Volcanic gases

Magma deep in the earth contains dissolved gases. As the magma rises closer to the ground surface, these gases are released and, because they are so mobile when compared to the sluggish liquid magma, they rise to the surface and are discharged through vents, fumaroles, and the soil. The gas temperatures, absolute amounts, and relative proportions of different gases give information on the state of the magmatic system. There are many types of volcanic gases, with the most common being water vapour (H2O); sulphur as sulphur dioxide (SO2) or hydrogen sulfide (H2S); nitrogen, argon, helium, methane, carbon monoxide and hydrogen. Many of these are poisonous or cause asphyxiation.