A new regime of volcanic eruption due to the relative motion between liquid and gas

In explosive magma eruptions, magma ascends through a conduit as a Poiseuil le flow at depth, and gas exsolves gradually and expands as the pressure de creases (bubbly flow regime). When the volume fraction of gas becomes suffi ciently large, liquid or solid parts of magma fragment into droplets or ash es, and the flow dynamics becomes governed by the gas phase (gas-ash now re gime). We propose a new flow regime, which we call fractured-turbulent flow regime, between the bubbly flow regime and the gas-ash flow regime. In the new regime, both liquid ma,oma and gas are continuous phases. The high con nectivity of the two phases allows the relative velocity between them to in crease significantly. We present one sample calculation, which displays bas ically explosive characteristics, but has three features distinct from prev ious models, The explosive characteristics are manifested as the fragmentat ion of the magma and the high speed jet that issues from the vent. The firs t distinct feature is a nearly lithostatic pressure distribution, which res ults from the increase of the height of the fragmentation surface. The seco nd one is the atmospheric pressure at the vent; the flow is not choked. The third one is that the relative velocity between the gas and the ash is lar ge at the vent despite the large interaction force between the two phases. The large relative velocity is established in the fractured-turbulent regim e, and is maintained in the subsequent gas-ash flow regime. (C) 1999 Elsevi er Science B.V. All rights reserved.