Volcanic gas is released from magma as it rises toward the surface and during eruptions. Just as when the cap is removed from a
pressurized bottle of soda and dissolved gas forms bubbles, magmas become bubbly as they rise to the surface and depressurize.
The most common gas released by magma is steam (H2O), followed by CO2 (carbon dioxide), SO2
(sulfur dioxide), (HCl) hydrogen chloride and other compounds.
Facts and figures: Because volcanic gas forms acid when it dissolves in ground water, it tends to attack most rocks, breaking them down into weak clay materials. The interiors of volcanoes often become seriously weakened by this hydrothermal alteration, which makes them susceptible to sudden landslides. Volcanic gas is one of the clues that scientists use to detect rising magma and a potential eruption. Volcanic gas does not necessarily leave the volcano through the central vent. It also escapes through cracks or vents called fumaroles.
Volcanic gas released from Bocca Nuova, Etna driven away from observer by strong wind. Foto: J. Alean