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 Poiseuille flow at depth, and gas exsolves gradually and expands as the pressure decreases (bubbly flow regime). When the volume fraction of gas becomes sufficiently large, liquid or solid parts of magma fragment into droplets or ashes, and the flow dynamics becomes governed by the gas phase (gas-ash flow regime). 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 magma and gas are continuous phases. The high connectivity of the two phases allows the relative velocity between them to increase significantly. We present one sample calculation, which displays basically explosive characteristics, but has three features distinct from previous models. The explosive characteristics are manifested as the fragmentation of the magma and the high speed jet that issues from the vent. The first distinct feature is a nearly lithostatic pressure distribution, which results from the increase of the height of the fragmentation surface. The second 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 large at the vent despite the large interaction force between the two phases. The large relative velocity is established in the fractured-turbulent regime, and is maintained in the subsequent gas-ash flow regime.