Popocatepetl, Mexico's most studied and dangerous volcano

29/06/2020

The word Popocatepetl means in Nahuatl "smoking mountain" and its history is documented for more than 400 thousand years, date from which it has registered explosive eruptions. Popocatepetl is one of the most active volcanoes in the country, considered one of the most dangerous on the planet due to the 25 million people who live less than 100 kilometers from the crater.

CENAPRED's monitoring report on Popocatepetl volcano
CENAPRED's monitoring report on Popocatepetl volcano

In the surrounding villages, the stories of its inhabitants are impregnated with familiarity with Don Goyo. Everyday life happens while scientists study Popocatepetl from various disciplines in order to describe and understand its current behavior.

Every day, when the sun casts its first rays in the village of Santiago Xalitzintla, don Tomás Jiménez, a strong and lucid 84-year-old man, dedicates a prayer: "Goyito, good morning, Rosita, good morning. How did you wake up? Take care of us, don't scare us or hurt us".

Don Tomás is one of the two thousand people who live in the closest town to the Popocatépetl volcano, which is located only 12 kilometers from its crater. They have all learned to live amidst spontaneous evacuations, tremors, and explosions, without thinking about moving their lives away from the danger of living so close to an active volcano.

"Nothing from fear, thank God nothing, I always have faith. Right now the volcano is doing its job, the eruptions that it does are because everything that damages it pushes it out. The truth is that in these 84 years of living here I haven't had any fear, because since it already has a hole, it's already breathing," says Don Tomás, sitting in the town's main square, while he takes a small drink from a glass with pulque.

Since he was very young, don Tomás has dedicated his days to the countryside. He mainly plants corn and beans. He had eight children, four women, and four men, and now has 58 grandchildren. In 84 years he has managed to forge a life in Santiago Xalitzintla, and despite the warnings of danger that sometimes come so close to a volcano, he does not consider leaving the village.

A smoking mountain

In Mexico, there are a little more than two thousand volcanoes distributed throughout the territory. However, most of these are no longer active and do not represent any danger to the towns or the ecosystems.

Currently, the National Center for Disaster Prevention (Cenapred) lists only 16 active volcanoes, in Baja California Sur, Revillagigedo Islands, Nayarit, Jalisco, Colima, Michoacán, Mexico City, State of Mexico, Puebla, Veracruz, and Chiapas. However, there are more volcanoes of which little is known about their level of activity.

One of the volcanoes that in recent years has received much attention is Popocatepetl, a colossus classified as a stratovolcano that has a height of 5,452 meters above sea level, the largest diameter of its crater is 900 meters and is located in the center of the country, surrounded by large cities in the states of Puebla, Mexico, and Morelos.

The volcano is one of the most active in the country and is considered one of the most dangerous on the planet due to the 25 million people who live less than 100 kilometers from the crater.

In Nahuatl, the word Popocatepetl means "smoking mountain" and its history has been documented for more than 400,000 years, since when it has recorded major explosive eruptions that have severely affected the surrounding settlements.

According to the document entitled History of the volcanic activity of Cenapred's Popocatepetl, an analysis of the volcano's morphology and geological evolution shows that it is a volcano that has been active for more than half a million years and has had several stages of growth, which formed at least three previous volcanoes.

These volcanoes were destroyed by extraordinarily large eruptions: V. Nexpayantla more than 400,000 years ago, Ventorrillo about 23,000 years ago, and El Fraile 14,500 years ago. The modern cone is built on the remains of these volcanoes. For this reason, Popocatepetl is classified as a stratovolcano, since its growth occurs from several eruptive phases.

In recent history, after seventy years of inactivity, a gradual increase in the fumarolic activity of the volcano was noted, which resumed its activity in December 1994. In these last 17 years, it has had effusive and explosive stages associated with the growth and destruction of lava domes inside the crater.

Its ashes have reached the cities of Puebla and Mexico and even more distant populations such as Querétaro and Veracruz. The incandescent fragments thrown by its explosions have reached almost five kilometers away from the crater and about 3.5 kilometers from the populations closest to the volcano. It has also presented on two occasions pyroclastic flows and lahars, which even reached Santiago Xalitzintla, Puebla, the nearest town.

