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Figure 1: Cold downwellings (left) and hot upwellings (right) in a 3-D simulation of compressible mantle convection with an endothermic phase transition at 670 km depth, by Tackley et al. [1993, 1994]. Isocontour show where the temperature is 110 K lower (left plot) or higher (right plot) than the horizontal average. A network of interconnected downwelling sheets in the upper mantle does not penetrate the phase transition, but cold material enters the lower mantle in the form of broad cylindrical 'avalanches' visible in several places around the sphere. These spread out in the deep mantle, surrounding the core with cold material. Broad hot structures are visible in the upper mantle and deep mantle, but plume activity in the midmantle is limited.

 
Figure 2: The effect of Rayleigh number on compressible mantle convection with phase changes. Representative frames from a sequence of simulations at 6 different Rayleigh numbers, taken from Yuen et al. [1994]. Simulations were performed in a 5x5x1 periodic box, with both the 660 km and 400 km phase changes included. Rayleigh numbers (based on surface material properties) are 2x10⁶, 1x10⁷, 4x10⁷, 6x10⁷, 1x10⁸, 4x10⁸. Episodic avalanching at the tower Rayleigh numbers is replaced complete layering at higher Rayleigh numbers.