What causes volcanoes to erupt with such explosive force?

Volcanic eruptions are driven primarily by dissolved gases in magma, particularly water vapor, carbon dioxide, and sulfur dioxide. As magma rises toward the surface, pressure decreases, causing these gases to expand rapidly, much like opening a carbonated beverage. The explosivity depends largely on magma viscosity. Thick, silica-rich magma traps gases and builds enormous pressure before releasing catastrophically, producing violent eruptions. Thin, low-silica magma allows gases to escape more gently, resulting in relatively calm lava flows. The sudden decompression can fragment magma into ash, pumice, and pyroclastic material. Additional factors include the introduction of groundwater or seawater into the magma chamber, which flashes to steam and dramatically amplifies explosive force, as seen in phreatomagmatic eruptions.