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

It is well-known that the shear viscosity of silicate-rich melts is ve ry sensitive to water content. Thus, as such, a material rises towards the earth's surface in an eruption, the pressure falls, water exsolve s to form bubbles or voids, and the melt viscosity increases. This is responsible for the (by now) well-known large departures of the pressu re gradient from hydrostatic. The exsolved gas is known to escape from the rising lava into the surrounding rock and since gas content is be lieved to have an important role in the transitions between eruption r egimes, it is of some interest to understand the mechanism of escape. A key difficulty is to explain how small gas bubbles could migrate lat erally across the conduit, in view of their small size and the large l iquid viscosity. It is shown here that the increase of melt viscosity with height produces a horizontal pressure gradient directed so that t he 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 conf iguration is (very probably) dynamically unstable. The resulting insta bility will have the effect of mixing the lava, thus tending to transp ort gas bubbles from the centre to the (depleted) regions near the wal ls. (C) 1998 Elsevier Science B.V. All rights reserved.