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Volcanoes form when magma, which is molten rock from beneath the Earth's crust, reaches the surface. The magma erupts to form lava.

Volcanoes usually form along plate boundaries, where tectonic plates are either moving towards or away from one another:

• Divergent (or constructive) boundaries - this is where two plates move away from one another. Magma rises up to fill the gaps between.
• Convergent boundaries - this is where two plates move towards one another. The 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 creates magma, which rises to form an explosive volcano.

Volcanoes affect different places in different ways. They cause more damage in poorer countries, where there are fewer resources to predict and prepare for them.