The role of magma composition and water content in explosive eruptions - 1. Conduit ascent dynamics

The role of anhydrous magma composition, water content, and crystal content on the dynamics of explosive eruptions is investigated by modeling the asc ent of magma along volcanic conduits and the subsequent pyroclastic dispers ion in the atmosphere, described in a companion paper [Neri, A., Papale, P. , Macedonio, G., 1998. The role of magma composition and water content in e xplosive eruptions: 2. Pyroclastic dispersion dynamics. J. Volcanol. Geothe rm. Res., 87, 95-115.]. The conduit model used is based on the solution of the fundamental transport equations assuming steady-state and isothermal fl ow conditions, and includes a composition-based description of magma proper ties and their variations along the conduit. This study stems from the well -documented vertical compositional variation of many pyroclastic deposits, often associated with reconstructed variations in initial water content. Th e results of the modeling show complex and sometimes non-intuitive dependen ce of the distribution of the flow variables on magma composition, crystal and water contents, in general, a water content decrease is expected to pro duce a decrease in mass flow-rate, decrease in pressure and velocity along the conduit, an increase in the exit gas volume fraction, and a decrease in velocity, pressure, and mixture density at the conduit exit. Reverse varia tions are expected to occur by decreasing the degree of chemical evolution of the liquid at a constant water content, apart from exit velocities which show more complex variations. The overall effect of increasing crystals is in general similar to that of increasing the degree of chemical evolution of the liquid, or decreasing the water content. The above results are to a large extent interpreted in terms of variations in magma viscosity, which i s recognized as the critical magma property besides water content in the dy namics of magma ascent. The common compositional trend of explosive eruptio ns characterized by chemically evolved, water-richer and crystal-poorer mag ma erupted first is predicted to be associated with variations in the evolu tion of the eruption dynamics, depending on the relative magnitude of the c hanges. However, the exit velocity always decreases in the above trend, and the mass flow-rate increases in most relevant cases, comparing well with t he results of chemical and stratigraphic studies of the deposits from explo sive eruptions. (C) 1998 Elsevier Science B.V. All rights reserved.