Why do some volcanoes erupt explosively?

The key to how a volcano erupts is the composition of its magma, especially how sticky it is and how much dissolved gas it holds. Thick, silica-rich magma is very viscous and traps bubbles of gas as pressure builds, much like shaking a bottle of soda with the cap on. When that pressure is finally released, the magma shatters and blasts out as ash, rock, and superheated gas in a violent explosion. In contrast, thin, runny basaltic magma lets gas escape easily, so it tends to erupt as gentle flows of lava, as seen in Hawaii. Subduction-zone volcanoes often produce the most explosive eruptions because their magma is both viscous and gas-rich.