During subduction, a plate's leading edge is forced to squeeze into a smaller total width than it occupied at the surface, yet the plate resists change of dimension in its own plane. A possible outcome is that the plate buckles as it descends: at a depth of 500 km, the amplitude of buckling could be as much as one-tenth the wavelength, and dip angles for a plate descending at 45° on average could range from 35° to 55°. Observed plate geometries, though not ideally regular, are compatible with these suggestions.
Before being subducted a lithosphere plate is convex upward at all points, whereas, to buckle, it must become convex downward in a series of plunging synclines. The “pop-through” is most likely to occur after the leading edge has descended about 100 km. At the moment of pop-through, the subduction hinge line changes from being smooth to being a series of arcs and cusps; back-arc spreading associated with the change can be 150 km or more in the arc embayments but should be zero opposite the cusps, where a compressive regime is sustained. In the pop-through event, elastic strain energy is released, and the associated back-arc spreading should be a comparatively sudden event, on a geologic time scale.