Well, the location of a volcano is dependent on several factors:

At subduction zones, the point at which a continental plate and oceanic plate collide, there may be the formation of a volcano of which viscuos/acidic lava is released. When these 2 plates collide, the dense material on which the oceanic plate lies[Sima], sinks hence the plate bends and eventually resubmerges into the mantle. At places where this occurs, a plume of magma is then allowed to rise up due to weak points in the crust,hence a volcano forms near to the edges of the continent, or in some cases,underwater. [check Peru-chile sea trench]

Similiarly though, it is possible for the magma within the mantle to simply travel through weak points in the crust and emerge from the continents when there is very hot magma between the margins of the crust and the mantle. These are called HOT SPOTS. [check the Hawaiian Islands]

Hope this helps...XOXOMrs.MathersXOXO