A MECHANISM FOR THE LATERAL TRANSPORT OF GAS BUBBLES IN SILICIC LAVA RISING IN A VERTICAL CONDUIT

Abstract

It is well-known that the shear viscosity of silicate-rich melts is very sensitive to water content. Thus, as such, a material rises towards the earth's surface in an eruption, the pressure falls, water exsolves to form bubbles or voids, and the melt viscosity increases. This is responsible for the (by now) well-known large departures of the pressure gradient from hydrostatic. The exsolved gas is known to escape from the rising lava into the surrounding rock and since gas content is believed to have an important role in the transitions between eruption regimes, it is of some interest to understand the mechanism of escape. A key difficulty is to explain how small gas bubbles could migrate laterally across the conduit, in view of their small size and the large liquid viscosity. It is shown here that the increase of melt viscosity with height produces a horizontal pressure gradient directed so that the pressure at the centre of the conduit is larger than at the walls. Thus the fluid in the centre is less viscous and more dense; this configuration is (very probably) dynamically unstable. The resulting instability will have the effect of mixing the lava, thus tending to transport gas bubbles from the centre to the (depleted) regions near the walls.