Why do volcanologists monitor gases like sulfur dioxide?

Volcanoes release gases such as water vapor, carbon dioxide, and sulfur dioxide from magma and hot hydrothermal systems. A rise in gas output or a change in the gas mix can indicate that magma is moving upward, cracking surrounding rock, or decompressing. Because sulfur dioxide is easier to detect remotely, it is one of the main gases monitored at active volcanoes. Satellites and ground instruments now track emissions at volcanoes around the world, including large eruptions and long-lived gas plumes. Gas monitoring does not predict every eruption, but it is a key part of modern volcano surveillance.