Predicting precisely when dormant volcanoes are going to erupt is fraught with difficulty. A new study published in the journal Nature Geoscience has revealed that a late-stage, rapid formation of bubbles within the magma may be responsible for triggering the most dangerous, unpredictable eruptions. Picking up on this at the surface may provide the early warning sign, possibly days to months before an eruption, that volcanologists have been looking for.

Volcanoes erupt all the time, but those that invariably cause the most damage and death are the enormous calderas. These cauldron-like volcanoes, such as Campi Flegrei underneath Naples in Italy, or Yellowstone Caldera in Wyoming, are tens of kilometers across, concealing unimaginably vast magma sources beneath their surface.

Volcanologists spend a huge amount of effort trying to work out why these volcanoes erupt – which is often on timescales of tens of thousands of years – by trying to identify geological markers that form every time they explode. This research team, from the University of Oxford and the University of Durham, decided to look for these markers within Campi Flegrei itself.

They spotted several fluorine, chlorine and water inclusions (“bubbles”) within several common minerals produced during an explosive, 4,000-year-old eruption of Campi Flegrei. These bubbles, formed at various stages of the magma’s chemical evolution, are “frozen” into place within these minerals when they form.

As such, they act as little time capsules, revealing to geochemists what state the magma chamber was in at the point of their formation. By looking at the state of the magma chamber in the run-up to the eruption, the researchers noticed that the magma remained bubble-free for most of its molten life. Just before the eruption occurs, the magma suddenly becomes bubble-rich.

^{Crystals forming within the magma may cause a late-stage bubble build up near the surface. Angelina Babii/Shutterstock}

Conventionally, it is thought that when the pressure of the magma overcomes the surrounding, confining pressure of the rock, the chamber roof crumbles. This causes a massive, sudden depressurization event – one that produces a plethora of bubbles – that violently propels magma and gas out into the environment.

In this case, the research team think that this sudden bubble formation at Campi Flegrei indicates that the magma spent most of its life “undersaturated,” lacking many of these dissolved gases. A late-stage delivery of dissolved gases “oversaturated” the magma; with no more room for these gases within the magma, they escaped, forming bubbles.

This bubble build up, within days to months, would have put sudden pressure on the roof of the chamber, causing it to collapse and trigger an eruption. The authors of the study cannot be entirely sure what caused the delivery of additional dissolved gases, but they suggest that the gradual formation of crystals, which would cause the molten magma to become increasingly saturated in dissolved gas, may be to blame.

These gas bubbles will sometimes escape to the surface before an eruption occurs. As the study’s lead author Mike Stock, from the Department of Earth Sciences at the University of Oxford, noted in a statement, a “sign of an impending eruption might be a change in the composition of gases emitted at the Earth's surface.” By looking out for this late-stage bubble formation at the surface, imminent major eruptions could potentially be detected, and lives could be saved.