According to Dr. Jaime Urrutia Fucugauchi, a researcher at the Institute of Geophysics, stratovolcanoes have the ability to remain active for thousands of years and to be built in stages. For this reason, many studies on Popocatepetl focus on trying to differentiate these stages.

"The old stages are no longer active so they do not generate seismicity. We need then to use different methods that are able to distinguish the changes in rock density and see the different stages of construction and the whole part of the volcanic duct system," he explained.

Don Goyo's internal structure, an enigma

Urrutia points out that the interest in studying stratovolcanoes like Popocatepetl is because they offer a different behavior to smaller volcanoes, in the type of eruption and internal structure. However, so far the models that exist to monitor them are based on the seismicity part and give an idea of the structure, but only in the parts that fracture and there is no idea of the structure of volcanic ducts underneath.

For this reason, in order to have a more exact idea of the internal structure of the volcano, Urrutia Fucugauchi is working on a joint project with the Institute of Physics and Dr. Arturo Menchaca Rocha to build an instrument capable of taking an x-ray of the internal structure of the volcano by tracking particles called muons that allow us to observe the interior of large volumes by capturing the differences in density of the mountain.

This ambitious project emerged after a similar exercise at the Pyramid of the Sun in Teotihuacan, when the team installed a muon detector inside the pyramid. When this project was in its final phase, the possibility of developing a similar study in Popocatepetl was raised.

According to Urrutia, it is complicated to analyze stratovolcanoes like Popocatepetl using traditional methods, since factors such as their large size and difficulties in access when active present additional challenges.

"The processes that trigger eruptions are generated at different depths in the volcanic ducts, so only muons have the ability to generate timing radiographs and one can monitor the change or access of magma and the fracture and gas triggering part of the eruptions," Urrutia said.

To be able to perform this X-ray, Urrutia explained that various detectors must be installed depending on the area to be studied, starting from the cone part, and moving them around to be able to have X-rays from different angles.

The project was based on various data from aerial geophysics, which allows us to see the structure in order to have an initial model of the interior of the volcano, from how the rocks are, to how the ducts are and the temperatures. On that model, we are working to make the design of the placement of the detectors. The advantage of the project is that the muons give a better spatial resolution.

Although the project was announced in 2016 and development is expected to take at least five years, progress is being made in building the geophysical models to construct the simulations for the muons.

"This already gives us an idea of what the best angles for detection are and also allows us to select observation sites and models of the internal structure of the volcano, although we would like to improve the spatial resolution," he said.

Barrancas, an ancient sign of the volcano's features

Just as various investigations focus on the internal structure of Popocatepetl, Osvaldo Franco Ramos, a researcher at the Institute of Geography of the UNAM, has focused his studies on geomorphology, that is, on the forms of the surface of the volcano, in order to describe, understand its genesis and current behavior, as well as to date erosion events or some lahars.

Various investigations focus on the internal structure of Popocatepetl. Images: Cenapred
Various investigations focus on the internal structure of Popocatepetl. Images: Cenapred

"We have been doing paleoclimatic type analysis to see more of those variations in time in that same area. Now we are carrying out a new project which is to date pyroclastic events that the volcano has also had. The last one it had was in 2001 that somehow affected the forest," he said.

In a research entitled Age, geomorphological dynamics and typology of ravines in the northern sector of the Popocatepetl volcano, Mexico, Dr. Franco made an analysis of which are the ravines that, by some event, are the most dynamic and where lahars can be generated with greater intensity and which ravines are not going to be so affected in case of a relevant volcanic eruptive event.

"It is important to know the age of the ravines because it is very much related to stability. In very old canyons, some that were formed 17,000 years ago and are still there, they are already stabilized. There are younger ones, like the ravines that are connected to the main building, which are about a thousand years old, that began to form after the last strong eruption of the volcano. These are still very active and are in the process of stabilizing," he said.

An example of this type of dynamic canyon is Huiloac, which drains towards the town of Santiago Xalitzintla, making it one of the most dangerous for the population. For this reason, says Franco, it is important to know the age and current dynamics of the ravines, in order to know the areas of greatest danger in case of a major eruption or an event of the weather type.

Another dynamic ravine is to the northwest of the volcano, Nexpayantla, which drains the town of San Pedro Nexapa, on the side of the State of Mexico. Franco points out that in that town there is evidence that at some point, which is not precisely dated, very large blocks came down to that town.

