What causes volcanoes to erupt suddenly and violently?

Violent volcanic eruptions are primarily driven by dissolved gases trapped in magma, particularly water vapor, carbon dioxide, and sulfur dioxide. As magma rises toward the surface, pressure decreases, causing these gases to rapidly expand and exsolve, much like opening a carbonated drink. If the magma is highly viscous, such as silica-rich rhyolite, gases cannot escape gradually and pressure builds until it releases explosively. Additional triggers include the sudden introduction of water, either from groundwater or seawater contacting hot magma, causing phreatomagmatic explosions. Structural collapses of a volcano's summit can also rapidly depressurize the magma chamber below, as happened at Mount St. Helens in 1980. The combination of gas content, magma viscosity, and external factors determines eruption intensity.