The link between magma and gases that rise out of volcanoes might be more complicated than previously thought, with the gas composition changing as it cools, reports a paper published online this week in Nature Geoscience. The findings may impact volcanic monitoring efforts, as gases injected into the atmosphere during eruptions are used to yield information about the magma source and the nature of volcanic activity.

As magma rises through a volcano towards the Earth’s surface its pressure decreases. This causes the gases dissolved in the magma to be released as bubbles. As the composition of the magma and volcanic gases are linked, studying the composition of one shines light on the other. The link, however, may be more complicated than previously thought.

Clive Oppenheimer and colleagues measured the composition of gases released from Kilauea, an active volcano in Hawaii, during periods of both gentle and more vigorous degassing in 2013. They find that when the gas bubbles rising to the surface get large, they cool down and the gas inside loses contact with the magma, resulting in rapidly changing gas compositions. Hazard and risk assessments based on the monitoring of volcanic gases, however, are based on gas compositions averaged over relatively long periods of time. The authors suggest that this time averaging may be missing important information about volcanic behaviour and the dynamics of degassing that should be factored into hazard assessments.