The "volcanic" gases

The "volcanic" gases are volatile compounds released by volcanic processes. They are improperly called so because they are usually a mixture of different gases that are not specific to volcanoes (they can occur naturally due to other processes). The main components are as follows (in descending order of average concentration):

• water (H₂O) is the most abundant component.
• carbon dioxide (CO₂), is a colorless, odorless gas (at standard temperature and pressure). This is one of the gases responsible for the "greenhouse effect" that keeps the temperature of the planet to an acceptable level for living organisms. CO₂ if dissolved in water, forms carbonic acid (H₂CO₃), making it more acidic. This is the component used to make fizzy drinks. It is used by plants for growth (photosynthesis). CO₂ is dangerous when the concentration is too high (over 5%). Denser than air, it can create pockets of gas that are death traps (see e.g. the Lwi lake, better known as the "Nyos" lake, Cameroon).
• sulfur dioxide (SO₂) is a colorless, dense and highly toxic gaz. At low concentrations, it is used as a disinfectant and preservative in food under the code E220 (in wine, dried fruits, meat ...). Oxidation gives sulfuric acid (H₂SO₄) and sulfurous acid (H₂SO₃ ). Before regulations to reduce its concentration in the air, he was responsible for "acid rain". In the air, it is irritating. The alert threshold is set at 300 mg / m ³ (300 micrograms per cubic meter). WHO recommends the 24-hour average concentration to be less than 0.5 ppm (parts per million). If the gas is injected into the stratosphere, it can lead to a decrease (reversible) of the temperature of the Earth.
• carbon monoxide (CO) is a colorless, odorless gas (at standard temperature and pressure) and very toxic. It is well known as responsible of domestic accidents (due to poor combustion).
• hydrogen sulfide (H₂S) is a highly toxic gas, flammable, with strong odor of rotten eggs. At low concentrations, it irritates the eyes and at higher concentration the upper airway. The attack of the olfactory nerve is insidious because it makes gas detection impossible. Other effects are headaches leading to unconsciousness. It may cause pulmonary edema after prolonged exposure.
• hydrogen chloride (HCl) is an irritant gas. It can cause acid rain when mixing with water (HCl is a strong acid).
• hydrogen fluoride (HF) is a colorless toxic gaz. In the presence of water, it forms hydrofluoric acid, highly corrosive. The exposure can cause conjunctivitis with corneal destruction. It also causes skin irritation. Although a small amount is beneficial, a high concentration of fluoride causes bone degeneration. Fluorine (F₂) is it a pale yellow gas.

Other gases are hydrogen (H₂) and helium (He). Gas is one of the main engines of eruptions. The example that follows is adapted from the USGS (and based onSparks et al ^[1]). Suppose we follow a block of one cubic meter of rhyolite at the temperature of 900 º C under high pressure (due to depth) containing only 5% dissolved water (by weight). No we bring it abruptly to the surface, so at atmospheric pressure. The magma cannot contain the water anymore, which forms bubbles, and this leads to a strong increase of the block size. The block then occupies a volume of 670 m³ . The length of the edges of the cube is multiplied by 8.75. In extreme cases, it forms a "foam", which, even when cooled, has a lower density than water and therefore floats . A typical example is pumice . This mechanism is - relatively speaking - similar to what happens when opening some bottles of soda or Champagne. The pressure drop also causes the expansion of bubbles, escaping violently.

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[1] Sparks, R.S.J., Bursik, M.I., Carey, S.N., Gilbert, J.S., Glaze, L.S., Sigurdsson, H., and Woods, A.W., 1997, Volcanic Plumes: John Wiley & Sons, Inc., England, 574 p.