Although apparently motionless, the rocks that make up Earth are constantly moving. This motion is partially accommodated by the small grains that make up the rocks, sliding past each other. Olivine, (Mg,Fe)2SiO4, is the most prevalent mineral in Earth’s upper mantle, which is why we have chosen it for our study. The sliding of olivine along the grains’ boundaries is an important mode of movement in the mantle. However, direct measurements of the amount grain boundary sliding have not been taken and this prevents accurate modeling of the
motion within Earth. Here we present the first direct measurements of the magnitude of grain boundary sliding in olivine. In our samples, grain boundary sliding dominates at low strains. As the amount of strain increases the effect of grain boundary sliding diminishes.