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We have conducted laboratory experiments to model important aspects of plumesin mantle convection. We focus on the role of the viscosity ratio U (betweenthe ambient fluid and the plume fluid) in determining the plume structure anddynamics. In our experiments, we are able to capture geophysical convectionregimes relevant to mantle convection both for hot spots (when U > 1) andplate-subduction (when U < 1) regimes. The planar laser induced fluorescence(PLIF) technique is used for flow visualization and characterizing the plumestructures. The convection is driven by compositional buoyancy generated by theperfusion of lighter fluid across a permeable mesh and the viscosity ratio U issystematically varied over a range from 1/300 to 2500. The planform, near thebottom boundary for U=1, exhibits a well-known dendritic line plume structure.As the value of U is increased, a progressive morphological transition isobserved from the dendritic-plume structure to discrete spherical plumes,accompanied with thickening of the plumes and an increase in the plume spacing.In the vertical section, mushroom-shaped plume heads at U=1 change intointermittent spherical-blob shaped plumes at high U, resembling mantle plumehot spots in mantle convection. In contrast, for low values of U (~1/300), theregime corresponds to subduction of plates in the mantle. In this regime, weobserve for the first time that plumes arise from a thick boundary withcellular structure and develop into sheet-plumes. We use experimental data toquantify these morphological changes and mixing dynamics of the plumes atdifferent regimes of U. We also compare our observations on plume spacing withvarious models reported in the literature by varying the viscosity ratio andthe buoyancy flux.

The role of viscosity contrast on plume structure in laboratory modeling of mantle convection