Volcanoes are mountains with melted rock erupting out at a certain time. Most of the famous volcanoes are in the ring of fire. The ring of fire is a line of  active volcanoes around the pacific ocean. A volcano has four things it needs to use erupt. First of all, it needs a big magma chamber. Then it needs a sill. That's like a mini magma chamber. Next it needs a vent for the magma to travel out of the volcano. Lastly, it needs a crater that is attached to the vent which lets the magma come out. When the magma is out, it is called lava.