The dynamics of bubble formation and growth in magmas: A review and analysis

Abstract

The mechanisms of bubble formation and the dynamics of bubble growth in common magmas have been investigated. Experimental and theoretical data suggest that the levels of volatile supersaturation pressure required for nucleation are low (< 10 bars) due to the occurrence of surface active components in common magmas. The growth history is, moreover, insensitive to the assumptions made on nucleation, except during early stages of growth. Nucleation theory indicates that there is a minimum diameter of bubble that can be preserved in tephra which is of the order of a few microns. A numerical method has been developed to determine bubble growth rates during volcanic eruptions of basaltic and rhyolitic magmas. The numerical solutions consider both diffusional and decompressional growth and the effects of magma ascent rates (0–400 cm s −1), magma viscosity (10 2 to 10 8 poise), gas solubility, gas content (0.25–5%) and gas diffusivity (10 −6 to 10 −9 cm 2 s −1) on growth rates. Bubbles are predicted to grow to between 5.0 and 0.1 cm diameter in typical basaltic explosive eruptions (Strombolian type) and between 200 and 10 μm in typical rhyolitic explosive eruptions (plinian and ignimbrite type). The smaller bubble sizes in rhyolitic tephras is shown to be due to the lower diffusivity of H 2O in acid melts and the generally higher rates of eruption in plinian and ignimbrite type eruptions rather than to the high viscosity. Non-equilibrium effects produce excess internal pressures within growing bubbles. These are a static surface tension pressure and dynamic pressure terms due to viscous resistance of the liquid to growth and the inertia of the liquid. These pressure terms are shown to be small compared to supersaturation pressures driving growth in freely growing bubbles in magmas with viscosities up to 10 7 poise and magma ascent velocities up to 400 cm s −1. For viscosities of 10 8 poise and above the viscous pressure term is calculated with values of tens of bars and has a major role in inhibiting growth. It is proposed that most bubbles cease growth well before explosive disruption of the magmatic froth. This is postulated to occur at volume ratios of gas bubbles to liquid of the order 3:1 to 5:1. At this stage bubbles cannot expand further because of the increasing viscosity of the melt as exsolution takes place and because high viscosity liquid has to be forced along very thin paths between closely spaced neighbouring bubbles. Vesicle walls are sufficiently thin that pressures close to equilibrium between gas bubble pressure and the vapor pressure of the gas dissolved in the liquid are established before the magma is disrupted. Equilibrium partitioning of gas between vesicles and liquid produces gas pressures of tens to hundreds of bars. The disruption of the magma is proposed to occur by bursting of the larger bubbles within the froth at the free surface of the magma due to a pressure difference across this interface.