In 1982, the Chichón volcano, popularly known as ‘El Chichonal’, starred in one of the most violent eruptive episodes in modern history of Mexico, even altering the global climate. Four decades later, the sleeping giant of Chiapas draws the attention of science again and not because it has begun to release lava down its slopes, but rather something more subtle and geochemical.
New signs. Recent data presented by the UNAM Institute of Geophysics It is showing quite notable physical-chemical variations, from temperatures that exceed the boiling point at the bottom to the appearance of sulfur spheres. And this is something that is causing geologists to call for more surveillance and exhaustive control of people who approach the crater.
From algae to sulfates. For years, Chichón Crater Lake has been a visually striking tourist attraction, often characterized by shades of green due to the presence of algae. However, Patricia Jácome Paz, researcher at the UNAM IGf, has revealed at the Volcanology Seminar that the lake’s ecosystem has been transformed.
Monitoring has detected a fairly aggressive transition: algae have given way to sulfates and silica. And this is something that informs us about what is happening at the bottom of the volcano, highlighting above all the great gas activity that is evidenced by the appearance of sulfur spheres.
Other factors. Beyond sulfates, extreme temperatures are also making an appearance at the bottom of the lake where up to 118 °C have been recorded. Additionally, there is an increase in chloride concentration, suggesting greater interaction between magmatic gases and groundwater.
The invisible danger. Beyond water chemistry, the biggest current risk to visitors and locals is not an imminent explosion, but what is not seen. Primary sources from the UNAM and Civil Protection reports warn about the emission of toxic gases that can end up in the airways.
The 2025 analysis highlights in this case the presence of a large amount of carbon dioxide and hydrogen sulfide. Their big problem is that, having a density greater than that of air, they tend to accumulate in the lower areas of the crater, creating deadly “traps” for those who descend without protective equipment. Inhaling it can cause dizziness or serious respiratory damage and that is why access to the crater is restricted.
Eruption imminent? Without a doubt the million dollar question, and the short answer is no. In these cases it is necessary to differentiate between magmatic activity and hydrothermal activity, that is, water heated by the residual heat of the volcano.
In this volcano, at the moment no deformation of the terrain has been detected that indicates that the dreaded lava is emerging, but a very active hydrothermal system has been seen that can generate phreatic events. These are nothing more than minor explosions caused by water vapor pressure, not lava, but they are still very dangerous in the immediate radius of the crater.
Listening to the volcano. So that these changes do not take anyone by surprise, science has deployed a reinforced monitoring protocol which includes additional stations installed from June 2025 to detect the increase in local seismicity. In addition, it is decided to make a millimeter measurement of the terrain to rule out swelling and have constant water sampling.
A great impact. And the truth is that all these measures are not nonsense, since they would affect a 30 km radius where approximately 100,000 people live. But the most important thing is undoubtedly the newspaper library, keeping in mind the year 1982, which determines that the population is already trained with the steps to follow in the event that this situation arises.
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