The acoustics of bulge rise and rupture at Strokkur geyser.

Geysers are natural geothermal features that episodically erupt hot water and steam due to the buildup and release of subsurface vapor bubbles. At Strokkur geyser, Iceland, eruptions begin with the growth of a surface bulge caused by rising bubble clusters, followed by gradual rupture and disintegration into a water fountain. In this study, we investigate the high-resolution acoustic and visual signatures of this initial phase of the eruption, providing detailed insights into the fluid dynamics that govern bulge formation and rupture. While similar eruptive behavior is sometimes observed in low-viscosity volcanic systems, Strokkur offers a uniquely transparent medium in which processes like bubble rise and clustering can be directly observed, providing analogies to otherwise obscured dynamics in lava or mud-dominated settings. We combine low-frequency infrasound and high-frequency audio recordings with high-speed video, using synchronized data to track the evolution of the bulge. The results demonstrate that infrasound effectively detects bulge growth, while the onset of rupture is marked by a rise in audio-frequency amplitude. A monopole model is used to simulate pressure variations during bulge growth. The observed decompression signal is associated with the downward water motion during bulge disintegration. These findings improve our understanding of geyser eruption dynamics and suggest how acoustic monitoring can provide valuable information about subsurface processes in both geysers and volcanoes, such as dome inflation or gas bubble accumulation beneath magma surfaces. Supplementary Information: The online version contains supplementary material available at 10.1007/s00445-025-01876-3.