GEOPHYSICS: Earth Under Strain

Large strains can build up in the slowly convecting mantle when it encounters less mobile regions, for example, at the bottom of rigid plates. In their Perspective, Mackwell and Rubie discuss new insights into the effect of this deformation on mantle rocks gained by Bystricky et al. Olivine, the major upper mantle material, is shown to decrease in strength without a change in deformation mechanism. Such a change was previously proposed to explain mantle processes.