cylindrical conduit (Del Bello et al, 2015;Capponi et al, 2016Capponi et al, , 2017. These investigations proposed three geometrical configurations of the slug-cap interactions based on the viscosity of the upper layer (the viscous "cap") and the size of the cap relative to the bubble volume (Figure 1).…”

The contribution of experimental volcanology to the study of the physics of eruptive processes, and related scaling issues: A review

“…Del also studied the influence of a rheological stratification in the conduit, which was representative of most natural cases, and they showed that slugs ascending through an upper viscous plug of degassed fluid exploded more violently as the slugs were pressurized. Capponi et al (2017) explored shear stresses and gas pressure variations as a function of the rheological contrast).…”