The most abundant gas released into the atmosphere from volcanic systems is water vapour (H₂O), between 70 and 90% of the total mass, followed by carbon dioxide (CO₂) and sulfur dioxide (SO₂).

Other gases released in small amounts are hydrogen sulfide (H₂S), hydrogen (H₂), carbon monoxide (CO), hydrogen chloride (HCl), hydrogen fluoride (HF) and helium (He).

Sulfur dioxide (SO₂), carbon dioxide (CO₂) and hydrogen fluoride (HF) pose the greatest potential hazard to people, animals, agriculture and properties. Locally, SO₂ gas can lead to acid rain and air pollution downwind from a volcano. Globally, large explosive reactions produce sulfur aerosols, which can become oxidised within weeks. This leads to the formation acids in the atmosphere, as given by the following equation:

SO₂ + OH +3H₂O -------> H₂SO₄ + HO₂

This promotes depletion of the Earth's ozone layer, and can also lead to atmospheric cooling.

Carbon dioxide gas is heavier in air, so it tends to flow into low-lying areas and collect in the soil. The concentration of carbon dioxide gas in these areas can be lethal to people, animals and vegetation.

A few historic eruptions have released sufficient fluorine-compounds to deform or kill animals that grazed on vegetation coated with volcanic ash; fluorine compounds tend to become concentrated on fine-grained ash particles, which can be ingested by animals.

To learn more about these gases, click here:

Sulfur Dioxide

Carbon Dioxide

Other gases are also released...see secondary gas emissions

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