Controls of conduit geometry and wallrock elasticity on lava dome eruptions

Abstract

Many lava dome building eruptions show periodic to complex non-periodic pulsatory activity. Typical time-scales associated with this activity range from hours to decades. Previous studies modelled the ascent of magma using a set of transient 1-D transport equations, accounting for degassing induced crystallization kinetics, gas exsolution and viscosity increase due to crystal growth. These models assumed flow in a cylindrical conduit with a fixed cross-section area. Since several observations suggest that extrusions are mainly fed by dykes, with cylindrical geometries developing only at shallow levels, here we generalised the model to the flow geometry represented by an elliptical dyke with major and minor semi-axes changing with depth. Quasi-static elastic deformation of the dyke is accounted by an analytical solution that couples cross-section area with the magmatic overpressure. The effects of the main dyke geometrical parameters and boundary conditions on the eruption dynamics were investigated. The presence of a deformable dyke can lead to a more complex periodic behaviour with a wider range of time-scales and cyclicity patterns with respect to a uniform cylindrical conduit. There is a regime where the period of pulsations is controlled by the elasticity of the dyke and a regime where the period is controlled by the volume of the magma chamber. Intermediate regimes are possible. Periodic variations in discharge rate are also possible for both fixed pressure in dyke source region and fixed influx rate into the dyke. Our study emphasizes the strong nonlinearities and complex behaviours of lava dome eruptions. From a forecasting and hazard perspective, intrinsic uncertainties in governing parameters may make volcanic systems in some circumstances unpredictable. On the other hand, lava dome systems may also develop episodic and systematic behaviours so that behaviour becomes predictable for a while.