Gases & Vog

Volcanic gases

Volcanic gases are dissolved in magma and released as the magma rises and eventually reaches the surface during an eruption. They are comprised mainly of water vapour (H2O), carbon dioxide (CO2), sulfur dioxide (SO2) and hydrogen sulfide (H2S), with minor but variable contributions from hydrogen fluoride (HF), hydrogen chloride (HCl) and other species. Where lava flows into the ocean, more substantial amounts of HCl can be present from the boiling of seawater.


The term vog (a combination of 'volcanic' and 'smog') refers to air pollution caused by the persistent SO2 emissions from Kīlauea volcano. As SO2 is released it reacts in the atmosphere with oxygen, sunlight, moisture, and other gases and particles and, within hours to days, converts to very fine particles which are mostly comprised of sulfuric acid (H2SO4) and which scatter sunlight, causing visible haze. Similar phenomena have adversely affected people near Nyiragongo (DR Congo), Masaya (Nicaragua), Poas (Costa Rica), and Miyakejima, Aso, and Sakurajima (Japan), as well as other locations. Information relating to the health hazards of volcanic gases and vog is available from the International Volcanic Health Hazard Network (IVHHN). See Hawaii Interagency Vog Information Dashboard to learn more about vog.

Interaction between gases and ash

Freshly fallen ash grains commonly have surface coatings of soluble components (salts) and/or moisture. These components can make ash mildly corrosive and potentially conductive. The soluble coatings are derived from the interactions in an eruption column between ash particles and volcanic gas, and may be composed of sulphuric and hydrochloric acid droplets with absorbed halide, sulfate and fluoride salts. The abundance of soluble components on ash surfaces varies greatly between eruptions of similar size and volume. An internationally ratified protocol for analysis of soluble components on ash is available in the Scientific Protocols section of IVHHN.

See Also:
Case Studies > Kīlauea > Kīlauea gas