Rheological controls on the dynamics of channeled lava flows

Abstract

The influence of a viscoplastic lava rheology on the dynamics of channeled lava flows is analysed using numerical methods and analogue experiments. A numerical solution for the flow of a Bingham fluid in a rectangular channel is found using a multigrid-based augmented Lagrangian scheme. The numerical results show that an internal viscoplastic rheology significantly modifies the velocity distribution within a lava flow through the development of plug regions whose size is determined by the magnitude of the yield strength. The flow rate, maximum surface velocity and central plug dimensions are determined as functions of the channel geometry and fluid rheology, and comparisons between these and several limiting analytical solutions confirm the accuracy of the numerical method used. The results are also compared to incorrect models which have been proposed previously in the literature. Several algorithms that extend the results to different sets of measured initial parameters are outlined.

The experiments used slurries of polyethylene glycol and kaolin, which flowed with a constant flux down an inclined channel under water. Three sets of complementary experiments are presented: isothermal, cooling, and solidifying flows which quantified the effects of the viscoplastic rheology on shear, internal convection and surface crust formation. The isothermal and cooling experiments showed the formation of unyielded central plug regions which were not broken up by the convective overturning. In the solidifying experiments flows fell into one of three regimes: a tube regime, in which crust covered the entire flow surface; a shear-dominant regime, with a mobile raft of crust in the channel centre and open shear zones near the walls; and a plug-dominant regime where the width of the central crust was determined by the width of the central plug region. The crust coverage is parameterized in terms of two dimensionless parameters: the ratio wp of central plug region width to channel width and a parameter {u03D1} which characterizes the relative importance of the strain and solidification rates. Finally typical lava flows on Mt Etna and the 1984 Mauna Loa lava flow are analysed to show that the parameterization agrees with lava flow crust widths observed in the field, and we find that even small yield strengths have a major effect on crust coverage. -- provided by Candidate.