What makes a volcano erupt?

The answer

Volcanic eruptions occur when magma from the mantle rises through the crust because it is less dense than the surrounding rock. As magma ascends, dissolved gases (mainly water vapor and carbon dioxide) come out of solution and form expanding bubbles, building tremendous pressure. When this pressure exceeds the strength of the overlying rock, the volcano erupts. The viscosity of the magma determines whether the eruption is gentle and effusive (like Hawaii) or explosive and catastrophic (like Mount St. Helens).