  We have conducted laboratory experiments to model important aspects of plumes
in mantle convection. We focus on the role of the viscosity ratio U (between
the ambient fluid and the plume fluid) in determining the plume structure and
dynamics. In our experiments, we are able to capture geophysical convection
regimes relevant to mantle convection both for hot spots (when U > 1) and
plate-subduction (when U < 1) regimes. The planar laser induced fluorescence
(PLIF) technique is used for flow visualization and characterizing the plume
structures. The convection is driven by compositional buoyancy generated by the
perfusion of lighter fluid across a permeable mesh and the viscosity ratio U is
systematically varied over a range from 1/300 to 2500. The planform, near the
bottom boundary for U=1, exhibits a well-known dendritic line plume structure.
As the value of U is increased, a progressive morphological transition is
observed 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 into
intermittent spherical-blob shaped plumes at high U, resembling mantle plume
hot spots in mantle convection. In contrast, for low values of U (~1/300), the
regime corresponds to subduction of plates in the mantle. In this regime, we
observe for the first time that plumes arise from a thick boundary with
cellular structure and develop into sheet-plumes. We use experimental data to
quantify these morphological changes and mixing dynamics of the plumes at
different regimes of U. We also compare our observations on plume spacing with
various models reported in the literature by varying the viscosity ratio and
the buoyancy flux.
