Scientists obtain first 3D images inside Mexico’s Popocatépetl volcano

POPOCATÉPETL VOLCANO, Mexico — In the predawn darkness, a team of scientists climbs the slope of Mexico’s Popocatépetl volcano, one of the most active in the world and whose eruption could affect millions of people. Its mission: to understand what is happening under the crater.

For five years, the group from the National Autonomous University of Mexico climbed the volcano with kilos of equipment, risking data loss due to bad weather or a volcanic explosion and using artificial intelligence to analyze the seismic data. Today, the team created the first three-dimensional image of the interior of the 17,883-foot (5,452-meter) volcano, which tells them where magma is accumulating and will help them better understand its activity and, ultimately, help authorities better respond to eruptions.

Marco Calò, professor in the department of vulcanology at the UNAM Institute of Geophysics and project leader, invited The Associated Press to accompany the team on its latest expedition, the last before publishing its research on the volcano.

Inside an active volcano, everything moves: rocks, magma, gas and aquifers. All this generates seismic signals.

Most of the world’s volcanoes that pose a risk to humans already have detailed maps of their interiors, but not Popocatépetl, despite the fact that some 25 million people live within a 100-kilometer radius and homes, schools, hospitals and five airports could be affected by an eruption.

Other scientists took some early images 15 years ago, but they showed conflicting results and did not have enough resolution to see “how the volcanic edifice was being built,” and especially where the magma accumulated, Calò said.

His team increased the number of seismographs from 12 provided by Mexico’s National Disaster Prevention Center to 22 to cover the entire perimeter of the volcano. Even if only three can alert in an emergency, it takes many more to understand what is behind these emergencies.

The devices measure ground vibrations 100 times per second and generate data that Karina Bernal, 33, a doctoral student and researcher on the project, processed using artificial intelligence to adapt algorithms developed for other volcanoes.

“I taught the machine the different types of tremors that exist in El Popo” and so they were able to catalog the different types of seismic signals, she said.

Little by little, scientists began to deduce what types of materials were found, where, in what state, at what temperature and at what depth. Later they were able to map it.

The result is far more complex than the drawings of volcanoes most saw in school, with a main vent connecting a magma chamber to the surface.

This first three-dimensional cross-sectional image extends 18 kilometers below the crater and shows what appear to be various pools of magma at different depths, with rocks or other material between them and more numerous toward the southeast of the crater.

Popocatepetl emerged in the crater of other volcanoes in its current form more than 20,000 years ago and has been active since 1994, spewing plumes of smoke, gas and ash more or less daily. The activity periodically forms a dome above the main vent, which eventually collapses, causing an eruption. The last one was in 2023.

Calò, a 46-year-old Sicilian, speaks passionately about El Popo, as Mexicans call the volcano, telling anecdotes.

He explains that its height can change due to eruptions and recounts how Popocatépetl, in the first century, had its own “little Pompeii” when a village on its flanks, Tetimpa, was buried in ash. In the early 20th century, it was human actions – the use of dynamite to extract sulfur from the crater – that caused an eruption. And even though El Popo emits more greenhouse gases than almost any other volcano, its emissions are still a small fraction of what humans generate in neighboring Mexico City.

For years, Calò studied volcanic activity from his computer, but trying to “understand how something works without touching it” brought a feeling of disappointment, he said.

This changed with Popocatépetl, a volcano that he describes as “majestic”.

After hours of hiking the volcano’s flank, Calò’s team set up camp in a pine forest at about 12,500 feet above sea level, a place seemingly safe from pyroclastic explosions, since the trees had managed to reach significant height.

A little further up the mountain, trees and brush give way to ash and sediment.

They must cross a lahar, a mixture of rocks and ash which during the rainy season turns into a dangerous mudslide carrying everything in its path. Today, the dry clearing offers a spectacular view: to the east, Pico de Orizaba — Mexico’s highest volcano and mountain — and the inactive volcano La Malinche; to the north, Iztaccíhuatl, a dormant volcanic peak known as “the sleeping woman”.

The sounds of Popocatépetl seem to multiply at night with the echoes. An explosion like a rocket may appear to be coming from one direction, but a blast of smoke from the crater belies the true source.

Karina Rodríguez, a 26-year-old master’s student on the team, said you can also hear small tremors in the earth or even ash falling like rain when the volcano is more active. On dark nights, the crater rim glows orange.

Having direct knowledge of the volcano gives a much more objective idea of ​​the limits of their analysis, Calò said.

“We have a natural laboratory here,” he said. It is “very important to be able to understand and give residents detailed and reliable information about what is happening inside the volcano.”

At 4,200 meters above sea level, their backpacks filled with computers, gas analysis equipment, batteries and water start to weigh more and their pace slows.

Ash, dark and hot, dominates the landscape here.

At a seismograph station, the team digs up the equipment and is happy that it still works. They download its data and rebury it.

A “volcanic bomb”, a rock a meter and a half in diameter and weighing tons, marks the path and gives an idea of ​​what the start of an eruption can mean. This is why the upper area of ​​the volcano is restricted, although not everyone pays attention to it. In 2022, a person died after being struck by a rock approximately 300 meters from the crater.

A bottle of tequila near a rocky hollow, known as El Popo’s navel, alludes to some of the traditions surrounding the volcano, including an annual pilgrimage to what some consider a point of connection with the underworld.

While digging up one of the last seismic stations, Calò’s face falls. The last recorded data dates back several months. The battery is dead. Sometimes rats chew through machine wires or an explosion causes more serious damage.

The project has provided some certainty and if repeated it will allow analysis of changes that will eventually help authorities make better decisions in the event of eruptions.

But Calò says that, as always happens with science, it has also generated new questions to try to answer, such as why tremors are more frequent on the southeast side – where magma is more accumulated – and what implications that might have.

This was the last expedition before publishing their years of work to map the volcano’s interior. Watching the inside of the volcano move in 3D on a computer screen is worth it.

“It’s what pushes you to start another project and keep climbing,” said Rodríguez, a master’s student.