The effective buoyancy ratio was introduced as a diagnostic tool to characterize the evolution of the thermo-chemical mixing in the Earth’s mantle. This parameter tracks the fate of the primordial compositionally dense layer above the core–mantle boundary such as (i) the transition phase of warming dense layer; (ii) the erosion and dilution of the dense layer; (iii) the effective thermo-chemical convection (mixing of layers) and (iv) the homogenization.