Geophysicist identifies a maximum practical viscosity

Per Physics World, a geophysicist at Ritsumeikan University in Japan has resolved a decades-long puzzle in Earth science. Tectonic plates should be rigid—nearly immovable—yet they bend and sink into the mantle. Masaki Yoshida used satellite data tracking tectonic deformations across vast regions, then cross-checked his observations with laboratory experiments and computer models. He found that the viscosity—resistance to flow—of rock under Earth's conditions tops out at ten to the twenty-eighth pascal-seconds, well below the long-accepted estimate of ten to the seventieth. That much lower limit explains why plates, though extremely stiff, can still deform over geological timescales. The work, published in Physics of Fluids, suggests this boundary may sharpen our understanding of Earth's dynamics and shed light on when and why plate motion first began.

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