The morphology of the Wadati-Benioff zone in the Tonga-Kermadec island arc verified the existence of an intermediate aseismic gap in the recent subduction zone and its relation to active andesitic volcanism. The gap, interpreted as a partially melted zone, appears to be the source of primary magma for andesitic volcanoes. A clear correlation between the depth of penetration of the Wadati-Benioff zone and the major physiographic units of the ocean floor implies a nonuniform rate of subduction along the Tonga-Kermadec trench due to the hampering effect of the main structural features of the subducting Pacific plate.