is where a tectonic plate composed of relatively thin, dense oceanic lithosphere sinks into the Earth's mantle beneath a less dense overriding plate. The overriding plate may be either another oceanic plate or a continental plate. The subducting plate, or slab, sinks into the mantle at an angle, so that there is a wedge of mantle between the slab and the overriding plate. The boundary between the subducting plate and the overriding plate coincides with a deep and narrow oceanic trench. This trench is created by the gravitational pull of the relatively dense subducting plate pulling the leading edge of the plate downward. Multiple earthquakes occur within the subducting slab with the seismic hypocenters located at increasing depth under the island arc: these quakes define the Wadati–Benioff zones. The volcanic arc forms on the overriding plate over the point where the subducting plate reaches a depth of roughly and is a zone of volcanic activity between in width. The shape of a volcanic arc is typically convex towards the subducting plate. This is a consequence of the spherical geometry of the Earth. The subducting plate behaves like a flexible thin spherical shell, and such a shell be bent downwards by an angle of θ, without tearing or wrinkling, only on a circle whose radius is θ/2. This means that arcs where the subducting slab descends at a shallower angle will be more tightly curved. Prominent arcs whose slabs subduct at about 45 degrees, such as the Kuril Islands, the Aleutian Islands, and the Sunda Arc, have a radius of about 20 to 22 degrees. Volcanic arcs are divided into those in which the overriding plate is continental (Andean-type arcs) and those in which the overriding plate is oceanic (intraoceanic or primitive arcs). The crust beneath the arc is up to twice as thick as average continental or oceanic crust: The crust under Andean-type arcs is up to thick, while the crust under intraoceanic arcs is thick. Both shortening of the crust and magmatic underplating contribute to thickening of the crust. Volcanic arcs are characterized by explosive eruption of calc-alkaline magma, though young arcs sometimes erupt tholeiitic magma and a few arcs erupt alkaline magma. Calc-alkaline magma can be distinguished from tholeiitic magma, typical of mid-ocean ridges, by its higher aluminium and lower iron content and by its high content of large-ion lithophile elements, such as potassium, rubidium, caesium, strontium, or