STROMBOLIAN EXPLOSIONS .1. A LARGE BUBBLE BREAKING AT THE SURFACE OF A LAVA COLUMN AS A SOURCE OF SOUND

Strombolian activity consists of a series of explosions caused by the breaking of large overpressurized bubbles at the surface of the magma column. Acoustic pressure has been measured for 36 explosions at Strom boli. We propose that sound is generated by the vibration of the bubbl e before it bursts. Oscillations are driven by an initial overpressure inside the bubble, assumed to be initially at rest, just below the ma gma-air interface. Inertia effects cause the bubble to overshoot its e quilibrium radius. Then the bubble becomes underpressurized and contra cts because of gas compressibility. These oscillations are only slight ly damped by viscous effects in the magma layer above the bubble. The bubble cannot complete more than one cycle of vibration because of ins tabilities developing on the magma layer that lead to its breaking, ne ar the minimum radius. Assuming a simple geometry, we model this vibra tion and constrain the radius and length of the bubble and the initial overpressure by fitting a synthetic waveform to the measured acoustic pressure. The fit between synthetic and observed waveforms is very go od, both for frequency, approximate to 60 rad s(-1), and amplitude. Th e initial bubble radius is approximate to 1 m, and the length varies b etween several and a few tens of meters. From the initial overpressure , approximately 10(5) Pa, we calculate the maximum radial velocity of ejecta, approximate to 30 m s(-1). The generally good agreement betwee n data and predictions of our model leads us to suggest that acoustic measurements are a powerful tool for the understanding of eruption dyn amics.