In the 1960s, plate tectonics, a model of the Earth based on the notion that the surface of our planet is paved with a mosaic of rigid plates that move large distances of geologic time, revolutionized the geological sciences and explained the distribution of dangerous earthquakes and hazardous volcanoes. The societal relevance of plate tectonics is hard to overestimate. Yet even then, there were hints that there are regions affected by diffuse deformation, where the rigid plate model does not entirely apply. Just how and why diffuse deformation occurs is not well understood, and a thorough analysis is long overdue. The project will address that problem, in a systematic way, for the first time by quantifying the rheology and dynamics of diffuse plate boundaries using thin viscous sheet models. The chief broader impact of this work lies in its implications for our understanding the working of our planet at the most fundamental level. The work also has more specific implications for improving hazards due to earthquakes and volcanoes.