How are the formation of volcanoes and plate boundaries related?

A volcano is usually formed by either a divergent plate boundary (two plates moving away from each other, such as the Mid Atlantic Ocean Rift Zone, which causes lava to flow to the surface from the mantle and make smaller volcanoes. In the case of the Hawaiian Islands (and many others), they were formed by a volcano brought about by a mantle plume. Basically, a formation of lava pushes to the crust, heats it, and forces its way through and deposits magma/lava on the surface. These volcanoes are usually harmless. But the predominant type of volcanoes (the really violent, nasty ones) are caused by convergent boundaries (two plates pushing together--perhaps an oceanic plate and continental plate). The oceanic plate, being more dense than the continental plate, is subducted underneath. Once it reaches a certain depth, the plate melts and the resulting magma pushes to the surface to form volcanoes, such as the Cascade Range Volcanoes along the west coast of North America. These volcanoes are particularly violent because they are not "free flowing" magma--it must push its way to the surface and usually ends in a violent eruption.

Since some volcanoes are made from plates, the boundaries definitely matter, if boundaries are big, then the volcano is most likely small.