ORIGIN OF THE SOUND GENERATED BY STROMBOLIAN EXPLOSIONS

During strombolian eruptions, large bubbles break at the surface of th e lava column and produce sound. Acoustic pressure recorded during sev eral explosions on the Eastern vent of Stromboli volcano, shows a patt ern consistent between explosions. The well-marked oscillation contain s only very low frequencies (around 7 Hz), and is followed by a signal containing now both higher frequencies and lower frequency (less than 4 Hz). The generation of sound is explained by the vibration (7 Hz) a nd bursting (higher frequencies) of large bubbles, followed by the dra inage of magma down in the conduit (less than 4 Hz). The interpretatio n suggests that: (1) the bubble vibrates before bursting and (2) the v ibration prior to bursting yields more energy than the bursting itself . The radius of bubbles responsible for explosions is determined to be close to 1 meter, in agreement with observations of a few conduit siz es.