Why do volcanoes erupt?
The short answer
As magma rises, pressure drops and dissolved gases expand. The expanding gas can push magma through cracks to the surface.
How it works
Magma contains gases such as water vapor and carbon dioxide. At depth, pressure keeps them dissolved, but near the surface they form expanding bubbles. Viscous magma can trap gas and build pressure.
A familiar example
Opening a fizzy drink is a small analogy: lowering pressure lets dissolved gas form expanding bubbles.