Most volcanoes occur on plate boundaries.
Plate boundaries are areas where Earth's shifting plates meet or split apart, usually with violent results. California's Mt. Shasta is a stratovolcano. Plate margins that are coming together are called convergent margins, while those that are splitting apart are called divergent. Volcanoes can occur on convergent or divergent plate margins or over a hotspot, a spot inside the mantle that heats areas of the plate above it. Colliding Plates Along convergent margins, two plates meet and one descends beneath the other, a process called subduction.