Volcanoes are complicated. Whether they’re acting destructively or they’re building new land from the depths of the ocean, volcanologists are still unable to pinpoint the precursors to many types of eruptions.

Progress is being made, however, and a new study published in the journal Earth and Planetary Science Letters details a new mechanism that could help researchers predict a particularly deadly type of eruption. Although they might not be the most well-known eruption style, phreatic or hydrothermal explosions kill a disproportionate amount of people, as they often erupt with no warning of any kind.

A team of volcanologists, led by Dr. Maarten de Moor from the Volcanological and Seismological Observatory of Costa Rica, have found that the composition of the gas emitted by the volcano prior to one of these blasts actually signals how close it is to initiating a hydrothermal explosion. Identifying this surface accumulation of gases could save hundreds of lives a year.

“Before this study, phreatic eruptions were primarily thought to… occur with no appreciable precursors,” said de Moor in a statement. “Our study shows that there are clear short-term changes in gas compositions prior to phreatic eruptions.”

^{The crater lake at Poás volcano. Peter Andersen/Wikimedia Commons; CC BY 2.5}

Hydrothermal explosions aren’t technically eruptions, as no new magma is ejected from the volcano. Prior to this study, volcanologists thought that they occurred when a pocket of pressurized vapor – often water – is quickly heated by nearby magma or hot rock, causing it to burst out from its shallow, subterranean hiding place and force its way to the surface.

This often happens with no seismological precursor signal, which is why they tend to kill many unsuspecting scientists and hikers, as the eruption at Mount Ontake in 2014 so horrifically showed. They can happen at all kinds of volcanoes, from the relatively calm to the extremely violent.

For this new study, the team were studying Poás volcano, a stratovolcano in central Costa Rica. It’s erupted almost 40 times since 1828 in a variety of ways, but it’s noteworthy for frequently exhibiting phreatic eruptions, particularly near its northern lake. There were 60 of them in 2014 alone, some minor explosions and some ejecting ballistic rocks at near supersonic speeds.

Using gas detection instrumentation placed around the crater lake for two months in 2014, the team noticed that the surface ratio of sulfur dioxide to carbon dioxide – two common volcanic gases – increases in the run up to most explosions. The composition of the crater lake gas also begins to match that of the expected magmatic gas explosions when a blast is imminent.

^{A hydrothermal explosion at Poás volcano. Moor et al./EPSL}

Although sulfur is normally removed from volcanic systems by fast-moving hydrothermal fluids, the researchers think that a significant increase in gas coming from the magma itself inhibits this action, which is why there’s a spike in sulfur dioxide gases at the surface before an explosion. This means that hydrothermal explosions can also be directly caused by the magmatic system, not just near-surface pockets of pressurized gas as previously thought.

Remarkably, these researchers have stumbled upon a new type of eruption trigger. However, there’s a catch: The instruments used to pick up on this chemical signature are often destroyed in the process. Hydrothermal explosions frequently obliterate the surrounding landscape when they occur, and the atmospheres around crater lakes are so acidic that the air itself can eat away at the equipment.