Mantle minerals won't share the strain The deformation of a mixed block of material depends on the strength of the components of which it is made. Weak materials will deform more than the strong ones in a mixture that is squished or stretched. Girard et al. find a large difference in strength between the two primary minerals making up Earth's lower mantle (see the Perspective by Chen). Deformation in the convecting mantle may occur only near boundary layers as a result, leaving large regions potentially unaffected. This could explain long-lived chemical reservoirs in Earth's interior and the lack of seismic anisotropy in the lower mantle. Science , this issue p. 144 ; see also p. 122