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The 2001 eruption of Etna volcano, prior to the flank eruption, was marked by 16 episodes separated from one another by few days of quiescence. Insights into fire fountain formation are provided by a close comparison of the sound produced by an episode solely involving a series of Strombolian explosions (4 July) and one also showing a transition to a fire fountain (12 July). The best fit between measured and synthetic waveforms gives the temporal evolution of the bubble length, 8–100 m, and overpressure, 0.2 MPa. Both episodes result from the coalescence of a foam layer trapped at the top of the reservoir. At the transition towards a fire fountain, the number of explosions and the bubble length increase simultaneously, suggesting that the foam destabilization is more efficient when a fire fountain is produced. Acoustic records give access to the gas volume trapped within the foam, called ‘active’ degassing, while the height of fire fountains also includes the gas from passive degassing. The small bubbles from passive degassing are carried to the surface via the wake of the slugs, coming from the depth of the reservoir. The proportion between active and total gas volume represents 38–48%.

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