Volcanic gasses are something that most people simply don't think about when they see an eruption or hear about a volcano. These gasses are the driving force for all eruptions, as well as one of their many dangerous aspects. This article will deal with the types of gasses that are present in magmas, how they are dangerous to humans and animals, and how they form within the earth. It is important to note that without a proper understanding of the gasses involved in volcanism, it is impossible to understand the subject of volcanology at all!
The first thing I’d like to do is cover the most common types of volcanic gasses and how they are dangerous to nearby peeople and populations (nearly every type of gas can be present during an eruption, but most are in very small quantities).
Water Vapour (H2O)Water is by far the most common gas in magma and lavas, but is not directly hazardous to humans. Water content, however, can intensely affect the explosivity of an eruption and can bind with other gasses to become acids.
Carbon Dioxide (CO2)Colorless and odorless, this gas is difficult to detect. Generally harmless unless in large quantities, carbon dioxide only becomes a problem in low lying areas such as valleys and house basements (gas may seep through the ground in volcanic regions) where high quantities can accumulate without significant disturbance. The eruption at Lake Nyos, in Cameroon, is a perfect example of what can go horribly wrong (see the Limnic Eruptions article). This is also a huge problem in Long Valley, CA, right now.
Sulfur Dioxide (SO2)The most detectable of all volcanic gasses, SO2 plays a vital role in gas studies at virtually all monitored volcanoes. The reason for this isn’t that it is the most abundant, but that it is easier to identify and measure. This is partly because equipment designed to detect sulfur dioxide have already been created to monitor smoke-stacks and industrial pollution. Consequently, we can use existing equipment to aid in our research. At more than 20 ppm, it irritates the eyes with a hard-to-ignore burning sensation and is dangerous to breathe. When inhaled, most SO2 is converted to sulfuric acid within the upper respiratory tract. Long-term exposure can damage the lungs as well as concrete, metal, cloth, and vegetation. SO2 is an important greenhouse gas.
Hydrogen Sulfide (H2S)Though colorless, this gas is easy to detect by its offensive odor (smells like rotten eggs). People are generally able to notice the odor at 0.77 ppm while it only becomes dangerous at 50 ppm (and that’s for long-term exposure). Most people have experienced the smell of hydrogen sulfide at Yellowstone National Park, where the volcanic geysers and hotsprings provide a noxious odor to the area. In large quantities, this gas can be deadly; in 1971, 6 skiers died on Kusatsu-Shirana volcano in Japan when they passed through a depression that was filled with this gas.
Radon (Rn)This noble gas is colorless, odorless, tasteless, and radioactive, making it a real hazard to humans, although no studies on the short-term effects have been performed. Long term effects include respiratory disorders and cancer. This is mostly a problem in low lying, undisturbed areas, such as house basements.
Hydrochloric Acid (HCl)Colorless, but with a rather “acidic” odor and taste, this gas is common around fumaroles and in eruptions. Just like the liquid form, this airborne acid will burn anything it touches (though it’s usually diffuse)- especially the respiratory tract. This acid is most common in acid rain and LAZE.
Hydrofluoric Acid (HF)In addition to being colorless, this acid is also transparent which means it cannot be visually detected. It does give a very strong and unpleasant odor and irritates the eyes and nose in small quantities. This acid is known for being especially nasty: contact with the eyes will cause deep burns and permanent blindness if not flushed with water. Skin exposure results in long-lasting burns and, if the acid is absorbed into the skin, can damage the underlying tissue and result in gangrene. Besides burning the entire respiratory tract, exposure can result in ulcers along the upper respiratory tract. In large quantities or over long periods of time, the acid can be absorbed into the bones and result in Fluorosis: a condition which is characterized by weight loss, degeneration of the bones, and anemia. This acid’s devastating effects on Iceland during the 1783 eruption of Laki fissure can be found in the Historic Volcanic Disasters article on the MIVO main page.
Sulfuric Acid (H2SO4)A range of shades of brown and odorless, exposure results in quick burns and dissolving of the outer layers of the teeth, as well as respiratory problems. With prolonged exposure, these symptoms can become permanent. It’s most common influence, however, is acid rain and VOG.
As should be obvious, all magma comes from the mantle at some point. This too, subsequently, is the source for most volcanic gasses. Most mantle-derived gasses are remnants from the Earth’s early history. Back in the day, when Earth was a spherical blob of congealing molten rock, most of our atmosphere was being degassed from the magma oceans. The crust hardened over these oceans, however, before they could become fully degassed (after all, that was quite a bit of magma). Remnants of those dissolved gasses are brought up through volcanoes. The most abundant one is water, though sulfur is also important (it is very reactive and so appears as various gasses and minerals upon reaching the surface). Also contributing water along convergent margins, however, are subducted oceanic slabs, which carry water between mineral grains and inside mineral structures (usually as lawsonite, chloritoids, and amphiboles). Finally, a very important contribution to the water vapour presence are meteoric waters (meteoric meaning originally from the sky). These are heated and moved in hydrothermal systems, and can have significant effects at the surface and on the eruption itself.
Hydrochloric acid, or simply chlorine (there is a difference, but we’ll ignore it) comes in from the subducting oceanic slab as well. Since the slab contains salinated water, salt is incorporated into the magma. Chemical reactions reduce the NaCl (chemical formula for salt) to simply Na and Cl. The chlorine then reacts with water molecules to form HCl, or hydrochloric acid. The formula, very simply put, is thus: NaCl + H2O = HCl + Na + OH-. Hence, the salt changes to become a magmatic gas that is carried up to the surface. The more appropriate term, however, would be volatile. This is because, deep within the Earth, the pressure is great enough that these chemicals are not actually gasses, but dilute liquids. It is only at or near the surface that these chemicals actually exsolve.
