Rheological controls on the dynamics of channeled lava flows

We report the results of a series of analogue laboratory experiments which investigate the cooling, convection and solidification of a channelized viscoplastic lava flow. In the experiments, slurries of kaolin and molten polyethylene glycol flowed with a constant flux down an inclined channel under cold water. Experiments with cooling but no solidification showed that thermal convection occurs in organised rolls aligned with the downstream flow. Three crust cover regimes were identified in solidifying flows: (a) a 'tube' regime, in which the surface crust covered the entire channel and jammed against the channel wall and (b) two 'mobile crust' regimes, in which a mobile raft of crust formed in the centre of the channel, separated from the channel walls by open shear zones. In the first mobile crust regime the crust width is determined by a balance between shear and solidification, while in the second it is determined by the width of the unyielded region in the flow centre. The degree of surface crust coverage and the transition between these regimes is quantified in terms of the Bingham number and a dimensionless parameter J, which parameterises the effects of shear and convection on crust growth.