<P> The reason for the existence of convergent boundaries are the mid-ocean ridges that were only beginning to be understood with the acceptance of the theory of plate tectonics in the mid-1960s . The radioactive decay of elements within the rocks in the interior of the earth generates large amounts of heat that moves magma to the surface of the crust in a process called mantle convection . This results in a new oceanic crust or sea floor being formed that slowly moves away from the ridges (sometimes called spreading centers) towards the subduction zones . In other words, new seafloor originates from the upwelling rock as it emerges from the depths of the mantle and it slowly cools as it is transported over the millennia to the collision or subduction zones where it is consumed or recycled . </P> <P> Large, shallow earthquakes on subduction zone interfaces are the largest types of earthquakes . They accounted for 90% of the total seismic moment released for the period 1900--1989, but these events usually occur offshore and far from population centers . Non-subduction interface earthquakes also occur in the region of the trench including shallow upper plate shocks, deeper events within the downgoing slab, and events that are related to outer trench swell . </P> <P> A volcanic arc is formed on the continental plate, above the location of the downgoing oceanic slab . The volcanic arc is the surface expression of the magma that is generated by hydrous melting of the mantle above the downgoing slab . Hydrated minerals (e.g., phlogopite, lawsonite, amphibole) within the oceanic lithosphere become unstable at certain depths due to increased temperature and pressure, causing the crystal structure of the hydrated minerals break down and release water . The buoyant fluids then rise into the asthenosphere, where they lower the melting temperature of the mantle and cause partial melting . These melts rise to the surface and are the source of some of the most explosive volcanism on Earth because of their high volumes of extremely pressurized gases . </P> <P> When two plates with oceanic crust converge, they typically create an island arc as one plate is subducted below the other . The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it . The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight - edge of the knife). A deep oceanic trench is located in front of such arcs where the descending slab dips downward, such as the Mariana Trench near the Mariana Islands . </P>

Where do volcanoes form near convergent boundaries why