According to the plate tectonic theory, there are three principal zones of volcanic activity.
Greatest volcanic extrusions at mid oceanic ridges where sea floor spreading is active. As the lithosphere moves apart, basaltic magma rise form the asthenosphere and fills the gap. Many successive eruptions, sometimes produces volcanic cone which may rise above sea level.
In convergent zones, oceanic crust is being suspected and melted. The andesitic magma thus generated moves upward and builds a volcanic arc adjacent to the trench. This chain of volcanoes subsequently develops into an island etc.
Since the continental rocks are less dense than the underlying material, they remain afloat for ever. This means that continental crust appears to be growing larger at the expense of oceanic crust.
Region within the Plates Themselves
Volcanism that occurs on continents away from plate boundaries is the most difficult to explain. Some of the volcanic activity that has occurred within a continent may be associated with a spreading center which is in the most initial stage of growing. The rift zone in Africa is considered to be the beginning stage of such a breakup.