Volcanic Gases | John Seach

Volcanoes produce 3 products. Tephra, Lava and Gases.
There are 5 major volcanis gases - SO2, H2S, HCl, H2O, CO2.

The cooling of magmatic gases ascending to the surface, is controlled by conductive heat transfer to country rocks, and convective heat transfer in the hydrothermal system of the volcano.


Properties and effects on humans



Carbon dioxide
Colourless, odourless gas. Dangerous because it may displace oxygen in topography depressions, leading to anoxia of people in the viscinity. Carbon dioxide release from a volcanic area has resulted in many deaths. Examples include Lake Monoun, Cameroon (1984), Lake Nyos, Cameroon, (1986), and Dieng, Indonesia (1992).



Sulphur dioxide
Colourless gas with pungent odour. Concentration of 6-12 ppm causes immediate irritation of nose and throat. People with asthma and other respiritary diseases can be severely affected by sulphur dioxide emissions. Examples of fatalities from SO2 emissions have occurred at Aso, Japan (1984, 1987), and Kilauea, Hawaii (1987).


Hydrogen sulphide
Colourless, flammable gas with pungent smell of rotten eggs. Exposure to 20-150 ppm causes eye irritation. Exposure to 1000-2000 ppm causes coma after a single breath. Hydrogen sulphide has been responsible for at least 10 separate fatal volcanic events in 20th century. These include Rotorua, New Zealand (1946, 1948, 1954, 1962, 1987).


Water vapour is the  main vapour emitted at volcanoes. It produces the characterisitc white plumes seen at active craters and fumaroles.


Hydrochloric acid
Colourless gas with an irritating, pungent odour. Detectible at 1-5 ppm. The vapour irritates the eyes severely, and may burn the skin. Concentrations of 35 ppm cause throat irritation after a brief exposure.


Helium is a noble gas, which forms from radioactive decay of uranium and thorium in the earth's crust. The 3He/  4He ratio is a good measure of the degree to which mantle gases contribute to volcanic gas discharges. Helium can be collected as a flank soil emission. Hotspots such as Hawaii have a large 3He/  4He ratio, and are tap pristine, ungassed parts of the mantle.


Radon is a colourless, odourless, radioactive gas formed by radioactive decay of unraium. Chronic exposure has been linked to respiritary disorders. Radon has a half-life of 3.82 days, and can be used to determine faults and fractures on a volcano, when derived from shallow sources.


Sulphuric acid
Sulphuric acid is a colourless, viscous liquid, which reacts vigorously with water. The acid irritates eyes and burns skin.


Hydrofluoric acid
Hydrofluoric acid is a colourless, fuming liquid, with a pungent smell. Irritation of eyes and nose occurs at 5 ppm. Concentrations of 50-250 ppm are dangerous for even a brief exposure. The 1785 eruption of Laki fissure in Iceland released hydrogen fluoride into the environment, destroying pastures, and a famine resulted which killed 10,521 people.