Three-dimensional convection for the spinel to perovskite phase change has been simulated numerically. Results for Rayleigh (Ra) numbers of 0(106) show intermittent layering with a strong robust plume rising through the phase boundary. Many descending instabilities are deflected but merging cold sheets come together at a junction. A pool of cold material accumulates underneath in the phase-transition zone. A strong gravitational instability results, which precipitates a rapid and massive discharge of upper-mantle material. ¿ American Geophysical Union 1993