In contrast to the upper mantle, the tower mantle of the Earth is elastically nearly isotropic, although its dominant constituent mineral [(Mg,Fe)SiO3 perovskite] is highly anisotropic. On the bads of high-temperature experiments on fabric development in an analog CaTiO3 perovskite and the elastic constants of MgSiO3 perovskite, the seismic anisotropy was calculated for the lower mantle. The results show that absence of anisotropy is strong evidence for deformation by superplasticity. in this case, no significant transient creep is expected in the lower mantle and the viscosity of the lower mantle is sensitive to grain size; hence, a reduction in grain size with result in theological weakening.