Remnant arcs are the submarine ridges which lie behind active island-arc systems. In simple cases, these ridges are bounded by scarp systems on both flanks, have volcaniclastic aprons on the rear flank, and are similar in composition to frontal arcs. Simple remnant arcs form as the rifted remnants of frontal arcs and are left behind as inter-arc basins widen. After creation, remnant arcs subside several kilometers. The simple morphology of some remnant arcs is obscured by sedimentation, by isostatic adjustment, and by tectonic reactivation. Occasionally, the polarity of an active arc system reverses, and, on the rear flank of the frontal arc, a new trench forms which consumes marginal basin crust. This reversed configuration results in arc-arc collisions and more complex forms of remnant arcs. The collision of a reversed arc with remnant arcs or continental margins is a likely explanation for the thrusting of oceanic crust over continental crust. Active and remnant arcs in which these processes are now taking place are located between New Guinea and the Solomon Islands. The oceanic crust on the leading edge of the New Britain ridge (reversed frontal arc) is colliding with New Guinea.