Bubble size distribution in magma chambers and dynamics of basaltic eruptions

In the shallow magma chambers of volcanoes, the CO2 content of most basaltic melts is above the solubility limit. This implies that the chamber contains gas bubbles, which rise through the magma and expand. Thus, the volume of the chamber, its gas volume fraction and the gas flux into the conduit change with time in a systematic manner as a function of the size and number of gas bubbles. Changes in gas flux and gas volume are calculated for a bubble size distribution and related to changes in eruption regimes. Fire fountain activity, only present during the first quarter of the eruption, requires that the bubbles are larger than a certain size, which depends on the gas flux and on the bubble content [1]. As the chamber degasses, it loses its largest gas bubbles and the gas flux decreases, eventually suppressing the fire fountaining activity. Ultimately, an eruption stops when the chamber contains only a few tiny bubbles. More generally, the evolution of basaltic eruptions is governed by a dimensionless number, tau* = tau g Delta rho a(o)(2)/(18 mu h(c)), where tau = a characteristic time for degassing; a(o) = the initial bubble diameter; mu = the magma viscosity; and h(c) = the thickness of the degassing layer. Two eruptions of the Kilauea volcano, Mauna Ulu (1969-1971) and Puu O'o (1983-present), provide data on erupted gas volume and the inflation rate of the edifice, which help constrain the spatial distribution of bubbles in the magma chamber: bubbles come mainly from the bottom of the reservoir, either by in situ nucleation long before the eruption or within a vesiculated liquid. Although the gas flux at the roof of the chamber takes similar values for both eruptions, the duration of both the fire fountaining activity and the entire eruption was 6 times shorter at Mauna Ulu than during the Puu O'o eruption. The dimensionless analysis explains the difference by a degassing layer 6 times thinner in the former than the latter, due to a 2 year delay in starting the Mauna Ulu eruption compared to the Puu O'o eruption.