Dynamical effects at the core-mantle boundary have so far been modeled in the frame of thermal as well as double-diffusive convection, yet accumulating evidence supports the idea that the mantle material looses part of its heavy component at the CMB due to combined chemical and physical processes. The segregation of the heavy FeO component from the lower-most mantle to the top of the core occurs inside the lower thermal boundary layer and leads to a new kind of boundary layer instability. A mathematical description of mantle convection including segregation of FeO into the core is developed and the relevant model parameters are estimated. The numerical investigation of the flow structure shows the forming and up-welling of thermo-compositionally driven plumes, as well as down-welling of pure thermally driven plumes even for conditions where the simple thermal convection case has a steady behavior. Periods of quasi-stable plume activity interchange with periods of chaotic reorganization of the flow structure connected with strongly time dependent Nusselt. ¿ American Geophysical Union 1996