A major factor controlling tectonic plate motion is the visco-plastic rheology of olivine, one of the most common minerals in the upper mantle. However, investigating the rheological properties of olivine in the high pressure and low temperature conditions of the shallow lithosphere is a difficult task because olivine tends to deform brittlely.
Kranjc et al.  use an ingenious microscale loading technique originally devised by material scientists but still new to geoscientists. Pillars about 1 micrometer in diameter and 2 micrometers long were fabricated in olivine single crystals and uniaxially deformed at room temperature. Many pillars developed brittle fractures, but some showed clear signs of visco-plastic deformation.
The visco-plastically deformed pillars were examined using the Scanning Electron Microscope and the Transmission Electron Microscope. In addition to identifying plastic shear bands with sub-micrometer thickness, the authors observed that the bands formed in Fe-rich amorphized material. The shear bands were enriched in ferric Fe3+ oxides compared to the ferrous Fe2+ oxides more common in the bulk of the olivine crystal. The relation between stress concentration in shear bands and oxidation from Fe2+ to Fe3+ still remains to be worked out.
Citation: Kranjc, K., Thind, A. S., Borisevich, A. Y., Mishra, R., Flores, K. M., & Skemer, P. . Amorphization and plasticity of olivine during low‐temperature micropillar deformation experiments. Journal of Geophysical Research: Solid Earth, 125, e2019JB019242. https://doi.org/10.1029/2019JB019242
—Yves Bernabé, Editor, JGR: Solid Earth