Japan has stratovolcanoes near the subduction zones of the tectonic plates. These processes occur primarily at magmatic arcs associated with subduction. The Cenozoic rocks represent offshore marine limestones deposited before subduction of the Tethys oceanic crust. Volcanism on Santorini is due to the Hellenic Trench subduction zone southwest of Crete. This development is presumed to have been linked to the subduction of cold oceanic lithosphere. Oceanic trenches are narrow topographic lows that mark convergent boundaries or subduction zones. Alkali basalts erupted after the cessation of subduction. This is caused by the addition of seawater into magmas formed at subduction zones. Downward motion of material occurs at convergent plate boundaries called subduction zones. Along the continental margin, volcanoes grew, coincident with the initiation of subduction. Diorite results from the partial melting of a mafic rock above a subduction zone. These islands arise from volcanoes where the subduction of one plate under another is occurring. Water in the mantle is responsible for the melt that produces volcanoes at subduction zones. The typical stratocone is an andesitic to dacitic volcano that is associated with subduction zones. Areas adjacent to the subduction zones have experienced large seismic activity.