Numerical modeling of magma withdrawal during explosive caldera-forming eruptions

We propose a simple physical model to characterize the dynamics of magma wi thdrawal during the course of caldera-forming eruptions. Simplification inv olves considering such eruptions as a piston-like system in which the host rock is assumed to subside as a coherent rigid solid. Magma behaves as a Ne wtonian incompressible fluid below the exsolution level and as a compressib le gas-liquid mixture above this level. We consider caldera-forming eruptio ns within the frame of fluid-structure interaction problems, in which the f low- governing equations are written using an arbitrary Lagrangian-Eulerian (ALE) formulation. We propose a numerical procedure to solve the ALE gover ning equations in the context of a finite element method. The numerical met hodology is based on a staggered algorithm in which the flow and the struct ural equations are alternatively integrated in time by using separate solve rs. The procedure also involves the use of the quasi-Laplacian method to co mpute the ALE mesh of the fluid and a new conservative remeshing. strategy. Despite the fact that we focus the application of the procedure toward mod eling caldera-forming eruptions, the numerical procedure is of general appl icability. The numerical results have important geological implications in terms of magma chamber dynamics during explosive caldera-forming eruptions. Simulations predict a nearly constant velocity of caldera subsidence that strongly depends on magma viscosity. They also reproduce the characteristic eruption rates of the different phases of an explosive calderaforming erup tion. Numerical results indicate that the formation of vortices beneath the ring fault, which may allow mingling and mixing of parcels of magma initia lly located at different depths in the chamber, is likely to occur for low- viscosity magmas. Numerical results confirm that exsolution of volatiles is an efficient mechanism to sustain explosive caldera-forming eruptions and to explain the formation of large volumes of ignimbrites.