Volcanoes can form anywhere the Earth's crust allows magma to reach the surface. Typically, this occurs around plate boundaries, either where plates are pulling apart or where one is forcing its way under another. Weak spots can also develop away from plate edges, creating magma vents called hot spots.
When two plates pull away from one another, they create a gap that allows large amounts of magma to reach the surface. This commonly occurs in undersea faults, where the new magma forms rock on the ocean floor instead of building up into a traditional volcanic cone. When one plate slides under its neighbor, this can create a number of potential magma vents as rock liquefies and pressure builds. These areas are more likely to create volcanoes due to the increase in pressure and sudden shifts that may allow it to reach the surface.
Hot-spot volcanoes can occur anywhere that the Earth's crust develops a weak spot. One notable hot spot is the vent that is under the Hawaiian islands and feeding the volcanoes there. That hot spot has been responsible for a chain of volcanoes more than 3,000 miles long underneath the Pacific Ocean. Most of the volcanoes created in this chain never reached the surface, but many islands in the Pacific are the result of its movement.