The inner workings of volcanoes that erupt with little warning are being revealed, exposing potentially new ways to monitor them.

The big picture: The science of volcanoes — and the monitoring and forecasting tools it informs — is largely based on what's known about a handful of the world's most studied volcanoes, including Mount Etna in Italy, that typically show signs of eruption.

• There are an estimated 1,350 volcanoes on Earth that are potentially active — and about 100 observatories. Some monitor multiple volcanoes but many volcanoes are under-studied and unmonitored.
• "Eruptions don’t happen often so being able to have different eruptions to study from a lot of different contexts and volcanoes will help us to understand the general physics of eruptions and improve our models of the insides of volcanoes," says Emily Montgomery-Brown of the United States Geological Survey.

What's new: In a paper published this week in the journal Nature, researchers used extensive satellite data, measurements from a nearby network of seismometers and other tools to study the 2021 eruption of Mount Nyiragongo near the city of Goma in the Democratic Republic of Congo. It left more than 30 people dead and hundreds more injured, and forced the city to evacuate.

• Nyiragongo is a bit of a special case: It behaves as an open vent with a large lake of lava in the crater at its top. Because magma isn't bottled up in these types of volcanoes, pressure generally doesn't build up over time and trigger signals of an impending eruption. Typically the shape of the volcano itself changes, small earthquakes occur and the volcano anomalously releases gasses in the days and weeks before an eruption.
• But in Nyiragongo, there wasn't a signal until less than an hour before the eruption.
• Delphine Smittarello of the European Center for Geodynamics and Seismology in Luxembourg and her colleagues describe what may have happened in the new paper: Magma stored close to the surface of the crater opened a path or dike on the flank of the volcano. The dike quickly ruptured the stressed surface of the volcano, the eruption started and lava flowed from the volcano — "less than 40 minutes after the first detectable anomalous seismic event."
• “Being already close to the surface, the magma only had to propagate laterally for a short distance before erupting, leaving little time to detect and interpret the associated signals,” Smittarello and her colleagues write.
• After the eruption, cracks formed in the surface nearby, the volcano deformed and small earthquakes occurred.

"We show that an eruption can start with only tiny and very short-term precursors," Smittarello said in an email.

• Volcanoes share properties and patterns but each has differences that can affect forecasting their eruptions. Nyiragongo "is an excellent case study" of this type of magma movement, Smittarello said. "There is still a lot to do to improve the detection and real-time interpretation of these tiny signals."
• Similar mechanisms occur in Ambrym in Vanuatu and Kilauea in Hawai'i, where signs of dike formation have been used to forecast eruptions. But it is unclear if the technique can work elsewhere, Montgomery-Brown notes in an accompanying article.

How it works: Most eruptions come with warnings that can be monitored by networks of seismometers and other tools. But many volcanoes are in remote areas without seismic networks.

• Satellite data is increasingly being tapped to try to forecast volcanic eruptions by observing the deformation of volcanoes and the flow of magma under the surface.
• A study published earlier this year used radar data from satellites to measure Nyiragongo's lava lake levels as an indicator of changes in the volcano's plumbing that could aid forecasting.

But, but, but ... Satellite coverage of the world's volcanoes is also spotty, the background imaging needed to measure changes for forecasting is limited and data sharing between remote sensing experts, scientists and local civil defense officials needs to be improved, says Matthew Pritchard, a professor of geophysics at Cornell University.

• "We have great opportunities with all these satellites up there but in terms of realizing a volcano monitoring system we aren’t there yet," he says, adding a volcano observatory in space is achievable.

What to watch: Some experts warn the world isn't prepared for massive volcanic eruptions that could disrupt the globe.

• The risk of a super eruption is greater than that of an asteroid strike, Michael Cassidy of the University of Birmingham and Lara Mani of the University of Cambridge’s Centre for the Study of Existential Risk wrote last month in Nature.
• A recent analysis of ice-core records indicates magnitude 7 eruptions — which are 10 to 100 times larger than the volcanic eruption in Tonga earlier this year — happen about once every 625 years, they write. (Previous estimates put the events at once every 1,200 years.) The last volcanic eruption of that magnitude was in Indonesia in 1815.
• But the locations of many of the nearly 100 large volcanic eruptions thought to have occurred over the past 60,000 years are unknown.
• For that reason, they advocate for a "comprehensive approach" to identifying potentially active volcanoes and for satellites dedicated to improving volcano monitoring.