HOW VOLCANOES FORM

Three types of plate boundaries have worked together to create the oceans and continents: divergent boundaries, convergent boundaries, and transform boundaries. 

Divergent boundaries: The crustal plates move partly because molten material wells up along some of their boundaries. This material pushes the plates apart, forming new crust. These plate boundaries are called divergent (spreading apart) boundaries and occur along mid-ocean ridges and continental rift zones. The Mid-Atlantic Ridge is one such boundary.
 
Convergent boundaries: At the opposite edges of the plates, the motion causes them to collide with other plates and either slide under them or ride up over them. These collisions cause volcanoes and earthquakes and create mountains. The Cascade Range in the northwestern U.S. is a volcanic mountain chain caused by subduction activity. 
 

Transform boundaries: The third type of boundary, a transform boundary, occurs when the edges of plates slide against each other. Earthquakes are also common along transform boundaries such as the San Andreas Fault in California.