Editing Oceanic trench (section)

===Mantle interactions===

Interactions with the [[mantle (geology)|mantle]] discontinuities play a significant role in slab rollback. Stagnation at the 660-km discontinuity causes retrograde slab motion due to the suction forces acting at the surface.{{sfn|Nakakuki|Mura|2013}} Slab rollback induces mantle return flow, which causes extension from the [[shear stress]]es at the base of the overriding plate. As slab rollback velocities increase, circular mantle flow velocities also increase, accelerating extension rates.{{sfn|Schellart|Moresi|2013}} Extension rates are altered when the slab interacts with the discontinuities within the mantle at 410&nbsp;km and 660&nbsp;km depth. Slabs can either penetrate directly into the [[lower mantle (Earth)|lower mantle]], or can be retarded due to the phase transition at 660&nbsp;km depth creating a difference in buoyancy. An increase in retrograde trench migration (slab rollback) (2–4&nbsp;cm/yr) is a result of flattened slabs at the 660-km discontinuity where the slab does not penetrate into the lower mantle.{{sfn|Christensen|1996}} This is the case for the Japan, Java and Izu–Bonin trenches. These flattened slabs are only temporarily arrested in the transition zone. The subsequent displacement into the lower mantle is caused by slab pull forces, or the destabilization of the slab from warming and broadening due to thermal diffusion. Slabs that penetrate directly into the lower mantle result in slower slab rollback rates (~1–3&nbsp;cm/yr) such as the Mariana arc, Tonga arcs.{{sfn|Christensen|1996}}

[[Image:Atlantic-trench.JPG|thumb|upright=1.4|The [[Puerto Rico Trench]]]]