Lava dome morphology inferred from numerical modelling

SUMMARY Lava domes form when highly viscous magmas erupt on the surface. Several types of lava dome morphology can be distinguished depending on the flow rate and the rheology of magma. Here, we develop a 2-D axisymmetric model of magma extrusion on the surface and lava dome evolution and analyse the dome morphology using a finite-volume method implemented in Ansys Fluent software. The magma/lava viscosity depends on the volume fraction of crystals and temperature. We show that the morphology of domes is influenced by two parameters: the characteristic time of crystal content growth (CCGT) and the discharge rate (DR). At smaller values of the CCGTs, that is, at rapid lava crystallization, obelisk-shaped structures develop at low DRs and pancake-shaped structures at high DRs; at longer CCGTs, lava domes feature lobe- to pancake-shaped structures. A thick carapace of about 70 per cent crystal content evolves at smaller CCGTs. We demonstrate that cooling does not play the essential role during a lava dome emplacement, because the thermal thickness of the evolving carapace remains small in comparison with the dome's height. A transition from the endogenic to exogenic regime of the lava dome growth occurs after a rapid increase in the DR. A strain-rate-dependent lava viscosity leads to a more confined dome, but the influence of this viscosity on the dome morphology is not well pronounced. The model results can be used in assessments of the rates of magma extrusion, the lava viscosity and the morphology of active lava domes..

Morphology of lava domes inferred from numerical modeling

<p>Several types of lava dome morphology can be distinguished depending on the flow rate and the rheology of magma. At an endogenous regime, magma is embedded inside the dome and fresh magma is not extruded on the surface; vice versa, at an exogenous regime, a fresh lava is extruded, and a lava obelisk is of particular interest. Sometimes obelisks reach hundreds of meters in height before they collapse. We present models of magma extrusion on the surface and lava dome evolution to analyze morphology of the domes. For this aim, we consider a flow of a Newtonian and non-Newtonian viscous inhomogeneous incompressible fluid in the field of gravity. The flow is described by the Navier-Stokes equations, the continuity equation, the transport equation of a two-component incompressible fluid, the heat conduction equation, and the rheological law. The lava viscosity in our models depends on the crystals concentration, temperature, and the rate of shear deformation. We show that the morphology of the domes depends on the characteristic time of crystal growths in the magma and on the rate of magma extrusion. In this case, obelisks are formed at a small value of the characteristic time of growth of crystals and/or low extrusion rates. At high values of the characteristic time and high extrusion rates, magma spreads over the surface after an eruption.</p>