The lithosphere is the shell of rigid rock that forms Earth’s surface, composed of crust and upper mantle, where olivine is the most abundant mineral. Beneath this rigid shell lies the asthenosphere, a ductile layer capable of slowly deforming over geological time. What happens at the boundary between these two layers whose mechanical properties are so different? Hosni Idrissi, a researcher at UCLouvain’s IMMC, co-supervised an international study to answer this question by investigating the mechanical properties of olivine. The authors of this study, published in Nature, used electron microscopy to observe the behaviour of olivine when it deforms under high stress: they found that the walls between the olivine grains present a thin vitrified layer resulting from the mechanical collapse of the crystalline structure (see illustration). This observation sheds new light on the mantle rock’s mechanical properties because glass has very specific mechanical properties, particularly a sudden drop in viscosity near a characteristic temperature called the glass transition temperature. The authors thus propose that the softening of this thin intergranular layer would be the trigger for the change in mechanical behaviour between the lithosphere and the asthenosphere. This scientific breakthrough represents an important step towards a better understanding of the origin of earthquakes.