What kind of gases escape from volcanoes?
The fumes escaping from a volcano consist mostly of water vapor (steam). Steam may be emitted from the hot interiors of volcanoes even when they are dormant. But steaming usually increases dramatically as magma intrudes and heats groundwater beneath a volcano. Magma gives off carbon dioxide (CO2) and hydrogen sulfide (H2S, rotten egg gas) that do not totally dissolve in groundwater and can therefore show up at the surface. As water inside the volcano boils away, other more water-soluble volcanic gases can reach the surface, signaling an increasingly grave situation. These gases include sulfur dioxide (SO2) and common halogen gases such as hydrogen chloride (HCl), and hydrogen fluoride (HF).
