Do an image search for "subduction zone", you will get some images that show you what is going on, schematically in cross section. A simplified summary: New oc

Do an image search for "subduction zone", you will get some images that show you what is going on, schematically in cross section. A simplified summary: New oceanic crust is created at mid ocean ridges, but since the earth is not expanding, when you create new crust, subduction must occur elsewhere. For instance, new crust is created in the pacific, and subduction is occurring beneath Japan in the east and the west coast of north america in the west. Once the oceanic crust subducts far enough into the mantle, minerals that make the oceanic crust change to more dense forms in response to the increased pressure, (basalt to eclogite transition) and the resulting rock is more dense than the surrounding mantle, meaning it wants to sink further and further, providing a "slab pull" component to subduction (as opposed to "ridge push" due to new crust formation at the mid ocean ridge). Oceanic crust will almost universally subduct beneath continental crust due to density (~2.3 g/cc for continental, 2.7 for oceanic, >3 for mantle). Continental crust is too low density to subduct into the mantle, so you make mountain ranges like the Himalaya when continents colide. After new oceanic crust is erupted at mid ocean ridges, it remains hot, and experiences sustained interaction with seawater forming new minerals with structural water/OH- (i.e. chlorite), as well as accumulating a veneer of sediment that includes clay minerals also with structural water. When that oceanic crust then subducts back into the mantle, it heats up, the hydrous minerals are no longer stable, and they release their H2O into the overlying mantle. This addition of water lowers the melting point of the mantle such that it will melt under the ambient temperature conditions, this melting is ultimately manifest as volcanic arcs, such as that that extends from the andes, north through central america, mexico, the cascades, the aleutians , kuriles, japan, philippines, indonesia, new guinea, solomons, fiji, down to new zealand. These volcanoes are characterised by high H2O contents, leading to explosive eruptions, in contrast to the style of eruption seen in places like Hawaii. Not all the water (structural OH-) is released when from the subducting slab, and makes its way deeper into the mantle, and aspects of the ultimate fate of this is the subject of the article.