Grain Scale Physics of Plate Boundaries: Tectonic Processes from Geological to Human Time Scales

 The motion of tectonic plates along our planet’s surface shapes the relationship between the solid Earth and its surrounding elements, including the atmosphere, ocean, and life. 

Examples include the chemical reactions between minerals and water at seafloor spreading centers, as well as volcanic degassing at subduction zones, both of which link plate tectonics to the global volatile cycles. Furthermore, volcanism and seismicity along plate boundaries have a clear impact on human life. 

However, Earth is enigmatic in that it is the only known terrestrial body that has plate tectonics. Understanding how plates and plate boundaries form and evolve is fundamental to our understanding of the Earth system as a whole. 

In order for a new tectonic plate to form, the cold and stiff oceanic lithosphere must be weakened sufficiently to deform at tectonic rates. The weakening mechanisms involve the microscale physics of mineral grains and their control on the strength of the lithosphere. 

In this talk, I will present the microphysics of lithospheric weakening by mineral grain size reduction, known as grain damage, and its application to tectonic scale processes, such as subduction initiation. 

I will also present the newly developed theory of grain mechanics, which couples evolution of grain size and intragranular defects. The new model predicts oscillations in grain size, and possibly material strength, on a time scale that is relevant to earthquake cycles and postseismic recovery, thus connecting plate boundary formation processes to the human time scale.