"On the one hand, when there are important lahars, these ravines are filled with material such as ash, these fragments fall very close to the headwaters of the ravines and with the rain, they are removed, especially above four thousand meters where the forest ends have been very filled with blocks of ash. However, in some sectors, it has eroded further. So it is a dynamic of erosion and accumulation that is very characteristic of these northern ravines, which are the youngest," he said.

The experts have studied the northern ravines with special attention because of their intense dynamics and because of their age, since the younger they are, the less stable they are. Furthermore, according to Franco, years ago in the northern part of Popocatepetl, there was a glacier that provided large amounts of water and favored movement.

Living in a yellow phase, a constant danger

It is Saturday morning and the main square of Santiago Xalitzintla looks half empty. Few children play at the kiosk, older people bask in the little sun that filters through, and a handful of 30 street vendors offer everything from traditional goods such as ceramic dishes and wicker baskets to pizzas and instant soups.

Popo map and evacuation routes. Photo: CienciaMx
Popo map and evacuation routes. Photo: CienciaMx

There are 786 homes in the village, home to approximately 2,196 people. Most of them are dedicated to the countryside and carry out all their activities within the same territory. Almost all of them know each other. When passing through the center of the village, one can hear from the formal "good morning" to the effusive "what happened, compadre!

Despite the peace and harmony in the village, there is a huge sign in front of the town hall that reminds people of the constant danger they may face if Don Goyito, as they call Popocatepetl volcano, increases his activity.

It is a map where the towns surrounding the volcano are located and the evacuation routes established by Civil Protection in the event of an emergency. A red dot marks the areas at greatest risk, orange those at moderate risk, and a green box indicates places that can serve as temporary shelters. Cholula, which is located one and a half hours away, is the closest temporary shelter to Santiago Xalitzintla.

In Mexico, there is a Volcanic Alert Traffic Light, which is the mechanism of the National System of Civil Protection that keeps the population informed about the different levels of danger presented by the activity of the Popocatepetl volcano.

The scientific community and the Civil Protection authorities permanently monitor the volcano and determine the color that the traffic light should show in each location. In recent years, this traffic light has varied between different levels of yellow alert, from phase 1 to phase 3. This means that the population has to be prepared at all times for a possible evacuation, because the volcano shows activity or increases it at an accelerated rate.

This means that the population has to be prepared at all times for a possible evacuation, because the volcano shows activity or increases it at an accelerated rate. These characteristics are given through volcanic seismicity and ash emissions in phase 1. In the next phase, the danger is manifested by falling incandescent fragments, the possibility of pyroclastic flows due to explosions, short-range flows of mud or debris, and water or gas vapor plumes. Phase 3 is manifested by the destruction of lava domes, persistent gas fumaroles, and medium-range pyroclastic flows.

María Dolores Gutiérrez is 66 years old and has lived in Santiago Xalitzintla since she was born. She says that several times Civil Protection personnel have arrived to evict them, but many people in the town refuse to leave.

"On some occasions, they have come to evict people. When they say that we are in danger, buses come to take people away, but if I have to be honest, we haven't left. When the land resonates here, that's when we get scared, but we don't go out because we think that nothing will really happen, because it already has. Maybe one day the danger will be real but in the meantime, we are sure that nothing will happen," she said.

María Dolores Gutiérrez is 66 years old and has lived in Santiago Xalitzintla since she was born. Photo: CienciaMx
María Dolores Gutiérrez is 66 years old and has lived in Santiago Xalitzintla since she was born. Photo: CienciaMx

María, who is dedicated to the traditional pottery trade, says that for them, in addition to the Volcano Warning Light, the real danger signal is when the volcano is already erupting lava. In that case, she says they already have a contingency plan: "What we would do would be to go out through a hill that we have here, where there is an auditorium that is supposed to shelter us. When the color and phase of the traffic light changes, there is an alarm and then they warn us if we are in danger," said Maria.

Maria and her husband live alone in the town because their three children decided to migrate to Mexico City. That is why they say they must take care of each other and prevent any incidents. At home, they both have a backpack ready with documents, birth certificates, receipts, and the deeds to their house.

Dr. Urrutia points out that one of the problems with people not evacuating is that Cenapred's risk indicator has been at that stage for many years, so people are already beginning to normalize it.

