Convection experiments in high Prandtl number silicones, Part 2. Deformation, displacement and mixing in the Earth's mantle

The evolution of structures in the Earth's convecting mantle is studied in a simple, but carefully scaled, laboratory flow. The geometrical scaling is constrained by the various geometries of convection cells possible in the Earth's interior. Two-layered mantle convection with a superposed small-scale circulation in the top is favoured here. The model flow has a Rayleigh number 10⁴, a Prandtl number 10⁸, and approximately axially symmetric cells with aspect ratio of unity. The rheology and boundary conditions are aimed to be dynamically similar to that occurring in the mantle. The flow is analysed on the basis on snapshots showing the progressive deformation and displacement of passive flow markers, which allow the reconstruction of velocity vectors, strain rate and vorticity. Inferences can be made about 1. (1) the progressive deformation in individual rock masses, and 2. (2) the nature of the large-scale mixing process caused by a combination of deformation and displacement in a convecting mantle. This is discussed in terms of the kinematic vorticity number. Both geological and fluid mechanical studies of low inertia flows may benefit from the new experimental technique and analyses presented here. Until now, it has been impossible to visualize and quantify the finite strains and rotation components of deformation in flowing fluids by other than numerical methods. The laboratory method advocated here is quick and involves equipment which is relatively cheap if compared with those required for numerical analyses. Basic understanding of elementary fluid dynamics is sufficient to perform innovative investigations in this new field.