Final answer:
Volcanoes can form when an oceanic plate collides with a continental plate at a subduction zone, due to the denser oceanic plate being forced under and melting into magma.
Step-by-step explanation:
When an oceanic plate collides with a continental plate at a subduction zone, volcanoes can form. This is because the oceanic plate, being denser, gets forced under the continental plate into the mantle where it starts to melt. This process generates magma which rises to the Earth's surface, forming volcanoes. This is a fundamental process in plate tectonics, shaping the earth's landscape over millions of years.
Learn more about Plate Tectonics