Degassing at low magma-viscosity volcanoes: Quantifying the transition between passive bubble-burst and Strombolian eruption

At many volcanoes, low magma-viscosities allow persistent degassing over a range of styles from explosive Strombolian activity to gas ‘puffing’ and passive degassing. It is generally accepted that Strombolian eruptions reflect the bursting of large bubbles (gas slugs) at the magma surface and that relatively quiescent ‘puffing’ indicates the presence of significantly smaller bubbles or slugs. Here, we address this qualitative range and derive a dimensionless parameter, Pslim⁎, to quantify and distinguish different regimes of ‘burst vigour’. For Pslim⁎ ≤ 1, ‘passive’ activity is anticipated and measurable parameters such as infrasonic amplitude will be small. For Pslim⁎ > 1, bursting slugs will be energetic, and surface effects are anticipated to increase with increasing Pslim⁎. Various physical parameters recorded during laboratory experiments and simulated through numerical models are shown to demonstrate the same trends when parameterised by Pslim⁎. Hence, Pslim⁎ provides a straightforward relationship between changes in measurable surface effects and the system's subsurface physical parameters and, for energetic activity of a particular system, burst effects are shown to be proportional to the square root of the gas mass involved. Effect magnitudes are also shown to be a function of atmospheric pressure and application of Pslim⁎ parameterisation to planetary scenarios produces results in line with our current understanding of planetary volcanism.