Today, in my why is gas important series, a brief look at 'Passive Degassing'. This is gas which is released at all times from a volcano, even when there is no eruption occuring. This can also be termed as quiescent. The major gases emitted include Water Vapour, Carbon Dioxide and Sulphur Dioxide (amongst others) and are usually in that order for amounts released.
Monitoring emissions of gas is important as it can inform volcanologists of amounts that are being released and whether these are in a normal range. An increase in emissions could indicate that an eruption is imminent, but a decrease could also indicate the same! It can vary greatly on a volcano basis. For example, an increase in emissions could indicate that a fresh batch of magma has entered the plumbing system with lots of entrained gas (the major driver of volcanic eruptions) and the increased emissions at the surface is showing this. Or there could be a large decrease in emissions, which could indicate that either there is a shortfall in gas supply from depth or that the gas cannot escape. This could indicate a build up in pressure and that a volcanic eruption is more likely. Of course, to make such assessments accurately a detailed history of amounts of gas released is needed to determine the normal behaviour for a given volcano. As with all volcanic situations, a large and unusual change in emissions doesn't necessarily mean an eruption will occur!
How do we measure these emissions? There are a variety of technqiues, those which involve collecting gas from fumaroles or a plume or remote sensing techniques be it via a satellite or more mobile ground-based cameras. Most monitoring is of Sulphur Dioxide due to its ease of measurement. Of course emission of other gases are also important. The eruption of Laki, Iceland released a large amount of Fluorine which is thought to have caused a large amount of disease throughout Europe.