Volcanic gas

Sulfur dioxide and other volcanic gases rise from the Pu`u `O`o vent on Kilauea Volcano, Hawai`i.

Magma contains dissolved gases that are released into the atmosphere during eruptions called Volcanic gases. Gases are also released from magma that either remains below ground (for example, as an intrusion) or rises toward the surface. In such cases, gases may escape continuously into the atmosphere from the soil, volcanic vents, fumaroles, and hydrothermal systems. The most common gas released by magma is steam ($H_{2}O$), followed by $CO_{2}$ (carbon dioxide), $SO_{2}$ (sulfur dioxide), (HCl) hydrogen chloride and other compounds.

Sources

USGS Photo Glossary