Why do volcanoes erupt?

Why and how do volcanoes actually erupt?

Volcanoes erupt when molten rock called magma rises to the surface. Magma is formed when the earth's mantle melts.

Melting may happen where tectonic plates are pulling apart or where one plate is pushed down under another.

Magma is lighter than rock so rises towards the Earth's surface. As the magma rises, bubbles of gas form inside it.

Runny magma erupts through openings or vents in the earth's crust before flowing onto its surface as lava.

If magma is thick, gas bubbles cannot easily escape and pressure builds up as the magma rises.

When the pressure is too much an explosive eruption can happen, which can be dangerous and destructive.

Another way an eruption happens is when water underneath the surface interacts with hot magma and creates steam,

this can build up enough pressure to cause an explosion.