Image courtesy of Jennifer Girard.
Tectonic plates and subducting slabs, portions of tectonic plates that have slid under other plates, hold the secrets to the movement of land masses. Previous evidence showed that older slabs, which are colder and supposedly stronger, deform in the mantle more than warm, and presumably weaker, plates. This phenomenon puzzled researchers and scientists. Professor Jennifer Girard of Yale University’s Department of Earth and Planetary Sciences, along with a team of researchers including Anwar Mohiuddin and Shun-ichiro Karato, aimed to uncover the basis for this unexpected deformation. They used a high-pressure and high-temperature press to simulate the conditions in the Earth’s mantle, allowing them to study the deformation and subduction of slabs on a smaller scale.
The team found that when a subducting slab made mostly of large olivine mineral plunges into the mantle, the increased pressure causes olivine to transform into fine-grained ringwoodite. “The study clearly shows that newly formed fine-grained ringwoodite is significantly weaker than the coarse-grained olivine,” Girard said. While newly formed ringwoodite in cold slabs grows slowly, higher temperatures in warmer slabs cause grains of ringwoodite to grow much faster; this causes the young slabs to become much stronger as the grain grows. In fact, the team believes that this inhibited growth rate may be the reason that cold slabs deform while warmer slabs do not. These findings will help researchers further explore and understand the unexpected behavior of tectonic plates.
Mohiuddin, A., Karato, S. & Girard, J. Slab weakening during the olivine to ringwoodite transition in the mantle. Nat. Geosci. 13, 170–174 (2020). https://doi.org/10.1038/s41561-019-0523-3