The initial pulse of volcanism on the Ontong Java and Manihiki Plateaus began prior to 123–124 Ma and had largely ceased by about 122 Ma, whereas the intervening Pacific-Phoenix spreading ridge probably was disrupted between 120 and 115 Ma by formation of the Nova-Canton Trough rift system. Plateau formation may have resulted from a large plume head that rose beneath each of the two plateau sites. During the plateau eruption phase, these two plumes spread beyond the plateaus and within the asthenosphere. They eventually coalesced beneath the intervening spreading ridge, where the Nova-Canton Trough rift system is now located, at about the time volcanism ceased on the plateaus. The two plumes combined to form the Nova-Canton Trough rift system by reheating the nascent lithosphere near the ridge crest and widening the plate boundary from the usual 1–10 km to a much broader region. The present rift system is a maximum of 500 km wide, and much of this area may have formed by normal faulting and crustal accretion across the broadened plate boundary as plate divergence continued. Eventually, coherent spreading resumed somewhere south of the Nova-Canton Trough rift system.