Tracing the differences between Vulcanian and Strombolian explosions using infrasonic and thermal radiation energy

摘要

Eruptive plume dynamics is a direct expression of explosive style, where duration and altitude of eruptive clouds and dispersion of erupted products is proportional to the degree of fragmentation. We present an analysis of infrasonic and thermal records for explosions at Villarrica (Chile), Stromboli (Italy), Santiaguito and Fuego (Guatemala) volcanoes. Across these four systems magma composition spans from basaltic to dacitic and explosive activity is typically described as ranging from Strombolian to Vulcanian. We use this analysis to provide a quantitative, geophysically-based description of, and discrimination between, the different explosive styles that characterize the four volcanoes. While infrasound is directly related to the emission of over-pressurized gas, and thus solely reflects the plume emission, both plume emission and ascent are detected thermally. Thus, the two data sets together provide a complete description of the plume dynamics. In particular, while infrasound solely reflects the gas-thrust phase driving plume emission, thermal radiation energy is also affected by buoyancy during plume ascent. Thermal radiation energy estimated for explosions at Stromboli and Villarrica (10~4-10~7?J) is lower than that for events at Santiaguito and Fuego (10~8-10~9?J), but infrasonic energies overlap. This suggests a greater contribution of buoyancy for eruptive clouds at Santiaguito and Fuego when compared with Stromboli and Villarrica. We further investigated the plume dynamics by comparing infrasonic energy, which reflects gas-thrust (E_(GT)), with the difference between thermal radiation and infrasonic energies, which mostly reflects buoyancy (E_B). Our data distribution reveals two separate clusters. Explosions at Stromboli and Villarrica share low values of buoyancy, pointing to a gas-thrust dominated emission, efficient coupling of the infrasonic source to the atmosphere, and a Strombolian-type source process to generate a plume rich in coarse fragments. In contrast, explosions at Santiaguito and Fuego share large buoyancy (E_B), and are consistent with Vulcanian-type events in which a large part of infrasonic energy is spent in magma fragmentation leading to ascent of buoyant, ash-rich plumes. We demonstrate that the comparison between thermal radiation and infrasonic energies of explosions allows a quantitative discrimination between, and characterization of, Strombolian- and Vulcanian-type explosions, which leads us to suggest that such an approach may be used as an index to further (and quantitatively) discriminate between eruptive styles.