The subduction of dense oceanic crust into the mantle plays an essential role in driving plate tectonics. However, the details of how new subduction zones form are poorly understood. In this model a section of buoyant oceanic crust arrives at a subduction trench and blocks it, causing the already-subducted slab to break off, and halting subduction. Then subduction starts in the opposite direction. The model shows not just a reversal, but how a new subduction can start, which is a matter of much