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. A

tmospheric 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.