Carbon Dioxide is a product of subduction, as well. Limestone, when subducted to a great depth with other silicate materials, releases its carbon (limestone is composed of CaCO3). Thus, we have carbon dioxide rising as a volatile also. At basaltic volcanoes, far from subduction zones, we have abundant CO2 also, though. This gas is, probably, composed predominantly of ancient carbon from the earth’s early history. As far as fumaroles go, most of these gasses are from hydrothermal reactions surrounding the magma, and are not actually from the magma itself!
As the magma and its batch of volatiles rise toward the surface, a number of events occur; moreover, the solubility of volatiles depends greatly upon the composition of the magma during the ascent. Here we’ll look at how the volatiles interact with each other during their trip toward the surface and into the atmosphere.
First is water. Traveling with magma is no simple thing, however, even for water. Instead of moving as H2O, as we normally see it, the water usually loses a hydrogen atom to become OH-, or a hydroxyl group. OH- is a reactive chemical that binds to other minerals. Most important is its relationship with silica. As you should already know, one of the most important controlling features in volcanology is the amount of silica, or SiO2, present in the melt. More silica means higher viscosity. So why is this?
Because SiO2 is unstable as a single unit. The stable configuration is SiO4, but let’s look at what happens. This is not going to be totally accurate due to limitations in time for putting this example together, but it should suffice to help you understand what’s happening. SiO4 wants to look like this: O-O-Si-O-O. The Si atom wants to be surrounded by four oxygen atoms. When you mix a large amount of Si and O (silicon and oxygen), however, what you get is: O-O-Si-O-O-Si-O-O-Si-O-O-Si. Do you see what happened? All of the silicon atoms are surrounded by four oxygen atoms, but count them. It’s not SiO4 anymore, it’s SiO2. 4 silicons and 8 oxygens.
And this is what our silica is doing in the magma. It’s forming chains, which are very durable and do not like to bend or move about. This is making our magma more viscous. But what does OH- do? It breaks the chains! Now, we have it interfering as HO-O-Si-O-OH. The hydroxyl group will latch on to oxygen, but not silicon. By breaking the chains, the hydroxyl groups (and thus water) reduces the viscosity, making the mix more fluid. The importance of this effect can not be over emphasized! As you may have guessed (though not necessarily), the solubility of water increases with silica content. Solubility refers to the amount of chemical a mixture can hold. With more silica, we can have more OH- in the mix, binding with our silica chains.
Water isn’t the only volatile in the mix, and it may not be the most complex. CO2 is present in large quantities, but generally much less than water. In fact, the solubility of CO2 (ability to stay dissolved in the magma) is 50-100 times less than H2O. Carbon dioxide, like water, actually exists as two types of chemicals in the magma. The first is regular CO2, as we’ve already talked about; the second is CO3, or carbonate. Carbonate is not volatile and behaves like any other melted solid in the magma…except that it’s solubility depends on the amount of SiO2 in the mix.
In this situation, silica and carbonate don’t get along: thus, silica rich magmas like rhyolite have very little carbonate: it’s all CO2 instead. In very SiO2 poor magmas like basalt, there is much more CO3 and thus less carbon dioxide. If the magma evolves a higher silica content during its ascent, as often occurs, the carbonate solubility decreases which causes the CO3 to lose an oxygen atom to become the volatile CO2.
Of course, it’s important to remember that the solubility of all volatiles are related to each other. For example, if our magma is maxed out in water content, it may not be able to hold enough gas to continue holding CO2, meaning it has to be released. This is true for all volatiles; magma can’t hold an infinite amount of gas, period.
Sulfur behaves rather strangely as well. It is present as sulfide and sulfate, which refers to the manner in which it bonds with other atoms. While the behavior of sulfur with other chemicals in the mix is complicated, it can be summarized rather simply. That is, the solubility of sulfur depends greatly upon the abundance of iron. Sulfur readily binds to iron, meaning that if you have more iron in the magma, it can generally hold more sulfur. Moreover, the solubility increases with temperature meaning you may have exsolution during cooling periods, or the precipitation of sulfur-containing minerals.
Chlorine, which can reach the surface as hydrochloric acid, is a bit simpler. Its solubility depends heavily upon the abundance of sodium (remember the chemical composition of salt). When the chlorine content of the magma is maxed out, our Cl is present as a molten salt within the magma! Where the magma has a very high content of water and carbon dioxide gases, the chlorine readily dissolves. By the time it reaches the surface, it has usually bonded with a free-floating hydrogen atom and become HCl.
Fluorine solubility is not very well understood, although we know that it rises dramatically with silica content. Fluorine is usually pretty sparse in magmas, though in some cases it has been devastatingly abundant (such as Iceland in 1784). To learn much more about fluorine solubility, you’ll have to do your own research!
So now you know quite a bit more about volcanic gasses and how they’re important to volcanology. We’ve looked at the impact they can have on people around them, as well as how they interact with the rising magma and each other. It was T.A. Jaggar, founder of the Hawaiian Volcano Observatory, who wrote in 1940: “The observatory worker who has lived a quarter of a century with Hawaiian lavas frothing in action, canot fail to realize that gas chemistry is the heart of the volcano magma problem.” There is much more to know about these gasses than has been discussed in this article. For more in depth information, I would suggest looking to textbooks and scientific journals.