However, the researcher points out that for an explosion to happen, the volcano must also go through various phases. The dome of the volcano has been built over the years by volcanic activity, when there is more emission of ash and pyroclastic flows.

"The dome grows until it explodes and the growth cycle begins again. Today it is possible to see this growth through the photographs taken periodically on the part of the summit inside the crater," he said.

Urrutia said the National Center for Disaster Prevention has a monitoring system that includes measurements of seismicity. This is the main monitoring method in the system because it detects fracture, gas movement, and interior magma, which are tremors of different types including what is known as tremor, which is a high-frequency vibration.

"Cenapred also sees mentions of the volcano's tilt because if the magma moves inside, the top of the cone deforms and changes the tilt of the ground. It also monitors the emission of ashes that give indications of where the material is being generated," he said.

The main objective of the monitoring is to take care of the populations that are settled on the slopes of the volcano, several of which have been affected in strong eruptions in the past and are now even built on material from lahars and flows of the volcano.

The Ministry of the Interior in its Plan of Operations of the Popocatepetl volcano classifies the towns near the volcano in three areas of danger, having the crater of the volcano as the central axis. Within a radius of 15 kilometers, it is called "high-risk area" and includes 16 municipalities in the state of Puebla, six in Morelos, and eight in the State of Mexico.

In a radius of 30 to 60 kilometers is the 'medium risk zone', which comprises 22 municipalities in the state of Puebla, nine in Morelos, six in the State of Mexico and 18 in Tlaxcala. From 60 to 90 kilometers is the 'low-risk zone', comprising the rest of the populations near the volcano in the states mentioned.

How toxic is a volcano explosion to the population?

The danger maps of the Popocatepetl volcano, which have been prepared by experts based on the reconstruction of the geological history of the volcano, list the emission of ash as one of the greatest risks to the population at short and long distances.

Volcanic ash is composed of fine particles of fragmented volcanic rock that, in suspension, block the sunlight. When they fall to the surface they can have an acid coating that causes irritation to the lungs and eyes. This acidic coating disappears easily with rain, but rain can wash it away, contaminating local water supplies.

In most eruptions, ash fallout has little adverse health effect; however, it can affect large areas around volcanoes and disrupt the normal life of people in the region.

The health effects of volcanic ash can be classified into several categories: respiratory effects, eye symptoms, skin irritation, and indirect effects.

Respiratory effects include nasal irritation and discharge, sore throat, and difficulty in breathing. On rare occasions, they can produce serious lung diseases. Eye symptoms include tearing, eye pain, and acute conjunctivitis. Although not very common, ash from the volcano can also cause skin irritation.

María and don Tomás agree that although it is common for the volcano to emit ash, it does not often reach the town because the wind carries it further, to Cholula or even to Mexico City. Both are firm in their belief that the risk they run is little, if not controllable.

"The truth is that we live in peace. Our grandparents from the beyond say that the volcano has always thundered and made noise. They say that when it gives off its smoke, it is because 'Goyito' is already giving off his cigarette. We are in a firm plan that nothing happens. Although we don't know what will happen next," concluded María.

Volcano warning light explained

Cenapred is the body in charge of constantly monitoring the activity of the country's volcanoes, to determine the levels of danger, in this way it can issue preventive safety measures.

Yellow

In the case of the Yellow Phase 2, this means that there is an increase in activity with the presence of water vapor and gas plumes, ashfall, incandescent fragments, creation and destruction of lava domes, pyroclastic flows and short-range mud and debris flows. The other two phases have the following characteristics:

Green

At this stage, there is little or no activity, and the only recommendation from the Commission is that people should be kept informed of evacuation routes, assembly points, and temporary shelters. In its first phase, the volcano is considered to be calm and in the second phase, it has minimal manifestations or spontaneous seismic activity.

Red

A Red Volcano Alert is not only an alarm signal, but it is also a sign that the authorities and the population should start evacuating the areas that could be affected in case of an explosion of the colossus. In its first phase, it is considered that there is an intermediate to the high danger that presents eruptive columns of several kilometers, as well as important ash fall and pyroclastic and mudflows in nearby areas.

By Janet Cacelín. This work whose author is Agencia Informativa Conacyt is under a Creative Commons Attribution 4.0 International license.