Abstract: Abstract: The advent of plate tectonics theory 50 years ago has revolutionized Earth Science thinking and provided a solid framework for understanding how the Earth works. This theory explains in simple clarity that all the geological processes are ultimately consequences of Earths cooling. This is well manifested by the origin of oceanic plates at ocean ridges, the movement and thickening of these plates, and their ultimate consumption back into the Earths deep interior through subduction zones, which provides an efficient mechanism to cool the Earths mantle, leading to large-scale mantle convection. That is, the immediate driving force for plate tectonics is the sinking of the cold and dense oceanic lithosphere, under gravity, into the deep mantle through subduction zones. Hence, there would be no plate tectonics if there were no subduction zones, but exactly how a subduction zone begins remains speculative. Studies on subduction initiation have been many and continue to this day by means of numerical simulation and geological inferences, culminating with three IODP expeditions (350, 351 and 352 in 2014) in the western Pacific to test the ideas of spontaneous and induced subduction initiation. All these efforts are welcome, but the ideas are not testable hypothesis. In this paper, we explain the only hypothesis that is geologically testable on subduction initiation, i.e., subduction initiation is a consequence of lateral compositional buoyancy contrast within the lithosphere. Such large buoyancy contrast is located at edges of oceanic plateaus in ocean basins and at passive continental margins globally. Because back-arc basins result from seafloor subduction, all the island arcs must have continental (or oceanic plateau) basement. Hence, this hypothesis can be effectively tested by sampling and studying island arc basement rocks that are exposed on landward trench slopes as the result of serpentinite diapirism.

Key words: completion of plate tectonics theory, subduction initiation, passive continental margins, oceanic plateau edges, island arc basement rocks