A fresh volcanic eruption near Vanuatu this week has again captured attention. When it comes to volcanic activity, what can we predict?

A study released by ETH Zurich in Switzerland attempted to uncover how to predict an eruption, and how big it will be when it occurs.

Predicting when a volcano may erupt is the simpler of the two. The team behind this study used monitoring data from La Palma in the Canary Islands for their case.

Using seismic data, Cumbre Vieja’s eruption in 2021 was tracked in near real-time as lava travelled to the surface, enabling an accurate prediction within days.

What is less simple is predicting how powerful the explosion will be. Size plays a role. Volcanoes like those on La Palma and Hawaii are too small for large eruptions but those in subduction zones like Japan and Indonesia are less predictable.

Analyzing 245 eruptions the team turned their focus to the volcanic conduit and the role of gasses.

Initial findings suggest that if magma in the conduit is low on water the explosion will be smaller, likewise if crystals are present which form channels for gas to escape before pressure builds.

If magma possesses a water content of more than 3.5% the risk of an explosive eruption is significantly higher although paradoxically if this surpasses 5.5% that risk drops again.

So, tracking the ‘when’ is simple, but predicting the strength of an eruption involves much more work.