Lake Chalco reveals the amount of volcanic activity over the past 400,000 years.

Mexico City has received, on average, one centimeter of volcanic ash every 900 years for the past 400,000 years, according to an international team led by the University of Edinburgh (Scotland). The reading was taken after evaluating the sediments of Lake Chalco , southeast of the capital.

The work identifies more than 450 ash layers , as well as landslide and lava flow deposits, in a 500-meter sediment core drilled in 2016.

" One centimeter of ash may not seem like much , but it is enough to cause respiratory problems for a large part of the population and collapse critical infrastructure such as sewage systems," volcanologist Alastair Hodgetts, a researcher at the universities of Edinburgh and Birmingham and lead author of the study, published in the international journal GSA Bulletin, explained to EFE .

In one out of every ten episodes, the ash exceeded ten centimeters in thickness: "If it happened today, it could damage buildings and force mass evacuations," he warns.

From Popocatépetl to distant ashes

Although most of the recorded eruptions come from nearby volcanoes, such as Popocatépetl or Nevado de Toluca, preliminary analysis from another study suggests that the core could also contain traces of ash from more than a thousand kilometers away, in Guatemala , although that finding has not yet been published or peer-reviewed.

The Mexican capital is surrounded by numerous active volcanoes and adjacent to the Sierra Chichinautzin volcanic field, which is home to more than 200 cones.

"Understanding the risk of these fields is complicated because their past activity is poorly documented. Our study allows, for the first time, an assessment of activity patterns and eruption rates over nearly half a million years," Hodgetts noted.

A natural laboratory for volcanic risk

The researchers used existing dating data and relative color differences to infer the origin of each ash layer , distinguishing between stratovolcanic volcanoes, which can erupt multiple times, and monogenetic cones, which only erupt once.

The result is an exceptionally complete geological archive that reveals peaks and pauses in eruptive activity and serves as a basis for estimating the probabilities of future events.

Hodgetts emphasized that these data do not allow us to predict the date of the next eruption, but they do allow us to gauge the risk.

"In the record, one centimeter of ash corresponds to a significant eruption, capable of closing airports, disrupting water and power supplies, and affecting the health of the population, especially vulnerable people," he said.

An exportable model

In addition to its value for Mexican civil protection , the method can be applied to other cities located in volcanic areas.

"There are cities in Italy, New Zealand, and Indonesia that could benefit from similar studies if they have lakes that retain sediment over the long term," the researcher says.

The project was part of an international effort funded by the International Continental Scientific Drilling Programme, with the participation of teams from Mexico, the United Kingdom, the United States, Italy, and Spain.

For Hodgetts, one of the most rewarding moments was seeing how painstaking laboratory work—analyzing the nucleus millimeter by millimeter—translated into key information for more than 20 million people.

"It's exciting to realize that the archive of ash in front of you is, in fact, a memory of how volcanoes have shaped and threatened one of the greatest cities on the planet," he said.

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