The magma (molten rock) found in volcanoes contains dissolved gases. Magma deep beneath the earth's crust is under tremendous pressures. As the magma rises towards the surface there is a decrease in the pressure that the magma is under, and the gases will come out of solution and begin to form bubbles. The formation of these bubbles renders the magma less dense and so it rises more quickly. The gases contained in the magma are released into the atmosphere through volcanic activity in a variety of ways.
Most obviously, gases are released in volcanic eruptions. In major pyroclastic eruptions many millions of cubic feet of gas can be released high into the atmosphere. Effusive eruptions, where there is an outpouring of lava without an explosive eruption, also can release vast amounts of gases, although these will tend to stay in the lower atmosphere. A volcano does not need to erupt to emit gases. Many volcanic systems have permanent vents (fumaroles) which constantly emit gases. Associated hydrothermal systems such as hot springs and geysers are also continually emitting gases. Where magma systems are close to the surface, gases may just seep up through the soil.
Geological research has shown that the major gas emitted by volcanoes is water vapour,and that significant amounts of carbon dioxide and sulfur dioxide are also released. Although the water vapour itself is not environmentally threatening, it is its combination with other released gases that can cause problems, and these will be discussed later in the article. Volcanoes also release the gases hydrogen, hydrogen sulfide,carbon monoxide, hydrogen chloride, hydrogen fluoride, helium, methane and ammonia in much smaller amounts. The composition of the amounts of gases released will depend on the particular volcano, and geologists all over the world are taking continuous measurements to monitor these gases.
Of the gases released by volcanoes it is probably sulfur dioxide that can cause the most damage to the environment. A major component of acid rain, sulfur dioxide is a colourless gas that irritates the skin and attacks mucus membranes. Up to 10 million tonnes of sulfur dioxide a day, can be released by a major volcanic eruption. The sulfur dioxide is usually released as an aerosol, associated with released water vapour, in effect tiny droplets of sulfuric acid. If sulfur dioxide finds its way into the stratosphere, it helps to accelerate the chemical reactions that lead to ozone depletion. The eruption of Mount Pinatubo in June 1991, and consequent release of approximately 20 million tonnes of sulfur dioxide, resulted in some of the lowest ever observed ozone levels in the stratosphere. Sulfur dioxide aerosols lead to a warming of the stratosphere and associated cooling of the troposphere. Thus major volcanic eruptions can be correlated to a drop in the earth's average surface temperature after the event.
Carbon dioxide is well known for its affects as a greenhouse gas, and the release of large amounts due to volcanoes can only reinforce this. The localised release of high volumes of carbon dioxide can cause direct threats to human and animal life. As it is heavier than air it can accumulate in low lying areas. Air containing more than 30% carbon dioxide causes unconsciousness and death. Overall, approximately 130 million tonnes of carbon dioxide are released by the world's volcanic systems every year.
Of the other released gases non is very pleasant. Hydrogen sulfide, also known as sewer gas, has the smell of rotten eggs and is poisonous. An irritant in low concentrations, it can cause major problems in the respiratory system when concentrated. The dangers of carbon monoxide poisoning are often highlighted in the news, so little needs to be said of its effects here. Hydrogen chloride gas readily dissolves in water forming hydrochloric acid, so acid aerosols are quickly formed with erupted water vapour. Again this is another form of acid rain in the making. Hydrogen fluoride is a similar compound, but more commonly attaches to airborne particulates (ash) in eruptions. Particularly dangerous if it gets in watercourses or if the particulates coat vegetation, there have been many recorded cases of livestock death after its release in eruptions. Hydrogen can be explosive when mixed with air, methane is a worse greenhouse gas than carbon dioxide, and ammonia is also an irritant and caustic in solution. Of the gases mentioned in this article helium is the most benign, being inert and emitted in very small quantities.
This article was written with reference to the web pages of the United States Geological Survey, and the British Geological Survey. Much more detailed information can be found on both these sites, and the reader is encouraged to explore their pages.