Ar-Ar Age Determination of Petit-spot, a New Kind of Volcano

INTRODUCTION: Tectonic plates move around on the Earth's surface with volcanic eruptions on their margins (tectonic boundaries). Undersea volcanoes erupted on an oceanic plate provide much information about the chemistry and tectonic history of the Earth. The fourth kind of volcanoes on the Earth, following three volcanic kinds which we have already known as mid-oceanic ridges, volcanic arcs and hotspots, are newly recognized on the oceanic plate off NE Japan prior to their subduction, named "petit-spot" submarine volcanoes [1]. They occur in a region of the plate, which is susceptible to fracturing due to plate subduction. The magmas produced by petit-spots originate from immediately under the plate.