Outer trench swell

The lithosphere is bent upwards by plate stresses, and is not in isostatic equilibrium (distinguish from the "outer ridge" of a forearc).

The bending of the plate is associated with tension in the upper 20 km, and shallow earthquakes, caused by tensional failure induced by the downward bending of the oceanic plate are common; about 20 extensional outer rise earthquakes with magnitude 5 or greater occur annually.

Faulting related to plate bending and stair-stepping of the descending slab into the trench may allow seawater to infiltrate deep into the crust and perhaps upper mantle.

The principal mechanism of frontal erosion may reflect combined effects of seamount tunneling, mass wasting and transport to the trench, deposition in a graben on the downgoing plate, and descent into the mantle.

Hirano et al., (2006) proposed that these small volcanoes erupted along lithospheric fractures in response to plate flexure during subduction.