A volcano is usually a mountain with an opening in the top. Molten rock, steam and ash burst out when it erupts.
The name volcano comes from the roman God of fire, 'Vulcan'. An eruption is caused by molten rock heating up.
This makes it grow bigger in size. Now it has expanded, it needs more room and moves in any empty spaces.
When these spaces are blocked the molten rock has nowhere to go.
It burst out through the weak part of the mountain.
It usually explodes rocks through the mountain and steam and gas
made by heat. An eruption is are very spectacular sight, but also
very dangerous. Some eruption have blown mountains apart.