How do volcanic gases help power eruptions and shape their danger?

Deep underground, magma contains gases dissolved under high pressure. As magma moves upward, pressure falls and the gases form bubbles, much like opening a carbonated drink. If the magma is sticky and traps gas, pressure can build until it fragments violently into ash and pumice. Water vapor is usually the most abundant volcanic gas, but carbon dioxide and sulfur dioxide are also important to monitor. Gas emissions can signal unrest and, in large eruptions, can affect climate by sending sulfur aerosols into the atmosphere.