Volcanism in Response to

Volcanism on Earth is known to occur in three tectonic settings: divergent plate boundaries (such as mid-ocean ridges), convergent plate boundaries (such as island arcs), and hot spots. We report volcanism on the 135 million-year-old Pacific Plate not belonging to any of these categories. Small alkalic volcanoes form from small percent melts and originate in the asthenosphere, as implied by their trace element geochemistry and noble gas isotopic compositions. We propose that these small volcanoes erupt along lithospheric fractures in response to plate flexure during subduction. Minor extents of asthenospheric melting and the volcanoes ’ tectonic alignment and age progression in the direction opposite to that of plate motion provide evidence for the presence of a small percent melt in the asthenosphere. T he northwestern Pacific Plate is cur-rently subducting beneath the Kuril and Japan trenches (1). This plate dates to the Early Cretaceous E150 to 120 million years ago (Ma) (2) ^ and is dotted with seamounts and oceanic plateaus, interpreted as having formed during Cretaceous times (3). The oceanic lithosphere cools as it ages (4) and moves away from the divergent plate boundary. Without the presence of a hot spot, new volcanism is not anticipated. During a dive (10K#56 in Fig. 1B) via the remotely operated vehicle Kaiko, the sampling of a young alkali basalt E5.95 T 0.31 Ma (5) ^ near the Japan Trench was unexpected, because there are no known hot spots in the vicinity and recent volcanism has not been reported. In light of this finding, we decided to conduct extensive surveys of the area with th