Abstract:
The ascent of gas-saturated magma in a conduit can lead to the transition from the laminar flow of a bubble-rich melt to the turbulent flow of particle-rich hot gas in upper part of the conduit. This process is investigated with a help of a disequilibrium two-phase flow model for steady and unsteady conditions. For the description of different gas-magma flow regimes in the conduit separated sets of equations are used. The main difference from previous conduit flow models is the consideration of the pressure difference between a growing bubble and the surrounding melt. For a bubbly liquid to evolve into a gas-particle flow a critical overpressure value must be exceeded. This transition region is simulated by means of a discontinuity, namely by a fragmentation wave. The magma flow calculations are carried out for a given conduit length and overpressure between the magma chamber and the atmosphere. The steady solution of the boundary problem is not unique and there are up to five distinct steady regimes corresponding to fixed eruption conditions. Within the framework of the quasi-steady approach (governing parameters being varied monotonically) the transition from one regime to another can take place suddenly and is accompanied by the fundamental restructuring of the conduit flow, resulting in rapid or abrupt changes in the intensity of explosive eruptions. Abrupt intensification of an explosive eruption occurs when the chamber pressure becomes sufficiently less than saturation pressure, and therefore corresponds to the case of shallow-depth magma chamber and high initial water content. A regime with minimum flow rate may relate to the growth of a lava dome following the explosive phase of eruption. These models can explain geological observations that imply large and sudden changes of discharge rate in large-magnitude explosive eruptions, particularly at the transition between plinian phase and ignimbrite formation.