Degassing behaviour at basaltic volcanoes: New insights from experimental investigations of different conduit geometry and magma viscosity

Abstract

Understanding gas-magma dynamics in volcanic conduits and linking them with the associated geophysical signals at the surface is of fundamental importance in monitoring active volcanoes. In the past, a significant number of studies have been devoted to this topic, addressing the onset, the key factors governing each degassing regime and the related markers at the surface, particularly at basaltic volcanoes. Here, we first review such a broad and increasing literature, focusing on the main approaches used in the attempt of deciphering conduit dynamics by indirect observations: 1) analogue laboratory experiments; and 2) seismo-acoustic measurements. Then, we combine the two techniques into a novel set of experiments, addressing a crucial, yet unexplored, issue: the irregularity (i.e. the departure from an ideal smooth cylindrical shape) of the conduit surface. We built a set of epoxy conduits with various fractal dimensions (Dc; i.e. irregularity) of the internal surface, using silicone oil as a proxy for magma. Different degassing patterns (bubbly, slug and churn-annular flow) were reproduced by changing systematically: 1) injected gas flux (5 to 180 × 10−3 l/s); 2) analogue magma viscosity (10 to 1000 Pas); 3) fractal dimension (Dc) of the conduit surface (i.e. Dc = 2, Dc = 2.18 and Dc = 2.99). The experiments were monitored by means of a video-camera and a set of sensor aimed to detect the seismic and acoustic signals. Results show that viscosity strongly influences the transition among degassing patterns and the recurrence rate of slug bursts at the surface. Moreover, we observed an increase of the exponent of the power law equation linking squared seismic amplitude to gas flow rate with conduit roughness; the opposite trend was noticed with increasing liquid viscosity. These results have important implications for linking seismic tremor to eruption source parameters such as the volume discharge rate at different volcanoes or for investigating its temporal evolution at a single vent.