Bubble nucleation during explosive eruptions starts in the magma chamber

Abstract:

Vesicles in pyroclasts from explosive rhyolitic eruptions typically have power-law size distributions over diameters ranging from ~1 micrometer to ~1 millimeter. We investigate under what conditions such vesicle size distributions (VSDs) are produced during bubble nucleation and growth within the erupting magma. We find that the observed power-law VSDs are the result of nucleation that begins already within the magma chamber and continues throughout the entire duration of magma ascent until fragmentation. Such continuous nucleation occurs in slightly converging conduit geometries wherein decompression rates increase gradually due to magma acceleration.

Using decompression experiments of hydrated rhyolite, we confirm that bubble nucleation in rhyolitic melt can be continuous. Experiments with similar decompression rates and magnitudes, but different experimental dwell times between decompression and quenching, result in bubble number densities that increase with experimental dwell time. A scaling analysis of all experiments combined substantiates a one-to-one relation between experimental dwell time and bubble number density, confirming that bubble nucleation in our experiments was continuous for durations of 20 to 120 seconds.