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Volcanoes form when magma, molten rock from the mantle, reaches the Earth's surface. The magma erupts to form lava at the surface, either on land or under the sea.
Volcanoes usually form along plate margins, where crustal plates are either moving towards or away from one another.
Constructive margin - this is where two plates move away from one another. Magma rises to fill the gap between.
Destructive margin - this is where two plates move towards one another. Heavy oceanic crust sinks beneath the continental crust at a subduction zone - a point where one crustal plate is forced beneath another. As the oceanic crust sinks into the mantle, it melts into magma, which rises to the Earth's surface to form an explosive volcano.