High‐frame rate thermal imagery of Strombolian explosions: Implications for explosive and infrasonic source dynamics

Explosive activity at Stromboli volcano is analyzed using a high‐frame rate (50 Hz) thermal camera and differential pressure transducers. We develop a thermal image‐based decomposition method to derive vertical and horizontal exit velocities of the explosive cloud. Peak vertical velocity ranges between 23 and 203 m/s, slightly higher than previous estimates and rapidly decreasing to a constant value of 30–50 m/s within the first ∼0.1 s. Plume velocities are consistent with an elongated cloud expanding much faster vertically than horizontally and indicating the interaction with the conduit wall. Considering a vent radius of ∼2 m we estimate a volumetric flux of 200–600 m 3 /s, which converts to total volumes of gas‐particles of 10 3 –10 4 m 3 for a single eruption. These volumes are proportional to the thermal energy recorded by the camera, providing a means to convert thermal radiance to volumes. Comparing the thermal onset of the explosions with the arrival time of the acoustic pressure, we demonstrate that infrasound is propagating 0.14−1.7 s ahead of the explosive front. The time difference between thermal and acoustic onsets constrains the infrasonic source within the conduit at 15–35 m below the crater rim. Peak amplitudes of acoustic pressure show a power law relationship ( p ∼ U 2 ) with the exit vertical velocities consistent with the energy balance of a two‐phase flow rapidly accelerated in the conduit by gas decompression. Our results support monopole isotropic acoustic radiation of a source embedded within the conduit walls and indicate that explosive dynamics undergo strong accelerations of 10 3 –10 4 m/s 2 .