Geophysicist identifies a maximum practical viscosity

Geophysicists have long struggled to explain why tectonic plates, which appear rigid, eventually bend and subduct into the mantle. A common assumption placed the viscosity of these plates at an incredibly high 10^70 Pascal-seconds. This number is so massive that it suggests the plates should remain immobile throughout their existence, creating a contradiction between theory and the observed reality of geological movement.

Masaki Yoshida, a researcher at Ritsumeikan University, recently tackled this issue by analyzing satellite data regarding tectonic plate deformation. By looking at strain rates as small as 10^-9 per year and combining these observations with numerical simulations, Yoshida identified a new, more practical upper bound for viscosity. His findings suggest that the limit is 10^28 Pascal-seconds, a figure significantly lower than previous estimates.

This identified maximum provides a clearer boundary between materials that demonstrate flow and those that maintain effective rigidity over geological timescales. While other experts note that this change in magnitude does not drastically alter current deformation models, it offers a necessary refinement to our understanding of Earth’s internal mechanics. This limit helps characterize how matter behaves under extreme pressure and temperature conditions within the Earth's crust.

Looking ahead, this work may provide clues about the early history of our planet. Approximately 4 billion years ago, the Earth was covered by a single, rigid lid known as the lithosphere. Plate motion only emerged after this period. Yoshida believes his findings on the limits of viscosity are closely tied to the transition from this early rigid state to the current era of plate tectonics, marking a step forward in geodynamic research.