Volcanic gases are gases given off by active (or, at times, by dormant) volcanoes. These include gases trapped in cavities (vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating from lava, from volcanic craters or vents. Volcanic gases can also be emitted through groundwater heated by volcanic action.
The sources of volcanic gases on Earth include:
* primordial and recycled constituents from the Earth's mantle,
* assimilated constituents from the Earth's crust,
* groundwater and the Earth's atmosphere.
Substances that may become gaseous or give off gases when heated are termed volatile substances.