What causes volcanoes to erupt?

Deep underground, there is a region hot enough to melt rock.

At times, some of this melted rock flows toward the surface and collects in large reservoirs.

Atmospheric pressure near the Earth's surface is less than deep underground; therefore, gases inside the melted rock expand when it nears the surface. This expansion propels the melted rock through a volcano.

Once the melted rock erupts, it is called lava.