GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.3 201 deformation of the conduit wall and the ensuing change in magma flow rate, determining the change in effusion rate at the conduit outlet. We consider a conduit having an elliptical cross section. This shape gives a great versatility to the model, because the elliptical shape can represent a wide range of cross sections, according to the value of eccentricity, from almost circular vents to very long and narrow fissures. We considered the deformation of a volcanic conduit following from pressure changes in the magma during an effusive eruption. The deformation changes the magma flow rate and can be a cause of the observed short-term changes in effusion rate at volcanic vents. Assuming that the conduit is embedded in an elastic medium, we presented an analytical solution for displacement and stress produced in the medium by an overpressure in the conduit. On this basis, the deformation of the conduit wall and the resulting change in magma flow rate have been calculated. The model shows that the wall displacement is proportional to the ratio between the overpressure and the rigidity of the surrounding rocks. The change in flow rate is instead a nonlinear function of the overpressure. If we consider an oscillation in pressure, the amplitude of flow rate oscillations increases with increasing eccentricity of the conduit wall: hence the greatest oscillations are obtained for very large values of eccentricity, corresponding to long and narrow volcanic fissures. The stress induced in the medium is proportional to the pressure change in the conduit and may be a cause of the seismicity observed in connection with magma ascent. References Del Negro, C., Cappello, A., Neri, M., Bilotta, G., Herault, A., & Ganci, G., 2012. Lava flow hazards at Mount Etna: constraints imposed by eruptive history and numerical simulations., Scientific reports, 3, 3493–3493. Lautze, N. C., Harris, A., Bailey, J. E., Ripepe, M., Calvari, S., Dehn, J., Rowland, S. K., & Evans-Jones, K., 2004. Pulsed lava effusion at Mount Etna during 2001, Journal of Volcanology and Geothermal Research, 137(1-3), 231–246. Piombo, A., Tallarico, A., & Dragoni, M., 2016. Role of Mechanical Erosion in Controlling the Effusion Rate of Basaltic Eruptions, Geophysical Research Letters, 43, 8970–8977. Vicari, A., Ganci, G., Behncke, B., Cappello, A., Neri, M., & Del Negro, C., 2011. Near-real-time forecasting of lava flow hazards during the 12-13 January 2011 Etna eruption, Geophysical Research Letters, 38(13). Voight, B., Sparks, R., Miller, A. D., Stewart, R. C., Hoblitt, R. P., Clarke, A., Ewart, J., Aspinall, W. P., Baptie, B., Calder, E. S., Cole, P., Druitt, T. H., Hartford, C., Herd, R. A., Jackson, P., Lejeune, A. M., Lockhart, AB, Loughlin, S. C., Luckett, R., Lynch, L., Norton, G. E., Robertson, R., Watson, I. M., Watts, R., & Young, S. R., 1999. Magma flow instability and cyclic activity at Soufriere Hills Volcano, Montserrat, British West Indies, Science, 283, 1138–1142. Wadge, G., 1981. The variation of magma discharge during basaltic eruptions, Journal of Volcanology and Geothermal Research, 11(2), 139–168. Whitehead, J. A. & Helfrich, K. R., 1991. Instability of flow with temperature-dependent viscosity: Amodel of magma dynamics, Journal of Geophysical Research, 96(B), 4145–4155. Woods, A. W. & Koyaguchi, T., 1994. Transitions between explosive and effusive eruption of silicic magmas, Nature, 370, 641–644. Wylie, J. J., Voight, B., & Whitehead, J. A., 1999. Instability of magma flow from volatile-dependent viscosity, Science, 285, 1883–1885.