The effects of multiple phase transitions on mantle convection are investigated by numerical simulations that are based on three-dimensional models. These simulations show that cold sheets of mantle material collide at junctions, merge, and form a strong downflow that is stopped temporarily by the transition zone. The accumulated cold material gives rise to a strong gravitational instability that causes the cold mass to sink rapidly into the lower mantle. This process promotes a massive exchange between the lower and upper mantles and triggers a global instability in the adjacent plume system. This mechanism may be cyclic in nature and may be linked to the generation of superplumes.
Using a spectral code, we have studied the time‐dependent regime of three‐dimensional anelastic compressible convection with depth‐dependent thermal expansivity, viscosity and thermal conductivity in a wide box of size 5×5×1. Surface Rayleigh numbers up to 5 ×l06, have been considered. Very few cylindrical plumes are developed at the bottom but they join up collectively to form strong upwellings, which pulsate chaotically. Major descending flows occur in sheets which form rectangular planform at the top. The thermal and flow fields are dominated by large‐scale features. The bottom 20% of the convecting layer is found to be superadiabatic.
We have applied spectral‐transform methods to study three‐dimensional thermal convection with temperature‐dependent viscosity. The viscosity varies exponentially with the form exp(‐BT), where B controls the viscosity contrast and T is temperature. Solutions for high Rayleigh numbers, up to an effective Ra of 6.25×106, have been obtained for an aspect‐ratio of 5×5×1 and a viscosity contrast of 25. Solutions show the localization of toroidal velocity fields with increasing vigor of convection to a coherent network of shear‐zones. Viscous dissipation increases with Rayleigh number and is particularly strong in regions of convergent flows and shear deformation. A time‐varying depth‐dependent mean‐flow is generated because of the correlation between laterally varying viscosity and velocity gradients.
Numerical simulations of three-dimensional convection with temperature-dependent viscosity and viscous heating at realistic Rayleigh numbers for Earth's mantle reveal that, in the strongly time-dependent regime, very intense localized heating takes place along the top portion of descending cold sheets and also at locations where the ascending plume heads impinge at the surface. For a viscosity contrast of 100, these localized heat sources exceed the internal heating due to the radioactive decay of chondritic materials by more than an order of magnitude. The horizontally averaged viscous dissipation is concentrated in the top of the convecting layer and has a magnitude comparable with that of radioactive heating.
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